The AM Forum

Hello everyone,
I am trying to get my station up on 20M and with some hit and miss tactics, got my tuner to give me a 1.2:1 match to the dipole. I am only seeing 1/2 amp per leg of antenna current, 200 watt carrier. A little bit of heating on the clamps of the capacitor. I'm using the famous balanced tuner in the parallel config. Series gets me thinking that is for a short dipole.

20M is a quirky band and calling CQ didn't get any attention. After field day, conditions changed and it got really quiet around 8PM. I was troubleshooting distortion in my monitoring system and spinning wheels wasting time on that, when it turned out to be an overloaded receiver.

I sent an email to Tom Vu with this info, and I forgot he is recovering his station from possible tornado damage or a microburst.

My concern is about the RF current. It seems a little light. Dipole is 70 feet in the air and 180 feet long, fed by ladder line, approx 120 feet. On 160M, I see 3 amps of RF current with 300watt carrier.

There's a lot of things that come into play here. Frequency, height above ground, etc, etc, that determine antenna current.
Should I keep trying to call CQ and hope it's working?




I'll keep checking here for any guidance

Fred

170 ft for the radiator sounds in the neighborhood of 3 wavelengths on 20 mtr .... if radiating conditions remain balanced, the feedpoint is at high impedance .... 120 ft for the feedline sounds in the neighborhood of 2 wavelengths and will tend to not transform impedance .... your observed feedline currents tend to support this since high impedance suggests by Ohm's law high voltage and low current ... ignoring feedline velocity factor ( which is .94 or so for ladder line ), try adding or taking away 16 ft of feedline and see if your feedline current changes to a higher value ... 73   John

Hi Fred,

You're probably getting out, it's just that AM on 20 may not be all that common.  I know some of the guys are working AM on 20, like Jack but I don't know if there's a lot of activity there yet so that may be part of the reason for the low response.

I don't know what you mean by "famous balanced tuner" so I can't comment on that.  If you know ur power and current you can calculate you're Z at the point where the measurement is being taken.  I would think your E and I distribution along the line is whatever it is, and as long as it's balanced, ur vswr on the coax is okay and you don't detect any heating in the tuner, then I'd think you'd be good to go.  you say a little heating on the clamps of the cap.  How's the inductor or inductors doing?  They getting hot?  You could try a CQ for a minute or two, switch over to rx and put ur hand in there near the coils and see if you feel any heat.  I don't know how much current you expect to see--500 mils for 200 w. seems pretty good to me--on 75, with 250 w. I was seeing 300 ma.  I was using one of those MFJ balanced amp meters.  On 40, my feed is an awkward length.  I can tune the antenna, a 1 w/l loop on 80 to 1:1 on 40 but it's a tight swr null.  I'd get it tuned, and commence calling CQ.  a minute or so later I'd see a big increase in power coming back.  Get up and find the vswr had changed.  Finaly found out on 40 that MFJ amp meter was getting hot.  Something in there didn't like the Z and/or the carrier.   My tuner is synched 15 uH (or maybe they're 20 can't remember) rollers and a 1000 pF vac variable.  maybe on 20 you need to add some capacitance, or like the other fellow said, make a small change in the feedline so ur tuner can get you to 1:1. 

I have also chased monitor distortion to find it's either too high level on a mixer channel to my cans, or a rx up too high.  Oh I just saw where you get 3 A on 160.  That's all dependent on the Z on the line, where on the line you measuring, frequency, and so on.  if you want to be certain measure at the actual feedpoint which you can't do with a dipole up in the air, but that's the way to find what you're actually delivering to the load b/c if you have that, you can probably get the f/p Z with modeling, then you can figure out the power at the f/p that's getting out.  Let's see...to calculate the R at the measurment point of the current, I think you divide ur power by ur current squared.  200 / .5**2 should equal the resistance on the line.  800 ohms?  that's actually not too bad but I guess that doesn't give us the reactive part.  Well, I'm starting to feel like I've been in over my head so maybe a real antenna expert can take over  :)

73

rob K5UJ

You feedpoint impedance will be about 193 + j422 Ohms. At the end of the 120 feet of ladder-line, the impedance will be about 119 - j102. You should see about 1 Amp total under these conditions, so it seems like things are OK FB OM.  :D

My inability to use math has haunted me all of my life. Thank you Steve for the calculations. I'll keep monitoring and calling the last couple of nights have been a little quiet.
Rob, Thanks for the input. It's not always a bed of roses using an untuned antenna system and ladder line. It seems once you get near a high impedance situation, there's high voltages that can be right at the tuner terminals.
The overload is from my R390A. Too much RF getting into the front end. Even though the receiver is isolated from the TX-RX relay with another antenna relay. 40M and down there's no problem. At 20M the leakage increases and overloads the receiver.
THanks to all.
Fred

Hi Fred,

I didn't have time to check this stuff last night.  I'm running the antenna and transmission line simulation programs, glad they exist, no math to do. 

I came very close to your results at 160 meters - 3.2 Amps.  I expected this to be close, not critical. 

For 20 meters though, the effective transmission line length and antenna impedance is much more critical.  As Steve said you should get 1 Amp ideally instead of the 1/2 amp you do.  This difference is due to tuner losses as you may be very close to an impedance peak; you may be just a few feet away from it. 

My thought - If you can, try adding 10 feet of line, this will move you farther away from the voltage node and the tuner losses will then drop.  This accounts for a couple dB.

You have a lot of lobing and nulls with this kind of antenna at 20 meters.  Another suggestion is to unfortunately add a separate 1/2 wave dipole at 35 feet for 20 meters to your antenna farm (or a vertical, got a pole barn roof?).  But this can be fed with some RG58 or RG8 and a 1:1 current balun at the feed point, no tuner needed.

I second that suggestion. A 1/2 wave, coax fed  20M dipole will have a clean, broad, figure-8 pattern.  The only nulls are off the sides.  Environmentally stable vs: open line - and no tuner needed.


Second choice would be to add a set of 20M legs (16' each) to the openwire fed dipole, but I wouldn't bother. Go with the coax fed dipole for 20M monoband operation. That antenna will work like a bomb at 35' high for USA contacts.  (1/2 wave high)

T

Tom, thanks for the suggestion to add some feedline.  I will have to try that on 40 m. on my antenna; it may make tuning on that band easier.

Fred, you could also try adding two more supports and turning your dipole into a loop which is what I did to get as much use out of one antenna as possible.  You'll still get all these lobes on 20, but they'll take off in more directions with a loop.

Rob K5UJ

I think I'll try the "second choice" of Tom Vu.
Less work and expense for me.
I have lottsa good antenna wire and dacron rope.

Fred

I followed Tom's advice on adding a 10M length dipole under my 135 ft ladder line dipole.   This was for 10 meters and it greatly improved the operation.    Adding two 16 ft legs should do the same for you on 20M.     I got some PVC pipe and made some spreaders, much like you would when making homemade ladder line, to separate the second dipole.    Drilled a hole through one end of each spreader and strung the new elements through.    Cut a slit using a saw on the other side of each spreader and slipped them over the existing antenna wire.    Make the slit only far enough so that you can glue a pipe cap over each one to hold it in place.  It's quick to do, only had to lower the antenna, of course you will have to solder or bug the new elements to the existing dipole.

Tom Vu vely smawt man!

