The AM Forum

Hello antenna gurus,

         I’d appreciate some thoughts on an installation. I am putting up an inverted V for 75m. The center will be at 60’, the ends will be at 20’.  I’m using 75 ohm RG-6 and 14ga stranded wire. How important is having the the 1:1 coax balun at the feed point as opposed to 30’ away at the bottom of the mast? 
        The reason I ask is I’m using a very light weight center mast. It’s a 30’ telescopic fiberglass pole made by Jackite. A coax balun will be too heavy up top without adding guys. I don’t want to use ladder line because it’s too difficult to run appropriately to my shack rack. (I’ve tried)
         My current antenna is 35’ at the apex. I’m hoping for a lower takeoff angle.
Will mounting the coax balun at the bottom of the mast affect the pattern/takeoff angle?
My initial thought is that since the antenna will be resonant with a low SWR, radiation from the shield of that 30’ section of coax should be minimal. ???   

-Phil

If the feedline does radiate is probably won't hurt the low-angle pattern since it will be vertically polarized. You may not need a balun at all. If the feedline drops down from the feedpoint vertically for 60 feet, you may be good to go without a balun.

If you can, look at a center fed inverted L. The angle of radiation will be a lot lower than an inverted Vee. The total length will be the same just 1/2 H and 1/2 V. So if you can get one half horizontal at 60 feet you should work better based on the simulation I have seen.

You could definitely get away without a balun - I did it for many years.  However, I did notice improvements with the balun, particularly with respect to noise immunity.

The dirt simplest balun you could probably use is simply some type 43 ferrites placed over your coaxial cable.  I use quite a few of them (about 12 inches worth) on my coaxial feedline, but you may not actually need that many.

This will add a small amount of weight to the antenna, but not as much as would a coaxial balun, and this one is a whole lot smaller !

Regards,

Steve

Hi Phil,

To answer your original question... yes, you want the coiled coax balun to be right at the antenna flat-top feedpoint to be the most effective.  Since you have a support problem, you might consider using a lighter coax for the balun, like RG-58/U or similar, then splice into RG-213 or whatever you use for the longer run. On 160-40M the tiny loss difference using the RG-58/U balun is less than 1/10 of a db.

Here's some more info on baluns. I have listed the various techniques in order of what I feel is most effective. Though the coiled coax technique is down the list, it is used quite often and is a good way to choke off unwanted shield currents.

http://amfone.net/index.php?pid=12

Feedline radiation on the lower bands has a small effect on a SINGLE dipole's pattern; whereas it is paramount to have minimal feedline radiation when phasing multiple antennas together. (when stacking)  A good stacked array pattern can be trashed real quickly otherwise. For a clean pattern, the designed phase relationship between antennas must be maintained.

ie, Changing the phase relationship between a dipole's legs OR between two antennas by randon feedline pickup into the radiating legs will produce an antenna pattern different than what you may desire.

Good luck.

Tom, K1JJ

The few valuable things I have learned about antennas is that low angle takeoff doesn't start happening until one uses a vertical Antenna or using a dipole or VEE (help me here please) 1/4 wave or 1/2 wavelength up in the air. Essentially most of are using cloud warmers and will make contacts out 300-500 miles. Really good condx might be the 2:00AM in the morning CQ.
I think GFZ has given some numbers where lower angle of radiation takes place.
Your dimensions are a little confusing, though. The VEE will be 60 feet high at the apex and 20 feet high at the ends? Or was the total length of the antenna was 60feet long?

Fred

Hi Fred,

Results depend on where the solar cycle is and the time of day, of course.  During this low spot activity you will find the 75m extremely long during the night and guys with low dipoles working Europe regularly on 75M.

But, on any given frequency, an antenna at 1/2 wavelength high (130' high on 75M) will produce a single low angle lobe at about 28 degrees take-off. Best for DX at 2000 miles or more.

At 1/4 wave high, (65' high on 75M)  most energy is in a broad lobe centered around 50-60 degrees or so.  Best for 100-500 miles.

