Length of new 160/80/40 dipole

[John K5PRO]

I have a question for the collective antenna gurus here:

We got 8 inches of snow yesterday, and tonight are expecting more. My Rohn tower is waiting for antennas - notes at http://amfone.net/Amforum/index.php?topic=17522.20. Over the holiday, I might be able to satisfy this need, if it drys out a bit. Heck, even if it doesn't.

I placed this tower along a diagonal line across my 1.7 acre lot, centered near the middle. There is 120 feet of length to the other tower near the house, seen in photos, and equal amount to the far end of the lot, where a 7200 volt overhead line crosses the corner, perpendicular to my diagonal line. If I dig a deep hole and sink a pole at this end, with a pulley at the top, can pull up whatever wire I will use. Have to be very careful though, when placing this pole, that I don't loose control of it and have it fall over to other direction to hit that power line. I think that a 5 foot posthole should do - i have a bunch of big alum. tubing from another SKs former vertical that I removed several years ago. Having a top line on it, under tension, should prevent the above mishap from 'happening'.

This essentially gives me a good 220 feet of length for a big dipole, leaving a 10 foot insulated tail at each end. The center point, the new 25G tower, would be hung at 40 feet, and the Universal tower at the N end is 30 feet plus about 10 feet due to the slope. So this half is nearly flat at 40 feet above ground. The S end, where the additional pole would be, might only get to be 30 feet tall for safety.

I have homemade open wire feeder that I scavenged with the 'new' old tower. How long should I make each leg of the dipole, if I want to be able to tune 160/80/40 meters? Are there some lengths that are bad choices as they will be tough to match? I plan to mount a waterproof box at base, where a balanced tuna (true balanced, using the variometer coupled output L from a Raytheon RA1000) will reside.





 

[WA1GFZ]

clamp a section of pipe on the side of the tower to get the center up as high as possible since there is no beam up yet. Stay away from the power lines with a metal pole....we don't want to add a SK section on Gary's BB. Get the dipole as long as you can have a safe run.

[K1JJ]

John,

The good news is that a 220' center-fed dipole fed with openwire will give a nice figure eight pattern on both 160M and 75M. It will actually have some gain on 75M with sharper lobes broadside. However, it will have deeper nulls earlier on the sides than a 1/2 wave dipole due to the gain.

The bad news is on 40M it will have a quasi 4-lobe cloverleaf pattern with sharp nulls broadside to where your other 160/75M patterns lie.  You will be PW broadside and in three other directions.

The best advice I can give is to add a pair of 40M legs (33' per side) to the center feedpoint to produce a nice broadside figure eight dipole pattern on 40M.  This way you will have the best of all whirls.

The 40M legs will represent a low impedance on 7mhz and "hog" the power from the other higher impedance 220' leg, thus
standard figure 8 pattern on 40M. I would have to model it, but there is chance that the 220' legs might be of low impedance on 40M and have an effect too, but it needs to be calculated out to see. Offhand, there shud be no problem.  On 75/160M, the 40M legs are basically invisible.

BTW, don't worry about matching. Get the flat top(s) up as high and flat as possible and if you need to play with matching, you can always add or subtract some openwire line to get the "problem"  band more in line with your tuner's range capabilities.

73,

Tom, K1JJ

[K3ZS]

Tom, K1JJ gives good advice.    I added a 16 ft dipole in parallel with my 135 ft dipole to get a much better pattern on 10M, as he suggested, and it improved 10M operation quite a bit.    Later used EZNEC and I could see what happened.

[John K5PRO]

Thanks for the advice, Tom. Maybe I could also just run a separate switched feed line to a 40 meter inverted V dipole from same center location, by putting another pole out on one side, 20 feet tall, and use the garage or fence for another end, not exactly in line with the 220 foot antenna.

What about if I load up the tower itself as a vertical? I would have to install radials of course. The base is grounded, but all the guys are insulated on the lower level and phillystran for the upper level. The ham I got this from had a single bare copper wire running up beside the tower, insulated at the top, with an LC network at the bottom transforming and feeding it from coax. It was about 3 feet from the tower. I presume he had some problems trying to load the tower. There was a copper braid from the tower to the mast that bonded them together, also had a box with low pass filter caps on every rotator wire.