Hi Fred,

Adding another antenna may not be a bad idea but I'd consider a smaller lower version of what you already have, such as a center fed dipole with each side around 22 feet long and fed with parallel wire feed.  You could probably use it with the famous tuner, and it would probably give you a good match for 15 through 40 m so there would be a lot of versatility there.  This is an attempt at Cebik's 1/3 w. dipole for 40 m. on up, a nice all around general purpose antenna and it might even be okay on 10.

Rob K5UJ

3/8 WL at the lowest frequency is preferable.

The cleaner pattern when using a shorter dipole has been mentioned. Take a look at the patterns for your exsting 180 foot long antenna on 20-meters. The broadside elevation pattern (the first image below) doesn't look too bad - a nice low angle lobe at about 14 degrees and another higher angle lobe. But that's not the entire story.

Look at the azimuth pattern, just at the 14 degree elevation angle. Lots of stuff going on here, including lobes off the side or ends of the wire and more of a cloverleaf pattern over all (not necessarily a bad thing if those leafs are pointed in the right directions). But still, there are lots of nulls, so those directions will not be covered very well. Even here, the entire story is not told.

Take a look at the bottom image - the 3D pattern. Now you are seeing everything. Look at all those crazy lobes and nulls. This makes it clear why adding a 20-meter dipole (or similar) might be a good idea.

Yep, that 3-D pattern is what I call a "garbage" antenna pattern.

Good for nothin. The funny part is there is no meaningful gain anywhere but you're STILL giving up huge broadside nulls to boot.   The ideal pattern is like a flashlight. It puts the RF where you want and is dead in directions you don't. The gain is big on the front and the null is huge off the sides and rear where you don't mind. (Assuming it's rotatable or you have a second one to switch too.) 

Even a 1/2 wave dipole has nulls off the sides - that's why two dipoles at right angles, switchable, is a good way to go to cover all four directions.  When there's nulls, there's a chance of gain.  But the thing to remember is it's not an instinctive thing at all. Running  a centerfed 180' wire on 20M does not produce big gain in any particular direction cuz the wire is not phased properly. In contrast, only 100' of wire in the form of a 3el wire 20M Yagi will blow away the 180' dipole by over 10db - with a huge f-b to boot. The hearing ability is no contest.

WOW that last image looked like RF hell. Tacking on a 3/8 WL for 20M onto the existing system sounds like a plan.
And Bob, thanks for the tips for separators to keep the two radiators from interaction and tangling. I just hope this extra stuff doesn't break on those high wind things we get around here in Winter.
But I guess that's the fun of Ham radio....................repairing antennas on a 5 degree day.


fred

I could be wrong, but I think Huz was referring to the absolute minimum length (3/8 WL) before losses start to mount up in case you wanted to use the antenna on a somewhat lower freq too. But in your case, there is no freq until you hit 7mhz. (or 10mhz)  Normally, you'd be better off with  1/2 wave dipole (+- 16' legs for 20M) to give a reasonable impedance for the openwire to work with, keep losses reasonable and provide a broad figure-8 pattern.

T

Fred,

When I recommended the smaller doublet I meant a totally separate antenna with a separate feedline; not one co-located with the big dipole and fed at the same point with the same feedline--I don't know how that would work out.  I know this kind of thing is done with coax (i.e. "fan dipoles") but with balanced line things may get more complex with two antennas close to each other affecting the tuning and balance on the line.   You'd have to fix it so the feedpoint Z of the desired antenna on a band you want to work would always most closely match the 450 or 600 ohms line impedence so that antenna is the driven antenna by default.  I would force the situation by simply using a separate antenna and feed.

As one who is risk averse and who wants antennas to work with a minimum of fooling around, I'd put it up at right angle to the big dipole and shifted a bit one way so the feedline drop is not too close to the other feed and bring it to a switch point.  The simplest way would be to have a common feed ending with banana jacks and your two antenna feeds ending with plugs and you simply plug in whichever antenna you want to use.  If you want to get fancy and avoid getting out in the winter to change antennas you could bring them both in and have a big knife switch arrangement.

Rob

Yes, 3/8 WL at the lowest freq you want to use. In Fred's case, this is 20-meters. So make the thing 25 feet long. Yes, a half-wavelength will be slightly lower loss, but it will also produce much more difficult to tune impedances on 15 and 10 meter (possibly 17 and 12 too, but I haven't checked).

If you only want to use the antenna on 20 meters, just go with the half-wavelength dipole and be done with it. Feed it with coax and rock and roll.

Rob,

The feed point impedance being matched to 400 - 600 ohm line has nothing to do with it.    Balanced line fed antennas hardly ever have an impedance matching the feed line.    It is the low loss of the feedline that allows you to run a high SWR between the line and the antenna.   That is why you can use these type of antennas on many bands.   Having a half wave dipole element in parallel with an otherwise higher impedance element, will take most of the RF current for the band it is cut for.    If you try paralleling a double extended zepp or other types, you won't get the same result.    Try modeling these this using the free version of EZNEC.    I did this after K1JJ set me on the path of looking at antenna patterns and you will see what happens with the different configurations.

Thanks for the kind comments, Bob.    BTW, you look very suave, debonaire and quite dapper in your tuxedo on QRZ.com... ;D   It wud make a good avatar here.

T

Hi,

Here's how I see it:   "It is the low loss of the feedline that allows you to run a high SWR between the line and the antenna."

That low loss is due to the line being balanced.  That's why vswr and a mismatch between a balanced antenna feedpoint and the characteristic Z of the line doesn't matter when it comes to line loss.  You can even operate with an antenna on a frequency below its lowest design frequency (up to a point). 
"The feed point impedance being matched to 400 - 600 ohm line has nothing to do with it."

I'm not convinced of that.  It _does_ matter in the unusual case of feeding two antennas from one balanced feedline.  If you have two antennas on the same feed, on a given frequency, the one with the f/p Z that comes closest to the Z of the line is going to get the most energy transferred to it whether that's the one you want to use or not.   Fan dipoles work on this simple principle because you have a bunch of 1/2 w. dipoles being fed by 75 ohm coax.  Whatever band you want to operate on, if you have a half wave dipole up in the fan that's cut for that band, that's going to be your radiator because its Z is gg to be near 75 ohms; none of the others will be (usually). 

My point, and I don't have the fancy modeling software so on that you have the advantage, is that if we have a big dipole and a 1/2 w. dipole for 14 MHz and you feed them both with some kind of parallel feedline and get on say, 12 meters, you're going to try to get your balanced tuner to find a match to 50 ohms unbalanced but both antennas will have some bizarre Z relative to the line, and, without doing some testing, or modeling, you might find that the unwanted antenna gets most of the RF because its Z is closest to the Z of the line.  If there's something I'm missing or something wrong with this reasoning please tell me.  Tnx,

Rob K5UJ

Feedline loss is directly related to its dielectric quality and type. Feedline can be balanced or unbalanced with no difference in loss figures.
The dielectric (insulation) separates and holds the two wires apart and in place. (or center conductor and shield) This wire placement is critical to maintain a certain characteristic impedance, like say 50 ohms.  *** Ideally, if the wires could be held in place with no dielectric at all, there would be almost no loss, like openwire.

The majority of the loss is generated from the secondary path (short circuit) from one wire to the other through the dielectric.... all along the feeder length. It will actually heat up the dielectric (and whole cable) if the swr is high with sufficient power.  Dielectric loss generally goes up as frequency increases.