At 3/8 wave high, (90' high on 75M) most energy is around 45 degrees or so.  Best for 200-1500 miles.

Once above 1/2 wave high, the radiation starts to break into two, then three or more lobes with the lowest lobe always being the largest. This is due to ground reflections in and out of phase.

In general, for local work on 75M, 1/4 wave high is FB. For all around work, 3/8 wave is perfect. For DX, (working Europe and S.A.) the antenna at 130' is ideal, but weak locally, which is what you want.


The best 75m antenna mix would be either a single dipole at 90' or two dipoles... one at 60' and the second at 130'.


For the higher bands, simply double the wavelengths above ground shown above. For example, on 75M, the optimum angle to Europe is about 40 degrees take-off or so. For 40M into Eu it is about 20 degrees. For 20M it is about 10 degrees and so forth. It's obviously easier to get a full wavelength high on 20M (60') and get that lower angle.

Later -

T

The Trees around here are 70 to 80 feet except for the 100 foot giant in the woods. I have been simulating stuff based on that height trying to get something low angle. 45 to 60 degree TO angle of the dipole is great for local work. Even my phased dipoles didn't work as well as the 160 meter dipole some 25 feet higher. 

I'm playing with a cF inverted L simulation with the end 5 feet off the top of the tower out to a tree 65 feet away the feed point then vertical drop. Working against the tower I see about 5.5 front to back with a couple dB of gain away from the tower. Not great, but plenty of radiation down to 15 degrees up to 45 degrees. It does face West to hit the US

Thanks to all for the input and comments. I have some serious spatial constraints at the old QTH. I've got a 22'x110' lot to work with. The L configuration will not fit, the house is in the way. The 30' mast is going on the roof on a tripod mount. It is a very light duty mast. It's designed for flying small decorative flags.

The RG-6 is super light, lighter than RG-58 but I'm still hesitant to coil it right at the feed point.  Maybe I'll situate it halfway up the mast.

I know I'm not creating a DX antenna, but I'm assuming I'll see an all around improvement vs. the 35' high inverted V currently in service.

The only semi serious DX antenna I can fit here will be a 5/8 wave elevated groundplane vertical for 15m. That will be up on the roof as well. I've got another one of those 30' poles for the vertical section.

The neighbors are already horrified. Oh well.

Good luck with it. Going from 35 to 60 feet should make a big difference.

Consider teflon coax like RG146BU or RG400. 1/4 inch and will handle a lot more power then RG58.
Screw the neighbors not like you are raising pigs.
yup 30 to 60 feet will make a big difference.

Yes, agreed!

Back about 10 years ago I ran some comparison 75M fast-click A/B tests with a REFERENCE dipole at 60' and a dipole that was raised and lowered from 10' to 35'.   I found the myth that a very low dipole (<30' high) performs better for local 75M work to be FALSE.

Yes, the low dipole may have a higher angle of radiaton, but the ground losses mount up faster than the apparant high angle gain.  When the test dipole was at 10' off the ground, it was almost deaf in comparison to the reference dipole at 60' high. Even at 30' high I could almost always see a 5-8 db difference when listening to stations 300 miles away or more.  Never once did the dipole at 30' appear louder than the 60'er EVEN when stations were within 50 miles.

I've concluded that angles above 70 degrees or so are useless on 75M, even for local work, thus a 30' high dipole is the absolute minimum one needs to strive for...  or expect to warm the worms.

I also believe that as much vigor and effort should be made in getting the dipole as flat as possible. You must do what the backyard gives you do work with, but a flat dipole will usually outperform an inverted vee at the same height. Lower ends simply cause  radiation cancellation as well as lowering the overall average height and effective take-off angle.

Given a choice, I'd rather have a FLAT 75M dipole at 45' than a sharp sloping inverted vee at 60' apex... anyday.   ie, If not flat, get out that bow and arrow and try for another 10' higher on the end legs if you can. BTW, it is better to have the ends HIGHER than the center than the center higher than the ends, if you have a choice.  Nothing beats a flat dipole with the feedline coming away at a right angle - textbook dipole.

