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[Articles Home]  [Add Article]  

Methods to Shunt Feed a Tower

Hal Williams (N6TZ) on April 18, 2005
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This is a follow up to an article that appeared on this site a few days ago titled "How Can I Have a Vertical and a Horizontal Antenna Too?"

In the comments that followed that article, were a few requests for details to Shunt Feed a Tower. I hope I may lend some advice to help others successfully load a tower on several bands. In the tips below, we will configure a shunt fed tower of the Monopole or Unipole design. With careful grounding and connecting of bonding points along with the use of a antenna autotuner, the Ham with some mechanical ability should be able to operate his tower on 30, 40, 60, 75-80, and 160 meters.

The tower or mast should be 40 feet to 65 feet, and the more beams and stuff on the top, the more top loading it will have to help it work on 160 and 80 meters.

The tower used for shunt feeding must be electrically conductive from top to bottom. In the case of a crank-up tower, you may require some of the following steps; and these steps should also be considered for push-up masts, etc also:

1. If you have a beam or other antenna on top of the tower, make sure the coax shield to that beam is electrically connected at the top of the tower (usually the beam is grounded to the tower and that does it). If this is not possible (like a quad), then run a tracer bare wire from a bonding point at the top of the top tower section down and maybe two or three times around the tower and bond it at the base of the tower. The tower must look electrically solid.

2. At the base of the tower, carefully open the jacket of the coax feeding the beam and solder a small bonding wire to the coax shield, waterproof the cut, and bond to the base of the tower. Also, bypass the rotator cable wires with .005 or .01 at 1kV caps to a bonding wire to the base. Also, bond the tracer wire coming down the tower if you used one.

3. Ground radials. There is no substitute for radials. Ground rods will help for lightning, but will not make a vertical work. In a small yard, put in as many of any length as you can. A minimum of 10 to 12 of 30 feet or more is just a starting point, but that number will work. At the base of the tower, bond by soldering them together and run a short lead from them to the bonding point on the tower. This bonding point is the same place you put the bonding from the coax, rotor bypass, and tracing wires. Use a stainless steel bolt and washers and clean the surface of the tower where you drilled or used an existing hole at the bottom of the tower. Lug and solder all the wires that will be bonded, and place them between stainless washers under the nut. Use at least a ¼ stainless bolt. Don't put the lugs directly in contact to the steel of the tower, let the stainless bolt and washers be the only contact with the steel of the tower. A little no-ox compound would be nice at this junction.

4. Purchase about 10 to 20 feet of copper flat ribbon strap, available from several ham sources. Get at least 2 inch wide stuff, but 4 inch is better. Now pick a place about 5 to 20 feet away from the tower base to use as your feed point. You will want to have a weatherproof box for your feed point. You will need the ability to run coax and control wire to that feed box from the shack. From this feed box, you will run the flat copper ribbon strap underground to the bonding point of the tower base. At the feed box, if you wish to do it proper, also run some radial wire from the flat copper strap to a couple of the radials that run nearby the feed box. If you solder ground connections that will be UNDERGROUND, get a propane torch and use the plumbing solder as it will hold up longer than the 60/40 electronic solder underground. Now you have a good grounded tower and feed point!

5. Next, you will need your shunt feed…. Remember the point at the top of the tower where the tracer wire or the coax shield bonded. That should be right at the rotor or at the top of the top tower section. Run a wire (12 or 14 ga.) from there down at the angle away from the tower to the feed box. This is the wire you will feed this UNIPOLE or MONOPOLE Vertical; or do you want to picture this as a "half of a folded dipole" standing on end.

6. Now to match and feed. The easy way for 100 Watt operation is to place inside the weather proof box an automatic antenna tuner. The feed will be the "single wire" output of the tuner, and the chassis of the tuner goes to that copper strap ground, and the 12 vdc operating voltage and control and coax lines go to your shack.