For example, RG-213 uses a polyethylene  dielectric that has about a 0.5db loss per 100' on 40M when perfectly matched. (1:1).  As the swr (mismatch) goes up, so does this db loss, until little power arrives at the antenna. This is heat in the dielectric circulating "short circuit" path.  PolyFoam is better, but cannot handle the power (heat) as well.  Zip cord feedline, using rubber or plastic insulation (dielectric) is poor, especially at higher freqs. Hardline with thin poly disks fused every 3" or so is exellent due to the small amount of dielectric surface contact between wires. It's not the wire size so much as the insulation used that dictates the loss.  Though wire size becomes more important at higher HF and VHF freqs due to skin effect. Also low impedance circuits need larger conductors.


Now compare this to a feedline with little to no dielectric like wide-spaced, well made, homebrew openwire line.  The only dielectric for loss is the few widely spaced insulators (good quality) or AIR.  Air is a great insulator and little RF flows across the wires as loss. Even better, an idea I use is where the two feeder wires are stretched tight between the tower and the ground, about 18" apart using no spacers. This uses NO dielectric except air and the four end insulators. To get better than that would require thick wires in a vacuum. 

With high swr, bigger conductor size can have a beneficial effect.

Connecting two coaxes in parallel will NOT improve a lossy feedline situation cuz the path splits and sees the same amount of dielectric in the end. It will produce a double sized set of wire conductors, but the dielectric loss overshadows the conductor size improvement, so improvement is slight.  I believe when we get into air openwire, then conductor size can have a more meaningful percentage improvement - once the dielectric gremlin is gone, especially when dealing with high swr and very low impedances, thus high RF currents and, of course, skin effect.  When using coax at high swr below 14 mhz, we're already dead anyway due to dielectric loss, regardless of conductor size.

Hope this helps.

T

It has nothing to do with the line being balanced and everything with the loss of the line. You could do the same thing with air spaced coax.

Open-wire line has far less loss when connected to a matched load than do nearly all types of coax (most certainly those in common use by amateur radios ops). As the load is changed from the matched condition, loss increases (and not linearly) on both the open-wire line and coax. But since coax starts out with so much less loss, it exhibits far less loss due to mismatch or high SWR.

This is all explained in detail in the ARRL Handbook. There is even a graph and a formula you can use to determine the amount of loss at a given SWR, knowing the SWR, the matched loss of the line in question (matched loss implies a given frequency). Check it out and run a few examples to satisfy yourself.

To futher clarify JJ's post, yes, dielectric loss is the issue, most often. One case where an additional loss factor comes into play is when feeding a very low-Z antenna. A typical case is using a dipole well below it's half-wave resonant frequency. Here wire resistance loss will become a factor due to the very high currents involved. Dave, W2VW has made some interesting measurements on the loss with open-wire line using different conductor sizes when feeding a 75 meter dipole on 160 meters. I'll let him fill in the details, if he chooses, but short story is that larger conductors had much less loss (not surprising).



You are correct in the 12 meter case. The big (in this case 180 foot) wire will take most of the power and dominate the pattern (and produce an ugly one). The situation may be different on other bands. Use the force Luke and DL any one of a number of FREE antenna modeling programs. No need to work in the dark any more and make guess, educated guesses, or even good engineering estimate. Model it and know for sure within minutes.

Hello again,
I think I'm interpreting what Rob is saying. I hope I'm close. Because there are two random lengths of antenna wire or 3/8 wave at 160M and 20M (eventually at my station) that there may be some frequencies that will go to both wires. I don't think so. I think Bob K3ZS has almost this same antenna system in operation at his station.

I can't explain the theory, but Tom and Steve and Bob, K3ZS have been trying to get us on track.

This may be a very easy way to keep the tuner happy and reduce losses and not have to hassle with resonating a 50 ohm system or even using lossy traps.

Fred

 

Okay that's cool--thanks for all the replies--this started as a problem for Fred and I feel like I wound up getting the benefit  :D

Fred, if the dual dipole setup has been done already and proven to work then go for it. 

Very interesting about losses and dielectric.  I have tried modeling once or twice with EzNEC but it was simply too time consuming--it took hours just to model a simple antenna.  I wanted to model my horiz. loop and I realized that would take hours over days and I had too much else to do so I gave up on it.  I have never heard of these other programs.  I'll have to check them out.

You sure know how to spoil my day. I was feeling pretty good about how well my 80 m dipole was working on 20.

Yuch!

js

Doesn't mean it won't work. It may mean another antenna will work better. You won't know until another is tried.

Jack,
You're doing pretty good on the air with your system. I heard you 10 over the other night in Western Pa. on 14.286.  As Steve says, a shorter dipole connected along with your 80M wire might be a little better. My problem using a 180 foot dipole on 20M creates a lot of ugly lobes. Less lobes are better.

And Rob, I know the frustration of computers and getting things like EZNEC and plugging in numbers and interpreting what the computer is trying to tell you. That's why I lean on the group here to feed me with info. They know my shortcommings....hi
And don't feel that you "hijacked" the thread. It got the grey matter working for all who have been reading and responding.
Sometimes there are nice neat shortcuts to get RF to work for you.

Fred

This has been an interesting discussion.   It seems that with parallel dipoles, the one that dominates with the antenna current is the one doing the most radiating.   As Rob has stated, I am not sure if the one having the best match will dominate or the one with the lowest impedance will.    The free software seems to do a good job of modeling the patterns in any case.    My antenna is a 135 ft in parallel with a 16 ft dipole.    It did change the tuner settings mostly on the bands from 20M to 10M.     The operation on 10M is much improved as far as I can tell after adding the dipole, just as EZNEC has shown.   Tom, my QRZ picture is getting dated, it was at my son's wedding before I ever had a digital camera.

I have been fooling around with EZNEC today.   According to it, the above described antenna will have about 3.5 times the antenna current in the 16 ft dipole as compared to the 135 ft dipole at 28.4 MHz.   I replotted the azimuth patterns at the maximum gain elevation angle.  Except for off the end, the dual antenna has less than 5 dB variation in the azimuth directions on 28.4 MHz.   Without the added 16 ft section, the nulls are many and go down to 20 to 25 db below the maximum points.   The best gain is about 10 dBi at the main lobes about 35 deg from the end of the antenna.    With the added 16 ft section the best gain is around 13.4 dBi pretty close to 90 degrees from the antenna wire.   Both azimuth patterns are best at a 16 deg elevation angle at the antenna height that I used, which was 30 ft.

I just plotted Fred's antenna with two 16 ft legs added to it.     The result is not as good as in my antenna.    The nulls are filled in but they are still about 10 db below the max, however the max gain is 12 dBi.   Off the end it is also about 10 db below max.   This all is at a peak elevation angle of 14 degrees and a frequency of 14.2 MHz.

Hmmmm I'm getting curious with this EZNEC. I was looking at the W7EL website and will try to find the free version. I thought someone here said it was free.
I promise I won't get frustrated if it doesn't work OR I don't understand what the computer is spitting out.

And thanks for the results Bob. I'll be adding the other element once this rain thing quits.