Another OT tip: Dipole longevity for the wind and ice - always support the center of the antenna. Hanging the dipole by the ends only creates a tremendous strain on the system when hanging the coax at the center. Just like an Iron Cross gymnist routine.. :-)

One last point for newcomers... some are impressed when hearing about very high array/beam  antennas on 75M. But they are surprised when the simple dipole at 60' outperforms the high Yagi for local work. The very best antenna for local 75M AM 100-500 miles work, which is the major activity on 75M AM, is a flat dipole at 60'. 


Just some things to think about... caw mawn.


T

Tom,
I have been looking at ways to reduce the TO angle when you don't have the height to make a dipole perform. Even at 60 feet phased dipoles didn't have the DX performance of a single high antenna. Any thoughts?

well this type of discussion is usually where i get me arse handed to me but let me give it a shot.

your jacklight pole will look like a fisher price fishing pole with a blue marlin on the hook if you dont center support it..  those poles are great for making a wire vertical or supporting the end of a #14 wire. but if you dont center support it the jacklite on the end will loose about 15 ft of height. go to skywalker.com  they are a wholesale dist but if you are not in indiana they will mail order to you.  they have a 30 and 40 ft channelmaster cuamawn telescoping mast for about $60 shipped.  take a 10 ft 4x4 post and sink it 2.5 ft in the gound and pour 80lbs of concrete around it.  buy 4 6.5 inch hose clamps from home depot to strap (cuamawn) the mast to the post.  If you need more than 40 ft you can buy 1 3/8 chainlink to rail from home depont which with a little mod will fit on the top pipe.  I weld mine together but ubolts would work. THIS WILL NEED TO BE GUYED IN 4 DIRECTIONS AT CENTER AND TOP.  STEEL ELECTRIC FENCE WIRE FROM FARM STORES WORK FINE. 

i FEEL YOUR PAIN BROTHER MY TOWNHOUSE LOT IS 20X120 AND THE BACK YARD IS 20X80  WHAT I HAVE DESCRIBED IS  WHAT I USE.  iTS HELD UP FOR YEARS AND MY TALLEST SUPPORT IS 60 FT. I HAVE A G5RV ON IT ONE END ON THE ROOF AND THE OTHER ON A 60 FT MAST.   THE CENTER IS ON A 60 FT MAST THAT SITS ON A BERM THAT IS 12 FT HIGHER THAN THE REST OF THE YARD.  one end of the antenna is bent down at a 90 becuase the lot was 20 ft too short

good luck

http://amfone.net/ECSound/K1JJ14.htm

Hi Frank,

You can "force" the take-off angle lower by adding elements, but by only about 5 degrees or so, depending on how long the boom gets. By using a 2el driven array like you have, maybe a few degrees lower is all you get compared to a single dipole at the same height. But alas, by raising the antenna higher, you will gain low angle at a faster rate.  When below 1/4 wavelength, adding 10'-20' in height is tremendous. It starts to get into slower returns as you approach 1/2 wavelength and higher.  The best of both worlds, a long boom at 1/2 wave high is optimum, of course for 75M DX.

So, I would guess your 2el driven array at 60' compared to the 160M dipole at 90' would favor the 160M for low angle. It cud be modeled easily enuff. But the F-B and lower receive noise might give the edge to the array for receiving.  Too bad you cudn't add a pair of end-end reflectors behind the 160M dipole used for 75M. That wud play well as 2 halves in phase wid reflectors. Bob/W2ZM uses this on 75M DX and is a channelmaster for sure.

Steve, while we're on the subject, I remember you did some nice studies on dipoles above ground and the losses using low dipoles. Maybe a link to that wud help.