It might be nice to have both the wire horizontal antennas and the option to load it vertical, on 80 at least? Comments? I appreciate hearing from youse guys for this. Reading articles and books all day is fun, but the direct experience gleaned here is priceless. Well, almost priceless.

[Bill, KD0HG]

John, lotsa good ideas here. Here's one more.

I'd suggest trying am inverted L off that tower. Run a wire up the side on standoffs (like PVC pipe), then run it over to any high support that's safe from power lines.

Bury a bunch of ground radials at the base of the L and use matching tuners feeding buried Heliax on the run to the shack.

Yes, you'll have to crank the tuner when changing bands. But you'll be safe from power lines and lightning.

I had extremely good results with an inverted L at my former QTH on a city lot. 60' vertical and 65' horizontal to the north to a tripod on my garage. Worked GREAT on 160, 80 and 40. Even worked Jack, KH6CC on 160 CW. On 75, the performance was similar to that of a 125' dipole with the center at 60'. You could almost look at it as an inverted Vee on 75, but not parallel to the ground and fed at the end of one dipole, if you get my drift. In effect an end-fed half-wave vs. a center-fed on on 75, a top-loaded vertical on 160.

Just another idea...You can't lose by installing a bunch of buried radials at the base of your tower, anyway, regardless of what type of antenna you use. Our ground conductivity in the west stinks. I'll bet your true RF ground is lossy and 5' to 10' down.

[K1JJ]

Hi John,

Yes, using a separate dipole for 40 M is a great idea. Not only do you get a perfect figure 8 pattern, but your "rowing around town" tuning of the transmatch is eliminated. Just one coax switch does it.  Plus you can favor a different direction than the other antenna, if it suits you.

BTW, I use all hardline and coax here there days and have simple switches to go from band to band.

As for using the tower as a vertical...  for some reason I get the impression that the tower is near to the house. If so, you will pick up computer noise and other garbage as well as the electrical wiring and plumbing having a huge effect on the vertical pattern.  The low angle will be non-existant cuz of the attenuation. Also, with poor ground conductivity, even though you have a fairly good local radial system, low angles need good ground out many wavelengths where you cannot cover with wire. This is pure loss.  ie, Verticals need an open area of a few wavelengths radius minimum to come close or surpass horizontal antenna performance.

Another thing to be aware of is if you shunt-feed the tower as a vertical, most likely your 220' dipole with openwire will couple to the tower and become part of the system. This will give you a vertical 'T' whether you want it or not. (Very heavily top-loaded vertical)
Feeding one leg of the dipole against ground may give you what you are looking for - the easiest way.


But aside from all that, if you do try a vertical, I would agree with my good friend Bill and try an inverted L. This has some horiz component in it and will give you a better chance of capitalizing on SOME low angle vertical component. Remember that on 75 and 160M AM, you will probably be working most of your stations within 1000 miles. A 1/4 wave high horizontal is usually best for this distance. Though, your dipole is going to be less than 1/8 wavelength high on 160M and less than 1/4 wavelength high on 75M, so this will create higher angle, close-in optimization.  With the down sloping ends, the average height will be even lower.

So, in summary, if you can keep the ground losses low and the proximity of houses away, an inverted L just might eek out enough lower angle signal to beat or simply complement the dipole on 160/75M for farther out stations. Tough call.   40M might work even better since you are up over 1/4 wavelengths and will have longer radials/wavelength on the vertical.

Playing and testing is the frosting on the cake after all your hard work.

Let us know, OM.

73,

T

[WA1GFZ]

That is why I thought adding a length of pipe on top of the tower would help the height issue. A couple plates with muffler clamps on each would hold it up. Heck just leave the gin pole up and use that for now. Then there is a way to raise and lower test antennas.
I have a center fed inverted l up and sometimes it is stronger than the dipole.
Dipole beats it most cases. I have not listened to any DX stations yet.

[K1JJ]

That is why I thought adding a length of pipe on top of the tower would help the height issue. A couple plates with muffler clamps on each would hold it up.

Yep, agreed!