Fred

The first time I tried EZNEC, I didn't want to spend the time learning what it was about.    The free version has a limited number of segments so the results are not that accurate, but it still gives a good idea what is going on.   If you have time, using the sample files that come with it, you can eventually make sense out of it.   
If I was to be an antenna hobbyist I would purchase the program.    I don't intend on making any changes now to my one and only antenna.

Fred,

Try EZNec and you will be able to try any old antenna you have dreamed about over the last 40 years without leaving your chair. It's like being able to climb any tower height and have perfect antennas... or model the limited conditions we all have.

After a week of playing with it, you will soon see how easy it is to anticipate patterns, mistakes, proper phasing and the like. It's really an education in antennas.


*** BTW, I finally got around to modeling the 160' flat top for 160-40M and second 22' flat top for 10-20M.  As a combined antenna with common openwire feed, I was very disappointed.  The 160-40M portion looks OK, but the 20-10M bands were not so good. 20M was barely acceptable, but 10-15M looked terrible.

The two antennas should be erected as two separate flattops fed with their own openwire and they willl perform beautifully, pattern-wise.     ie, ONE feedline feeding three bands (160-40M)  and a second feedline feeding three bands. (10-20M)


Personally, I like separate antennas fed with coax, but one needs the room to separate them to quench interaction.

My rule with FIXED mono-band Yagis is to have NOTHING in front of them, nothing on the sides, but I load up the field behind them with other Yagis looking the other way. A Yagi sees nearly nothing behind it due to the reflector.  However, a dipole is sensitive to front and rear, so actually takes more room to isolate it.

The good thing about dipoles is you can hang them at right angles to each other, using the same support, You CANNOT do this with Yagis or the interaction is severe.  (This assumes the same band antennas)


So, bottom line is to MODEL every antenna before trying it. Some patterns seem logical (like my 160', 22' idea) but turn out to be real garage.

Fred, I would STILL recommend using a separate coax fed 33' flat top at 35'-40' high  for 20M. Wind up 6 turns of coax on a 4" ABS pipe at the feedpoint  from Home Despot and you have a balun and clean pattern.  Use the ABS pipe as the center insulator to secure the leg wires, too. Use a rope to hang the abs pipe on a side-arm on your pole that hangs out horizontally a foot or so..  Instant band switching - leave your openwire antenna tuned up on 160-40M. Compared to the openwire version with the common legs, it is superior in pattern by 6db in spots. It is smoother and predicable.  It is down only 4-5db from a full-size 3el Yagi at the same height. Put it up as I described and I guarantee you will start making lots of contacts on 20M, reliably, or your money back... ;D

T

The EzNEC I got was on a CD that came with either the ARRL Handbook or the ARRL Antenna book, probably the latter.  Maybe it  is easier to use now.  What I got had a few samples and I tried to model a dipole on 20 M and it took hours.   So I decided I had to be retired to use it  :)  But maybe I should save it for a cold winter holiday.  Now that I think about it, I believe what I got had to run on windows and I now use mac and linux machines.   Are any of the other packages out there written for linux?  Probably not for macs.   I'd love to model my loop.  It's sort of a rectangle (or more trapezoidal) with two 80' sides and the other two about 40' to fit on my 50 x 100 foot lot.   It's up about 25 - 30 feet and fed in the middle of one side, the south side (one of the long sides) with 600 ohm ladder line.  I wanted to model it but after finding out the dipole took forever, I figured the loop would never happen.  I want to try to raise the loop another 15 feet or so to try to reduce capacitive loss.  You'd think this is trivial but it involves four man-made supports that can't be guyed because they're very close to the property line.   City ordinances prohibit lattice towers.  I'm looking at some aluminum tubing ($$$). 

Fred, so T says you should use separate feedlines.  heh heh heh...(can't resist gloating  ;D).  K3ZS ur photo looks Mahvelous.
Has anyone seen this weird video of people on towers in wedding clothes singing?  I saw it at Dayton a few years ago.  Never did figure out what it was about.

73

Rob K5UJ

Not sure what program you were using. Any of the spreadsheet entry type programs out there would take about a minute to model a dipole. Just enter the two end points in X,Y,Z coordinates, specify wire size and type, frequency and ground type and hit run. Some programs even allow you to draw the antenna, which can be easier for loops and sloping wires, etc.

Dunno about Linux but CocoaNec is available at no charge for the Mac.

Rob,

 ;D  Don't gloat until you've looked at your OWN pattern... ;D

Your loop fed with openwire probably has a similar "octopus"  look as on Fred's 10-20M dipole.


His exisiting dipole for 160-40M with an additional coax fed dipole for 20M would be a good compromise.

I also like the idea of using one openwire feeder to feed a 160' flat top for 160-40M.  Then a second feeder to feed a second 22' flat top for 10-20M. The resulting patterns are beautiful figure-8's on all bands.   Yes, more effort, but cleaner results.

The interesting thing is you can place the 160M-40M dipole higher than the 10-20M dipole to fine-tune the take off angles.
Model those when you can and see what they look like.  There is a way to model your own loop and then compare the pattern against other antennas - slick and revealing.


Tell ya what - since you're not set up with EZNec yet, I'll model your loop as described and let ya know what it looks like on the various bands. Maybe later tonight -

T

Steve and T:  cocoaNEC that is fantastic!  I must find out about it.  Modeling my antenna THANX!! I can't wait to see it.  I'm sure there are crazy lobes on the high bands but a net I check into from time to time on 14.340 always works better with everyone especially the guys in the south and southwest when I'm on the loop.  But the other 20 m. antenna is a Gap Titan, so it isn't much to compare the loop to.   the Gap vertical is mainly being used now to hold up part of the 160 m. inverted L.

I was just listening to W0HRO on 3880 with my 12 KHz passband rx feeding the heathkit EA-2 from the product detector and did he ever sound fantastic on a big speaker! 

73

Rob K5UJ

WOW yous guys caught me as I was going out the door to start measuring and planning the 20M add-on.
This adds a little twitch by having to run another ladderline for 10-20M. Can I run it along with the existing ladderline? Does there have to be separation from the two runs?
If there has to be any kind of separators to keep them apart, then this will be a bad problem during Winter as we get a lot of steady to heavy winds for days blowing from the West. The ladderline system runs along the back of the house from the shack, then hooks on the back side and goes up to the feed point on the utility pole.
I was able to dig up some old pics for the install of the utility pole and the feedline runs and the dipole and the broken DAM Yagi.
And the quest continues. THanks for the input

fred


ROB, I used to have an EA-3 Heath amp. That was my very first electronic kit.

Fred,

In your case, wouldn't it be far better to fix the triband Yagi instead?

If not, if you plan to use 10-20M, then the second 22' flattop with another separate feedline will work FB.  Keep the two sets of openwires at least 2-3 feet apart from each other and the isolation shud be fine. Add a slow twist to one or both of them and the coupling will be even less. You don't want the same two wires looking at each other for too long of the run.

If you build a second tuner dedicated to 10-20M, that wud be pretty slick and cornvienient.


If you desire adding ONLY 20M, then the 20M dedicated coax dipole is by far the easiest solution.



Rob, my modeling doesn't have downloadable pics like Steve's, but I can describe the patterns I see on the screen. Maybe Steve will also model your loop and show the pics.

T

Okay, that's fine--what do you see?