To the guys with limited antenna space... I also feel your pain. There was a period of 10 years where I bounced from apartment to trailer campsites trying to pull it off. I think the best arrangement was putting up a 40' mast on the top of a 4 story apt house in Denver, Colorado. At the top was a coax fed inv vee stretched out to the other trees in surrounding apts. Four 4-400's in parallel.  It worked like gangbusters for a month until the neighbors found out where the RFI was coming from and formed a lynch mob. I had to climb a tall tree to get to the top of the apt roof. People wud see me coming by the window and freak. Hey, anything to get on the air, right? I finally got an eviction notice demanding I leave, take down the ant and it also complained of "drilling noises" at 3AM in the morning... :-) 


Tom, K1JJ

:) :o :) I totally agree. The jackite pole is going to be the center support. I'm trying to keep it as light as possible. I've got a solid 20' mast on one end and a tree on the other. With stable supports at the ends it should remain vertical. What it does side to side will be interesting. I've never used these poles before so a lot remains to be seen. I could turn out to be a big waste of time.

Drilling noises, eh? What was her name?

I'll dig up the loss graph. I didn't do the study. I found it in some old radio/engineering book. I may do some modeling runs with NEC4 and Sommerfield Ground to further amplify ( ;D ;D ) the point.

Tom,
I did some more simulation with the CF inverted L working against the tower as a second element and found by adjusting the antenna length the front to back could be increased. No big gain but the TO angle was nice down to 15 degrees. I want to simulate a cage with 2 or 3 wires to see if I can broad band it. It fits perfectly in my back yard between the tower and tree line.
Remember the rope I showed you up over 80 feet that I sent up for a support.  That is about 70 feet from the tower. If it doesn't work I can go back to the phased system.

Sounds like a good experiment, Frank.

The only problem I see is you will need a ground radial system to keep losses low for the added vertical components. In addition, the antenna will perform only as well as the earth around you out many wavelengths to support 15 degrees. To get that low 15 degree take-off angle shows that you are using a fair amount of vertical component.  Normally a  fully horizontal antenna at 70' would give about 45 degrees, so you see my point.

Even using a high impedance feed like a 1/2 wave radiator to keep IR losses low, low angle efficiency still depends on real Earth out far from the ant - out of your control.

In addition, with the many houses surrounding your Enfield QTH, I'd give it a low probability of working better than your high horiz array.  Stand at the base of the L and draw an imaginary line moving up at 15 degrees and see what you hit... :-)  There's not much to work down at 15 degrees anyway. Better to put it all up at 40 degrees or so. East coast to Eu has been shown to be a 45 degree path. Lately it is much higher, like maybe 50 degrees or so as shown by the long condix and guys with dipoles at 50' getting big reports.  ie, A real low angle vert needs not only the right condix, but a good radial field AND an open field for at least 1/4 mile radius to support those angles. Otherwise you take a bite out of the low lobe like Pac-Man.

BTW, For 75M DX, I've tried many verticals and vert arrays here over the years with some having elaborate ground radial systems. They were always 10-15 db below the high horiz Yagi, even with a big, clear field.   All depends on your Earth conductivity out for a mile or so.  Mine here is poor being rock ledge, so I run ALL horizontals on all bands. It's all a matter of discovering what works best at our own QTH's.

T

Well the CF inverted l is fed at the top but yes ground is a consideration. 15 degrees is actually only a problem to the East, West I have about 300 feet and N and S the yards and stream bed.  Well if I build it and it doesn't play I can use it as a horizontal.  Also it makes good radiation at 45 degrees.
Old Dick W1JF, suggested this antenna over 30 years ago but I never tried it.
He suggested the shield to the horizontal wire and the center conductor to the vertical.  Who knows? I should simulate it with a crappy ground just to be sure.

Crappy ground lifted it up a bit peak is between 30 and 60 degrees and 15 is only down 2 dB within a quarter db of 90 degrees. This actually looks better for medium range.

He sure is! I spoke to him briefly this morning as we were working a VK station. Needless to say, the VK heard Bob much better than me!

Here's the efficiency graph is posted previously. Note that is is for 18 MHz and poor ground. Things will be different for a different frequency and different ground. But I bet the curve will be just about as steep at the lower heights, no matter what.