Even try to clamp on a 20' pole to the top, John. That would give you 60' to work with. That would make a huge difference on 75M.


There was a time in 1973 when I lashed a 70' telescoping pole to a 50' tree. At the top was an inverted V for 75M fed with openwire. The ends stretched way out to the neighbor's trees on both sides making it almost flat.  The Tron came over for the frst time and commented that it was the highest installation he had seen to date for the AM crowd... :-)  Worked like gangbusters.

Of course, later that year an ice storm snapped the top off.

I'm a big believer in mounting/lashing a pole to the top of any structure to eek out all the height you can get, caw mawn.

T

[John K5PRO]

The building seen about 50 feet to the north of the tower is garage/workshop. In there are the big iron, a BC1H1 that came from KD0HG from Denver and a Continental 314R1 Power Rock that came from Jim, W5JO and Steve K7KMT in Casper, WY. Neither has been converted to amateur though. Both work fine on medium wave.

The main studio/shack is in the house, 120 feet away. I have to run some sort of crossover hardline from shack out to the big rigs, so that they can run remote controlled, but the receiver will be in the house.

I will follow your advices here, and start with something besides loading up the tower. Adding a pole to the top, I got plenty of big aluminum mast I could use, all this cool hardware came with the tower, including some HD brackets for paralleling masts off towers. BUT, I also have this Tennadyne LP10 that came with it, right now taken apart. I might refurbish that, or might ignore it outright and just use the wires and low bands for now. The main shack has an old Cushcraft A34B yagi that works up on 20-10. Tanks 73

[K1JJ]

Now you're talking, John!

Yes, get up the high mast with a pulley and play around. A dipole supported at the center really is not a big mast load cuz it is somewhat guyed by the dipole itself. If you use a tapered mast, you might be able to pull off 25', giving you 65' total. I've done that here.

Later on, for the log periodic... a good technique Ive seen done is to mount the log at the top of the tower using the conventional rotator, etc.  Use the same mast as the dipole support for the log. Then have a rotating bearing at the very top of the mast to support the dipole. The log will rotate and the dipole will stay still as the bearing turns at the top.  Pull the openwire feedline out far enuff so the log els miss it, etc.

BTW, 120' from the house is a reasonable distance - probably the absolute minimum, but workable for a vert. Give it a try.

Good luck.

Tom, K1JJ

[Jim, W5JO]

John, given what I know about wind in your area and altitude, I suggest anything you add must be capable of withstanding 100 MPH.  If not you may, at some date, be cleaning up a mess. 

I have had great success with an inverted (Lazy) L that is 44' vertical and ~125 horizontal.  The answer is a good radial field (I have about 60 of them).  You could have a 25 ft. verticle with a great radial field and do wonderful.  Using a good tuner at the shack allows me to tune this antenna on all 5 (AM) bands.

Any number of installations will work for you where you are, provided you install a good ground screen.  Don't worry so much about the height as the ground system and be careful putting something that is fragile on top of your tower.

[K1JJ]

100 MPH? Yes, agreed -  at least that high.  I use YagiStress software to design all my antennas and usually set the minimum for 130 mph WITH 1/2" of radial ice.


That guyed 40' tower, the way it's installed is probably good for 150 mph or more.  With a load, it will be somewhat less, of course.

The aluminum mast can be of 1/4" wall, if need be. Sleeved if using thinner wall.  It can be 3", tapered down to 2"  diameter. The mast can be supported in the center of the tower using thrust bearings or other comparable steel bracketing center supports designed to handle a large sheer load.

No reason to make it flimsy. Build it overkill to last. I've had 36 homebrew Yagis in the air here since about 1998 and have not lost a single element to date. Been through high winds and ice stroms. YagiStress software tells you where to put trussing, where to double up the wall thickness of tubing using sleeves, etc.
The same philosphy can be applied to wire arrays, masts, etc.

Yes, a good ground screen will make that 'L'  play.

Good luck.