Rob

How about juggling the lengths of the 160 footer and or resonant 20 meter parallel dipoles so that they have different impedance characteristics as possible on 14 mhz. I would think the 20 dipole would show more of a classic dipole pattern if the 160 footer's load was in a different neighborhood on the Smith chart. The job would be probably be easier if one loses the resonant idea for the 20 radiator.    

That's an interesting apporach, Dave. Change the ingredients around a little to get some different results.
I don't have EZNEC so, I'll impose on some who does, if juggling some antenna lengths around, would make the 160M antenna work with a second radiator on the same feedline. I have heard that the free version won't display what I think we're trying to do here.

A little re-cap of present system: Ladderline is approx 130 feet, 160M antenna is 180 feet, want to add a 20M element using same feedline.

BTW, Dave the modified TS-440 is still a happy radio.

Fred

Hi Fred,

     Step one would be to see if 180 feet is OK for a broadside pattern on 40 meters. You may want to take 15 or so feet from that length. Stay under 5/4 wavelength for that wire. Then try what I said in my last post. Unless I'm missing something that system properly configured should exhibit 2 different patterns on 20 depending on which system resonance you adjust the coupler for. We could call it the TFPD Tunable Fred Parallel Dipole.
   
     Glad the TS-440 is doing well.

73,
Dave.

Will do, Rob -

Got busy last night but will get it done for you today - and post it tonight.

T

Hi Rob,

I modeled your loop as a rectangular 80' X 40' X 80' X 40'  loop, and fed it in the center of one of the 80' legs. The height is at 27' high.

Generally, it IS a high angle antenna for 160-40M and the angle gets lower on 20-10M. 

On 160M the peak take-off angle is 90 degrees and almost ominidirectional. However the efficency is only about 13%, being too short for 160M as you'd expect.

On 75M, this is probably your best looking pattern and for local work. It's a broad figure eight, with TO angle at about 88 degrees, 95% efficient. Good cloud burner for general use.  Max radiation off the front and rear. (front and rear being the 80' legs)

On 40M, the maximum radiation switches to the sides and the front is down -6db while the rear is down -2db.  TO angle at 70 degrees.

20M, broadside best, (front/rear) big butterfly pattern. TO at 30 degrees, which is good for USA work and maybe Eu. A 1/2 wave dipole would be better for general use.

15M  Big butterfly with lots of nulls. -10db down in the front and rear. Poor pattern, pot luck.

10M: VERY useless octopus pattern with many deep nulls - pot luck.

All in all, 80M is probably your best band followed by 40M for local work.  20M has a decent take off angle, but has a "nully" butterfly pattern horizontal.  160, 15 and 10M look poor.

Bottom line:  If you're serious about 10-20M, I'd put up a 22' flat top fed with openwire and you will get a nice sharp figure 8 pattern on all three bands, even better looking than the loop has on 75M.

I imagine this is pretty much what you expected.  It agrees with the idea that it takes several specialized antennas to get optimum patterns across many bands. One will not do it all.   You will still be heard and make contacts on the higher bands, of course, but in many cases you will be 6-10db weaker than when running a simple 1/2 wave dipole dedicated to the band.

Hope this helps, OM.

Tom, K1JJ

Wow Tom this is fantastic information; I can't thank you enough!   I was going to try dipoles on 20 15 and 10 but one fed with ladderline will be much easier.  I have everything to make one and put it up right away! 

This gives me some incentive to get the loop higher up.  I have an inverted L on 160 because the loop was much too small and low for that band.  This explains why with the loop I'm good on 20 out to N.M. and So. Cal.  yes, it's the bomb on 75 for local work and I often get 20 to 40 over 9 reports from hams within 200 miles.  the 40 report was a bit of a surprise. 

>On 40M, the maximum radiation switches to the sides and the front is down -6db while the rear is down -2db.  TO angle at 70 degrees.

TO angle is a little lower than I expected but not complaining.  Can you say which long side is the front and which is the rear?

I have been thinking about co-locating a 75 m. inverted L with the 160, use the same ground system and feedline, LDF4-50, and switch it out at the feedpoint.  Hopefully I'd get some low angle on 75 to the east coast. 

Anyway, it's starting to sound like it's time to say goodbye to the Gap Titan, it got me back on in 2001 and seemed like a pretty good antenna until I put up the 88' ladderline fed dipole and compared the two.  I gained 15 dB right then and there, as if I had fired up a 10 kw amp.  the loop extension added another 10 dB. (this was all on 40 with close in hams.)

tnx again & 73

Rob K5UJ

For the sake of conversation. Here is my current antenna modeled on EZnec by Joe.  The only real change is that I went to 600 ohm open wire line.

This antenna works very well on 80.. SUPER on 40 and pretty nice on 20m.  I have made alot of contacts on 20 with it but its the only 20 m antenna I have owned so its hard to compare.

Clark

Glad to help, Rob.

On your loop model, the rear is the driven 80' length and the front is the opposite 80' side.


Let us know how the 22 foot,  20-10M open wire dipole works out.  Get it up as high as possible. At 35' high it will work like a dream on all three bands for the USA.  Higher will only lower your angles more.  Angles of 15 degrees on 20M are very useful for DX.  Down to 7 degrees on 10M are FB too.  You can get these angles if the dipole is up in the 50' high area or more.


Most hams are satisfied with the patterns I described for the loop for 10-20M. But when you see the difference in performance using a high dipole that produces CLEAN figure eight patterns, you will be amazed how competitive you are against the triband Yagi boys, especially the trap tribanders.  If you get ambitious, put a reflector behind it for your favorite band and direction to make it a 2el Yagi. (Beam it east from w5?) Your ears will improve markedly. Raise and lower the ref on pulleys for versatility.  The other bands will not see it at all.

You could even have three different reflectors hung together or one at a time.

T

Okay Tom FB thanks again!  Well, I have been dramatically reevaluating the antenna plan.  I'm gg to go ahead and try to get the loop higher up, but I've decided to take down the gap titan and put up a 50' pushup mast in its place.  I'll attach pulleys on it to hold ropes and insulators to hold up part of the inv. L, and one end of the 22' dipole.  the other end will be held by a 2nd 50' pushup mast that will primarily old up the feedpoint of the loop.  this will give a dipole facing southwest and north east.  I will also put a short pvc arm on the mast that replaces the titan and  use that to hold up a pulley with a rope/insulator holding a wire that will hang down to make a 1/4 wave vertical on 40 and I'll use the titan's counterpoise.  This should give me all of the coverage but much better efficiency compared to the Titan, which as a multiband vertical did not do very well so I only used it on 40 cw, 20 and 17 and 15.  It was very poor on 10 and 80.   There's something about the design of caps, stubs, and linear loading that caused problems.  There simply is no such thing as a single antenna that works well all across HF.   I think we all know that here.   

73

Rob K5UJ

Good, Rob.

Always nice to see someone who takes action.  It's too bad 10 & 15M aren't wide open to see how well this system will work at 50' for you. You will be amazed what you can do on 20M with it.  I'll bet when you call CQ on 20M, you will almost always get a reply. Hams tend to call louder stations cuz they think THEY will be loud too and get a good report... ;D  Also, many apt dwellers have so much noise, they can't hear but the loudest stations, or have given up calling anyone that is not loud due to poor response.

Anyway, sounds like a good plan you have now.

Looking forward to the 20M reports.  BTW, are you gonna use the same tuner or build a dedicated one for 10-20M?