I did some simulations of an 80 Meter Dipole at various heights above ground. I started with a dipole at 60 feet and trimmed it to resonance. I used this length for all the other heights. That length was 126.646 feet (this doesn't matter that much). What's important is to notice in the table below is that the Real portion of the dipole impedance drops as the height decreases. The imaginary portion changes very little.

The ground used for all the simulations was Sommerfield Ground (best simulation of losses when close to ground) with parameters ("average" ground) - Conductivity (S/m): 0.005   Permitivity: 13.


The table below and the attached elevation pattern plots summarize the simulations.


Height (ft)      Gain (dB)    Take-off Angle (deg)   Real (Z)   Imag (Z)
20               3.741         90 (straight up)      46.321      -3.090
30               5.607         90                     51.952      2.638
40               6.314         90                     61.963     5.886
50               6.441         90                     72.605     4.883   
60               6.232         72                     81.497     0.002


You can see the 60 foot high dipole has almost 2.5 dB more gain. Don't be fooled by the higher gain number of the 50 foot high dipole. That number is still straight up, where the 60 foot high dipole's maximum gain is at 72 degrees. If we compare the gain of the 20 foot high dipole and the 60 foot high dipole at 72 degrees, the 60 foot high dipole is better by 2.82 dB, almost 3 dB! And as Tom noted, this is more in the range of angles best for "local" coverage out to 300-500 miles. That's like running 200 watts instead of 100 watts and your tubes don't have get any hotter!

The reality is that at the longer distances of the "local" coverage range, the received signal with the 60 foot dipole will likely be far stronger than just 3 dB, for several reasons.

One is that the signal will get there in a fewer number of hops, assuming it takes more than one (anyone good with VOACAP or some other  propagation software want to check). For every hop, the signal suffers loss (ground reflection, and additional absorption and dispersion in the ionosphere, and just plain spreading loss).

The second possible cause of greater loss for the 20 foot high dipole is that these simulations assume there are NO other objects near the dipole (like your house, trees, overhead wiring, etc). The lower dipole will most likely couple more to these real-world objects than the higher dipole.

The differences are even greater between the 20 foot high dipole and the 60 foot high dipole if the ground is less conductive. I ran simulations with the following ground parameters (described as Urban/Industrial) - Conductivity (S/m): 0.001, Permitivity: 5.

The difference in gain is now 2.74 dB at 90 degrees and 3.08 dB at 72 degrees. At 60 degrees (the take-off angle of the 60 foot dipole over poor ground -  remember poor ground lowers gain but also lowers take-off angle), the difference is  3.62 dB. And the difference increases as the angle decreases (e.g. 5.3 dB at 30 degrees). So the 20 foot high dipole is about another dB worse than the 60 foot high dipole over poor ground.

Of course, if you are lucky enough to have good ground (Conductivity (S/m): 0.02, Permitivity: 17), then the differences will be less, something on the order of only 1.54 to 2.4 dB (depending on the elevation angle in our area of interest), which is about a half a dB less than the average ground case and more than a dB than the poor ground case. And, remember the two additional loss mechanisms mentioned above (real world object near the antenna and additional hop loss) apply no matter what the ground is like under your antenna. No matter how you slice it, 60 feet is better than 20 feet.

Vely, vely well thought out, Steve.

The points you mention concerning house absorbtion and less hops are more reasons to do everything possible to get to the magic number of 60' above ground on 75M. We all have our backyard limitations and do what we can.

It must be kept in perspective that this  ~ 3db improvement is a lot. Hams kill themselves to put up 3el Yagis that give only a 4-5 db improvement over a dipole.  Antenna db is hard-earned... probably one of the most difficult things to pull off since there are so many variables and things that can go wrong.

The end result is to listen to the many stations on the air. Over time you can tell the stations that are doing things right and the ones who are not by their average signal strength consistency.  The one station that comes to mind is Ed, WA3PUN who had a dipole at 60' for years. He was always the Channelmaster here for the 350 mile trip.