T

[John K5PRO]

I took a long look outside today, walking in the mud from melted snow, and measured the following dimensions, See attachment. The bigger box is my home, with h'shack in a spare bedroom. The other box is the garage, where the future broadcast TX conversions are, for 160. Right now, I would like to just get a solid 80 and 40 meters, and the coax is already running underground from shack to the center of an old B&W trap dipole using RG8X since it is self supported at the center. That antenna is only 20 feet high and will be replaced with new wire, tied from tower to tower, putting it about 40 feet above ground at S end and 30 at N end. The ground slopes uphill to the north (top of diagram). That 7200 volt line in the SE corner worries me. If I get within about 15 feet of it, I meaured 100 feet to the tower. If I stuck up a pole would have to make sure it doesn't fall over to the SE. Also, if i tied it with some guys or stays from the opposite side, they would be under the power line i think. The simpliest and fastest antenna would be to put a true 80 meter half wave dipole from tower to tower, and erect a mast at the point marked "40'pole". This could be tapered alum tubing, buried in the ground or some such technique. I do have some tower sections of an ancient steel tower, a 22 and 27 foot piece, that used to telescope. But then I gotta pour mo'concrete. But the mast might be enough, if it has a pulley at the top where I can raise and lower the stuff. It would then hold a piece of RG8 instead of 8X.

For 40 meters either make it a multi-band dipole hanging the shorter wire below the longer one like i did in the 1970s as a novice. It would droop a bit below the 80, but still be like 35 feet above ground. This scheme isn't elegant like had been discussed here, with the open wire feed line, the long wires and additional gain. But it gets me on the air without needing a tuna for these two bands. If'n I want 160 meters, then I could resonate the second tower as a vertical, adding radials of course. It would be at the end instead of center point of the horizontal flatwire dipoles.

Obviously will loose some of the gain of the longer wire, but simplicity might reign. Comments?

[WA1GFZ]

You have 160 feet between two towers? Hang some vertical drops and tune it up on all three bands.

[Steve - WB3HUZ]

Yep, 180-190 feet is a good length for those three bands.

[John K5PRO]

OK, so if I hang some vertical components, where do you mean? Would I feed it from center across an insulator or from the end. Are you referring to making it an inverted L?

[WA1GFZ]

Feed it in the center with open wire line. Hang the vertical off the ends and tie them off so the wire doesn't hit the tower.

[W2VW]

You can get on 160 just fine with a length of 160 feet. No loading coils or other airborne gizmos needed. Currents will be high. The feed must be homemade open wire line made from heavier than normal wire. The center of the doublet out 30 to 40% should also be pretty heavy stuff. The tuning unit should be built from components rated for high current. Myself and several others are currently on 160 with that exact setup. They work OK. Design the whole thing like a car alternator B+ line and losses will be kept to a minimum.

160 feet is actually better on 40 as it behaves like a double extended zepp. When you make the antenna a little longer the pattern breaks into a figure 8.   

[K1JJ]

You can get on 160 just fine with a length of 160 feet. No loading coils or other airborne gizmos needed. Currents will be high. The feed must be homemade open wire line made from heavier than normal wire. The center of the doublet out 30 to 40% should also be pretty heavy stuff. The tuning unit should be built from components rated for high current. Myself and several others are currently on 160 with that exact setup. They work OK. Design the whole thing like a car alternator B+ line and losses will be kept to a minimum.

160 feet is actually better on 40 as it behaves like a double extended zepp. When you make the antenna a little longer the pattern breaks into a figure 8.   

Yes, this is good advice from Dave.  John, if you choose to go from the original 210' to 160' length fed with openwire, then this will be a good solution for 40M too. I think Dave meant to say, "clover-leaf" pattern, not "figure 8" for the longer 210' length on the 40M band above.

As Dave said, the disadvantage of using 160' is the heavier current requirements on 160M, but that is a minor issue - you just need to beef up the overall system as described.

Another advantage is that usually we can get a 160' antenna up higher and flatter than a 210' antenna. In addition, with a shorter length, your overall ground coupling will be lower producing slightly lower ground losses and a slightly lower take-off angle.  The pattern on all three bands is basically a figure 8, (desireable) with sharp lobes on 40M, so aim it broadside correctly for 40M's sake.

You now have many options for antennas, OM.

Good luck -

Tom, K1JJ

[W2VW]

TNX Tawm.

    St. Pattys day is a long way off.