T

Rob,

Thinking about it, I have a suggestion about the loop....

Since you will no longer use it on 10-20M, 160M is too lossy, and you may have that vertical for 40M, I'd replace the loop. 

For 75M, if you can get it a little higher, then all the better for an openwire center fed dipole.  If space is a problem, just make the legs as long as possible and drop the ends down vertically. It might even work decently on 160M too, with longer dropped legs.  Two small center loading coils are also a solution to bring up the input impedance for 160M operation. You could use simple coax if only for 75M when loaded, too.

Then use the tuner dedicated to 10-20M with the ant 22' ant.


Also, the loop takes up your whole lot. There is no hope of isolation for other antennas. A simple 75M dipole, even though the RF footprint is similar, you may be able to put something at right angles to minimize things. With the rectangular loop, you're scewed.

Just something to think about, OM.

T

Okay Tom thanks; I'll use the same tuner.  Interesting thoughts re the loop.  Well, I'd certainly save money on supports with a dipole  :D

The only way to fit the dipole would be to have one about 95-100 feet long running east to west so north and south would be off the sides and about 6 feet dangling down on each end.   From your description I kind of like the pattern of the loop on 40 but yeah, it's a cloud burner on 75.  And I do operate on 75 a lot more than on 40.  I'll have to think about this.  Any idea of the efficiency of the 100' dipole on 75 at 35-40 feet and the efficiency and pattern of the same thing on 40? 

tnx

Rob K5UJ

Put up that 100 foot long dipole, except add 10-15 feet on the ends and let them hang down vertically. Now you have a full-sized half-wavelength dipole on 75 meters. The little bit hanging down on the end has minimal effect on the pattern or efficiency

I've given this some thought while sitting in the parking lot of the Jewel before going in for groceries and what I decided I'd do is time honored ham experimentation.  I'm gg to go ahead with the plan as is, and do some testing/operating/measuring then clip the loop wire and isolate the unused part and see what life is like with all dipoles.   :)  That's the great thing about wire antennas--you can have it both ways with a pair of dikes--if you screw up, you just have to do a little stripping, twisting and some work with silver solder and a torch and everything is right back like it was.

Rob

Rob,

Just a quick note:  I have been using an 80 meter full wave horizontal loop for my vintage gear operations for about 5 years now.  The loop is roughly square although the sides are not exactly even and the highest corner is around 35 feet high and the lowest corner is around 30 feet high.  It is fed at one corner with RG-8 using a homebrew reversible L network at the operating position.  I also use it for local work on 160 with the coax shield and center shorted where it goes up vertically (although I did work several S. Americans and Hawaii during a recent 160 contest). 

It seems to work fine on the higher bands also (both stateside and EU) on 40 thru 10 but it doesn't compare with my Quad on the higher bands.  The loop is noticeably quieter than the 160 meter Carolina windom I used earlier and the Hy Tower I also use currently. 

Of all the antennas I have used over the years, the loop definitely scores high on low maintenance and versatility.  The Carolina Windom I used earlier with the vintage gear worked pretty well but I don't think its line isolator would have held up to the Desk KW.

Just an additional data point to help with your decision.

Rodger WQ9E

Thanks Rodger; I hope no one get's offended by this but I think those carolina windoms are so-so antennas (they work so-so on a bunch of bands).  They are an example of the multi-band snake oil being sold. 

My guess with the loop i have is that on 75, it is too low and the short sides are too short.  Therefore it is marginally a loop and that's why it works better (more loopy) on 40 with a slightly lower t.o. angle and more pattern to the east and west.  On 75, it would probably be okay if it were twice as high and all sides closer to equal.  I kind of hate to give it up on 40 but I could perhaps gain the east west with the 1/4 w. vertical on 40.   part of the game is choosing carefully because there's a limit to what I can have in a small patch of property.  Go over the limit and everything plays poorly.   Hence...the experimentation.

73

Rob

Rob,

I like your idea of using all dipoles. Also consider Steve's suggestion of the hanging ends for the 75M dipole with the dimensions he gave. That will be the best antenna you cud put up for 75M on your lot. 

Heck, since the 75M dipole is now gonna be a 1/2 wave antenna fed with openwire, use it on 40M too as two-half waves in phase. Hopefully it is broadside in the right direction  (NE?) cuz the pattern narrows to a sharp figure 8.

Otherewise find a way to stick in a 40M dipole. It is very hard to beat a dipole on 40M. It is down only 4-5 db from a full-size 3el Yagi at the same height. A single vertical cannot compete with that.

Your ideas are starting to gel. You gonna be whirl-wide!

T

Thanks; it will be a few weeks as I need to schedule some time off and order a few supplies (dacron rope).  I usually get everything ready to go (everything measured and cut, soldered etc.) before I take time off.  The low band dipole is going to have to be broadside to the south and north.  No room to put up one right angled to that and if I tried to cram one in, there'd be too many dipoles in a small space ergo, the 40 m. vertical, which I might also setup as a 75 m. inverted L with some sort of switchable insulator at the 33' point.   Hmm but if I ponder this for a while I may think of something  ;)
Oh yeah, be careful of comparisons between antenna x and a yagi--those dB comparisons may be true on tx but half the job is receiving and a yagi, lp, quad always beats a simple dipole or vertical hands down on receive.


73

Rob K5UJ

WOW! Page 3
I have a dream! somewhere in my reading. It was either QST or the ER mag that there was an article about using vacuum relays to reduce the length of a dipole for a different band. I don't know if it was on a resonant 50 ohm system or open ladder line.
The writer did his home work and bypassed the DC lines going to the vac relays. And the "control lines" didn't seem to bother the actual antenna element.
I'll do some research through Google or ARRL and report back.
Any one ever read about anything like this?

Fred

Yep, I'm always talking about raw forward gain on transmit when referring to 4-5 db over a full size Yagi at the same height. Signal to noise ratio is a different matter.  The front to back of a Yagi on receive reduces the noise from the full rear hemishere. Even the sides are sharper.  In fact, my high 75M Yagi/Loops hear slightly better into Europe than the terminated beverages. Using a simple dipole to receive puts the Beverage way in front in comparison.


Question:  How long would the 75M flat top be (and dropped legs to equal 123')  if you oriented it NE/SW?  Give us the numbers and maybe Steve can model that to see how close it comes to a full size flat top.   Just the orientation (and side nulls) may make it more worthwhile to do when compared in the end.  I mean, if the shortened flat top is down 1 db but the side nulls are down 15-20 db, the better choice is the shortened flat top oriented in the desired direction.

T

to get n/e off the side the flat top would have to be no more than ~45 feet long;  the narrow sides of the lot face east n/e unfortunately, but I could throw a big signal at K4KYV and into the Gulf and Brazil  :)

Rob

Maybe you'll be OK since low dipoles tend to exhibit a BROAD figure 8, almost omni-directional at the lower angles.  Come up with the flat top dimensions at max span and maybe Steve can re-model it to give the exact vertical drops needed to resonant on 3850, in case you wish to feed it with coax.

T

Hmm, the flat part would probably be 88 to 90 feet, leaving 15 on each end to dangle down.  or they could be strung out horizontal to make a [ looking dipole.  not interested in coax except for unbalanced antennas i.e. verticals fed against ground.  anything balanced I want to use for more than one band.  no room for luxury of all monoband antennas, so it's got to be ladderline and bal. tuner.  That's okay, the big boys do the same thing with their curtains, they just have bigger tuners and more dipoles  :D

R.