A db here, a db there - they start to add up after awhile... :-)

I think most newcomers instinctively know they need to get the dipole up as high as possible, but usually don't know the real reasons why or the specific parameters to shoot for. Your info gives all the right reasons to plan for the best possible erection from the start. (Viva)

I think many simply throw up a G5RV in the trees and then slowly make improvements from there over time. This thread gives a specific plan to do it right the first time.  How many times in new hobbies I wish I started out right, but had to learn as I went along.

Another point to remember is if we are hard-pressed to reach 1/4 wavelemgth above ground, it is better to use a simple 1/2 wave dipole rather than two half-waves-in-phase, extended double Zepp, etc. The shorter antenna will have less ground coupling and a lower angle of radiation, thus less loss in comparison and better angle. Also, it is probably easier to get the ends higher using a shorter antenna.  Plus, for a FIXED antenna, attempting to obtain a narrow pattern with longer legs resulting in deep nulls elsewhere is not a good idea for local AM work.


Frank, I'll be curious how your L system works out. Be sure to insure you are using a reference antenna (the 160M ant?) and they are not coupling, giving false readings. That will be hard to prevent due to their near proximity.  A min 300'-400' separation is required OR they need to be at perfect right angles from one another (or collinear, not broadside) OR one needs to have a reflector separating the two to keep interaction minimal.  If you see little difference between the two, I wud be suspect.  Obviously, making a meaningful determination between the driven array and L will not me possible if the driven is down since condix change minute to minute.

Tom, K1JJ

Tom,
I will be able to compare it to the dipole in the woods. The ends will be close to each other but they will line up  If it doesn't work it will be the new dipole required to get the phased array working again. I keep coming back to the CF inverted L every time I get into simulation. 30 to 60 degrees TO is not bad and still OK at 90 degrees. It just keeps coming up as a winner. I want to play with spacing from the tower. I have not tried putting it broadside to the tower which would put it 90 degrees from the reference. That may be an even better test. There is plenty of room in the yard to put it either way and up at least 70 feet.
I also want to try phasing a pair of them. The nice thing about that is it only takes 65 by 65 feet.
Also tried a 2 wire cage with a BW good 3.5 to 3.9 MHz at 2:1 VSWR.   

Sounds like you gots it covered, Frank.

A pair of phased L's is very worthwhile, assuming your ground conductivity is OK. Otherwise I'd switch back to the proven horizontal phased dipoles.

Another way to test it is we can get down in the DX window and do a series of comparison tests into Europe over a coupla nights. If I run exactly the same set up each time, you will start to see a very close difference number that can be used when you try the next ant, etc.  It's amazing even with changing condix how the differential between two stations remains pretty close, esp for long haul DX.  Local tests are much more difficult and unreliable in comparison.

T

I was looking at a Cebik article and he also says the ground effects are less with a CF inverted L compared to base fed. I guess comparing a vertical GP to a vertical dipole.
Yup, if it doesn't work no harm done just hang it horizontal in the woods.

Well I just got down from the roof.  The Jackite mast does not function well as an inverted V center support.  The gusty winds today were snapping it around like a whip. Mission aborted.

As some have already implied, I will have to use something more substantial. 

Thanks guys.

well, WX too good becuse  I just gave the CF inverted the last yank to pull it into place and a ceramic end insulator snapped at the top of the tower.
I didn't even see it break. I was about to tie off the vertical lead and looked up and the other 1/2 of the antenna was missing. Dropped the whole thing back down and got another insulator. (the old one must have been dropped because that never happened before). Then the sky opened up. Weird warnings for twisters and the sky is a funky color.

what a dope I am. The high winds would have cleared the branches for me had I not given that last yank. I could be on the air with the new ant testing tonight,. The end was off the ground and I wanted more.

No place like homer out there tonight

well got the CF inverted L up but had to put the feed point near the tower. Just too many branches in the way. End is a good 80 feet and feed point about 60. Vertical  is on a slight angle. Faces West. AWA net Herb K2VH is about even money with the diople but most others louder on the reference.

WX not bad enough. Simulation shows VSWR messed up and pattern screwed with feed point near tower. I did notice a tree fell in the woods that was blocking the new position of phased array. Maybe it is time to go back to it.