OK, openwire fed, 88' long flat top. How high at the center and at the ends?    Maybe by extending the dropped ends to the ground we can get a working 160M dipole there too. It may require series tuner feed on 160M, but wud be a good local 160M ant too. 75M would work well and 40 would be a sharp figure 8.   On 40M, the height would start to make it less omni-directional, however, and your NE/SW wud suffer.  It wud happen even with a 1/2 wave 40M dipole, however.

I wud say a total of 160' (88 flat top and 36' on each end) would do the trick on all bands, 160-40M. The ends wud drop down near to the ground, but that's OK.  On 75M you still have the majoity of 88' flat in the air. You will be amazed how well that system will work on all three bands.

T

Lat night, I modeled just 88 feet and then 88 feet with 15 feet ends hanging down vertically last night on both 80 and 40 meters. I'll post the results when I get home tonight since it was late and I can't remember the numbers.  :P  I'll also have some pattern plots.

The above arrangement would propably be too short to work effectively on 160 meters, unless you built a really strapping tuner and some home-made open-wire feedline with #10 or larger conductors. I'll run some numbers on 160 too, just to see what would happen.

I'm told a 44 foot lazy H works great 40 through 10M Vertical spacing 22 feet.

That'll be interesting to see thanks.  Ends and middle would be about 40' high.  I do have the inverted L on 160 which seems to be a pretty good ragchew antenna.  I have doubts about the dipole being worthwhile on 160.  Too low for one thing; and too small, too much loss at the ends near ground....a matching network might make it look okay to the rig but that's another trap you have to watch out for...the right network can match a coat hanger on 160 but ur antenna is still a coat hanger  :D

Losses in a tuner would probably be high.   

Rob

If you already have an inverted-L for 160, then you are set. The low, short dipole will not work any better, especially if you have a decent groud system under the L.

Tuners can have low loss but need to be large and fugly

For a small lot, the additional inverted L for 160M will add interaction and needs radials.  It has both horizontal and vertical components.  There is also a 40M vertical, 75M dipole and 10-20M dipole too.  Yikes.

Clean house first - start with a clean field.  Then lengthen the 75M dipole drop legs and make it work 160-40M. (36' end drops and openwire fed)   Then put up that 10-20M openwire fed 22' flat top and that's it. TWO antennas with figure 8's on all bands.  Minimal interaction.  All horizontally polarized. Great performance.

If ya really want an inverted L on 160M, then tie the openwire together of the 75M dipole and feed against ground. It will work a little like a vertical 'T' against ground.  Still, only TWO *unrelated* antennas to muck up the RF field.... a good compromise.  Each antenna sees nothing to interact with.

** Another point:  The 22' flat top would make a great vertical 'T' on 40M . Tie the feeder leads together at the ends and feed against ground.**

T

Rob. You are talking about the antenna I use and I even posted the EZnec of it..   Open wire fed, Flat top and ends turned down.

Clark

Yes, I mentioned the 160 L back at the beginning of the month but you all probably forgot or missed it.  I mentioned it in passing.  It's an inverted L with 101 radials and a tuner out at the feedpoint for 1985 KHz.  feedline is ldf4-50.   the f/p Z is 14 ohms in the middle of the band at the vswr null (~3.5:1 or so).  the driven part is around 120 feet long.  I plan to lengthen that and get the vertical part higher this fall.  The radials vary from 10 feet to 120.     

R.

Hi Clark thanks;
looks like a higher t/o angle than I expected on 75, probably what I'm currently getting with the horizontal loop i.e. straight up :)   

Hi Tom; well, on top of all that, there's the metal pushup pole supports and a garage and house with aluminum siding on them.  I just have one tree here :) so something else has to hold up this wire and it has to be steel.  I have real doubts about a low in wavelength compromised dipole on 160.    I'm seeing an inefficient antenna there with a lot of loss to ground.  I like the idea of feeding the high band dipole against ground on 40 though.   but there is no way you're going to get a setup here that's has these clean patterns.  That's just not gg to happen.  To much metal from the house, garage, supports....I'm back to considering keeping the loop since it seems to have a good pattern on 40, continuing to run the inverted L inside the loop, and going ahead with the plan to take down the gap titan and put up the higher supports for the loop and the 50 foot mast in place of the titan and stringing up the 22 foot dipole but forgetting about the 40 and 80 vertical hanging down from the 50 footer.  I can experiment with the drooping dipole idea like what you have there Clark too.

Rob

Here is the info on your potential center-fed system at a height of 40 feet over average ground (Rel. dielectric constant 13.000, conductivity: 0.00500 mhos/meter) for 80 and 40 meters.  Length is 88' + 10' ends (total length is 108 feet)

3.8 MHz

Feedpoint Z: 38.961 - j251.740   about 14:1 SWR on ladder line (around 400 Ohms Z)
Efficiency:  96.47%
Max gain: 5.98 dBi
TO angle: 87 degrees
Loss in 100 feet of ladder line: 0.91 dB
Z at the end of 100 feet of ladder line: 171 - j853.26
Loss in 50 feet of ladder line: 0.79 dB
Z at the end of 100 feet of ladder line: 67.9 + j427.16

For comparison, here is the data on a full-sized, half-wave dipole at the same height.

Feedpoint Z:  61.247 + i 0.017, about 6.5:1 SWR on ladder line
Efficiency:  97.75%
Max gain: 6.33 dBi
TO angle: 87 degrees
Loss in 100 feet of ladder line: 0.39 dB
Z at the end of 100 feet of ladder line: 95 - j289.25
Loss in 50 feet of ladder line: 0.29 dB
Z at the end of 100 feet of ladder line: 640.74 + j1189.39

Not accounting for the tuner loss (which could differ some between the two antennas since the impedances at the end of the feedline are not the same), your short dipole is down about 0.87 dB with 100 feet of feedline and 0.85 dB with 50 feet of feedline. The short dipole may also suffer some increased loss not shown in the modeling due to near field coupling, since it is close to the ground. But since it is not extremely short, it's likely these losses are minimal.

So, on the air, not often that less than 1 dB going to be noticed. This setup is essentially as good as a full-sized half-wave dipole.


7.2 MHz

Feedpoint Z:   657.33 + i 1372   about 9.3:1 SWR on ladder line
Efficiency:  99.79%
Max gain: 6.47 dBi
TO angle: 49 degrees
Loss in 100 feet of ladder line: 0.68 dB
Z at the end of 100 feet of ladder line: 63.5 - j60.58
Loss in 50 feet of ladder line: 0.41 dB
Z at the end of 50 feet of ladder line: 83.96 + j290.63

See the radiation patterns for each band below. The green line on the azimuth pattern is taken at the take-off (TO) angle and the red line is at 60 degrees elevation, just to show some of the pattern at a mid/high angle.

wow that's great Steve thanks.  Lots to think about.  I also like Tom's suggestion to think about a clean sheet of paper, i.e. take everything down and start over.  That has occurred to me before since the antenna farm is a collection of three antennas put up piecemeal and this might be a good time to start over with everything I've learned.  I need to let things simmer for a few days.

73

rob

Hello Everyone
I thought I could GOOGLE or look in the ARRL archives for an article I read about a Ham using vacuum (small) relays to switch in or out lengths of a dipole antenna. I did not have very good luck on the ARRL website and Google is not cooperating.
In my case, I would want to switch out the excessive length of my 160M dipole to become more friendly towards the 20M band.
Anybody remember seeing this article or could verify that it would work?

Thanks 

Fred

How do the OCF antennas model on the higher frequencies? A lot of guys rave about them, and I've been curious as to their performance. I'm also looking to string a "one dipole, all band" antenna between two tall oaks...

Pete k1zjh

Hi Pete,

The OCF dipoles (asymmetric dipoles) I have heard about are the ones that are coax fed and have some kind of common mode choke and/or balun at the feedpoint.  I have never used one so perhaps I should not comment.  I have not used one because I have not been interested in them for the following reasons:

1.  They seem to be one of these attempts at finding a design (point on the half wave wire and balun in this case) where if you break the line into two sides and feed at that point, you'll have some sort of impedance on several bands that can sort of be transformed to 25 to 100 ohms maybe 120, with a vswr <= 2:1 to make it a multiband antenna.  I don't like that because it seems to be just another attempt at giving a lot of coverage for the-price-of-one, type of deals.  These things always flash red lights and sound bells for me. 

2.  I don't like baluns in general where they're going to be used in a wide variety of voltages and impedances out side.  I've heard too many stories of heating frying baluns or at best, lost power in them. 

3.  I don't like using coax for the same conditions in number 2.

I know these antennas get the same positive comments, you know, the usual "I work everything I hear" type stuff but I always suspect the users of these antennas have nothing to compare them to and are running < 500 w. SSB or CW.   And to be sure, they aren't dummy loads and may be satisfying for some purposes but I would not want to use one for QRO AM.

I am from the school that says a dipole should be balanced and fed with balanced line that's tuned with a balanced tuner. 

73

Rob K5UJ

OCFs may yield easier to match impedances on multiple bands, but their radiation patterns will be little different from a dipole of similar length and the same height.

The except would be the OCFs that have part of the feeder (vertical portion) radiate. This arrangement will tend to fill in the nulls on the higher bands and create a more omni pattern.

THanks John,
Now I know that I wasn't dreaming. Pneumatically operated(good work-around to avoid wires to key the relays), and low power. NOPE it won't handle the legal limit radio.
Thanks
There is an article in JULY QST for an HGSW (high gain single wire),wire antenna that is like a non-rotatable beam antenna. The author claims that he ran EZNEC on this design and there is 11dBi gain. Consists of various lengths of antenna wire with two each 13 foot lengths of ladderline hanging down, can be used for other bands with less directivity and gain, depending on feedline length. There are options to use it as a single band 20M antenna with coax or multiband with ladder line for 80-10M. The author claims that certain lengths of feedline would prevent tuning on 40 or 30 and 12M.
It would be interesting if one of our famous aerial gurus could prove this article out.
I don't want to take a chance of scanning it and posting on AMFONE.


Fred

The best way to use EZNEC to determine gain is to compare the dBi of the antenna in question with a resonant dipole at the same height.

Exactly! 11 dBi is pretty meaningless unless comparing two antenna in free space. 11 dBi at what angle? How high was the antenna? Over what sort of ground.

The QST thing sounds like some sort of collinear array (some number of half-waves in-phase). No magic but it will give some gain in two directions over a dipole. Of course, you have less gain in other directions, so properly consider the trade-offs.

I think Steve is on track with the QST article. Beings that I don't have schematic drawing software try to imagine the following:  5 egg insulators, and imagination used for the feedpoint described below.
There are two coils wound on a 1" dia PVC 3" long 19T 10 Gauge solid copper. The center and ground from the coax connect to these coils. The other end of the coils connect to 17.5 feet of wire to an egg insulator.
At these insulators hang a shorted piece of ladder line 13 feet long.
From this point connect 45 feet of wire to an end insulator.
Remember that this is a dipole, I'm just describing one side of the total dipole  ::)
The author recommends a slightly longer wire length to allow for connecting all of the elements together. But the above mentioned lengths must be accurate for the antenna to give you the performance calculated by EZNEC.

Maybe with the dimensions given here from the QST article and the description how they are connected together, someone could run EZNEC and prove out the article.
1/2 wave above ground it's supposed to have a 29 degree takeoff angle and 28 degree beamwidth.

I'm building this today (SUN) and I'll see what happens here at MOP radio


Fred


BTW What is wrong with AMFONE when I post a message??? sometimes as I get to the bottom of a message or edit the bottom line of typing keeps jumping up and down as I type?? This happens on several computers at home and one at work.

     sorry to bust a few balloons here guys, but in my real world situation with no antenna bridge or computer amtemma modeling I am actually foolish enough to  operate a 180 foot long dipole at 60 feet fed with 4 inch spaced #12 open wire line. it's good on 160, excellent on 75, and outstanding on 40 and 20. yes, it's showing very low antenna current on some bands and the voltage at the tuner terminals is very high, but it just works. I use a 275 watt matchbox with it, and all is well. a few nights ago on 40 SSB I was told by a DL4 I worked (that's Germany for you DX challenged AMer's) that I was the strongest station he was hearing from 'stateside. this was at 7PM with all of 150 watts SSB. the "garbage" radiation pattern has served me very well with long open wire fed dipoles for years on the higher bands. So, mr. Flintstone Mop- don't sweat the details. just run it and on a good day with decent conditions you will strap. remember we still have no real sunspot activity...yet.

"just slam the hood and drive it"
Junior Johnson

LOL. Any antenna will work when the band is open. I've worked several VK stations running a mere 10 watts and low dipole in recent months on 40 meters. That said, I'm not taking down my delta loops or giving away my amp.  ;)

Well,
Junior Johnson, didn't bust any balloons. It was a reality check, though. I guess we have been sanitized by the computer modelling software and want to get all we can out of our station with an effecient design.
If my A3S Yagi wasn't out of service, I wouldn't have bothered getting the grey matter working as a work-around. I'm still proceeding with the QST antenna and the Yagi will be dismantled and resurrected to a more suitable mast that can be easily maintained instead of hiring a crane truck for the utility pole.
WE had two bad wind storms and the last one musta been over 80mph and killed my Yagi.
Fred

LATEST UPDATE FROM MOPRADIO   :o

The HGSW that was in last month's QST is a reality and it does work 20M. I had some flaky solder joints. Anyway it was built to the specs from the article and the modification that a group of Hams did building this thing was to use a 4:1 balun for a better match.
It brings back memories when my Yagi was working. Nice strong RX sigs. I will be trying for some A.M. on 20M in the days ahead

Fred

Hey, cool. Glad you got it working.

Congratulations Fred, that's great.  My own antenna projs are progressing.   I plan to post a full report maybe with pics once everything is finished and I've had time to check it all out.

73

Rob K5UJ

Right on Rob!!

I'm RE-locating my 160M vertical a little closer to the house with the addition of a motorized tuner to use the ant anywhere on 160M.
Got to clear the hornet's nest from the base of the mast.

I have a tilt-over ability to get at the "stinger" to change freq and it was a scary surprise to see these things "crawling around" where I was standing to lay the mast down. The signal slowly came to my brain that these things were a hornet or wasp nest inside the mast. I didn't think they would have a nest so close to the ground........geesh that was close!!
Fred