7. If you have problems matching on 160 or sometimes 80, you may have to add a coil of 10-15 turns at 3 inch diameter in series with your feed wire. Use at least 12 ga wire for that coil. But lets hope that the auto-tuner can get down to the low impedance that 160 will present. You will have to tap that coil to see just how little of the coil you can get away with, as less is better. Also, this coil may have to be switchable in and out for use of the upper bands above 80. If your tower is 60 feet or more with top loading, you probably won't need that coil.

8. If you want to build your own tuner, it can be done, but I will warn you, this kind of matching project with all the complexities for multiband operation is not for the beginner or the faint of heart. It will keep you busy and is the kind of situation, which I cannot even begin to give values for parts for your configuration. The impedances exhibited by this kind of system will vary GREATLY from almost identical installations, and there is no way to duplicate a tuner for two sites. Just plain way too many variables. I built my own tuner, but my occupation is broadcast engineering and I have a big advantage over the average ham on this kind of thing.

You can see my system at my friend Steve's site: http://earthsignals.com/N6TZ

Good Luck, Hal, N6TZ n6tz@arrl.net

Member Comments:
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Methods to Shunt Feed a Tower  
by G3RZP on April 18, 2005 Mail this to a friend!
I have shunt fed the crank up tower very successfully. But I don't use radials! I tried radials: adding radials one at a time while measuring the feedpoint impedance made no discernible difference, and I could not measure any current in them, either. My tower is grounded through four 4 foot long ground rods, but this is definitely case where your mileage will (not even may!) vary. The sub soil here (starting about 6 inches down) is thick, heavy, wet blue clay, however.

I got better bandwidth by using a wide 'feedwire' up the tower, consisting of three 16 gauge wires spaced about 8 inches apart, using copper water pipe as spreaders. This looks like a large conductor, RF wise. It certainly proved easier than omega or gamma matching, both of which are pretty narrowband for a non resonant antenna.

The installation originally consisted of a 5 element monoband Yagi for 20 at 62 feet, and interlaced 4 element monobanders for 10 and 15 at 68 feet. This provided enough top loading that the resonant frequency was below 3 MHz and the matching could be achieved with an L network with a motor driven capacitor on 80, and was flat across 160 with an L network. However, I've just changed to a 4 element Steppir, so the amount of top loading is very different, and the resonance has moved to about 3.7MHz. This has led to a rebuild (not yet finished) of the matching network to have a L network with both variable C and L. Because of the way the values vary, there has to be some elctronics to relate the values of the C and the L - I don't have enough wires to control them separately, and anyway, my automatic tuning system won't handle that. So the position of one of them (derived from the potentiometer mounted on the component) will control the position of the other, using an A-D converter and a suitably programmed ROM, DAC and servo system.

Complicated! But then, I enjoy home brew, and have a lot of test equipment as well as a well equipped workshop with lathe and mill etc.

How does it work? Fine. It's not a local contest winner on 160 - it's a DX antenna. Similarly on 80. It's worked such niceties as Heard Island on 160, 3B9C on 160 and 80, and run JA pile ups on 160. Back in 1986 or 7, conditions in the ARRL 160 contest were worse than lousy, but it got me the top world wide single op score outside the US.

It's important to bond those coaxes top and bottom. I have a side arm attached to the tower: all the coaxes are terminated in bulkhead connectors on this side arm, so they connect well at the top, and at the bottom, I have a 6 by 8 foot shed with my home brew automatic antenna switching and tuners in it - the 'antenna tuner shed'. The coax outers get bonded back to the tower through 6 inch wide aluminium flashing (from the Sacramento branch of Home Depot, believe it or not!). Incidentally, use a leader rope to support the weight of the cables coming down the tower.

I had a problem early on with the direction indicating potentiometer in the T2X rotator burning up. I think this was caused by the large RF current through the capacity hat formed by the beams. I got around this by putting bypass capacitors - 0.1 microfarad 50 volt - from each side of the pot to ground, and putting a clamp on the stub mast and several wide braid straps to connect the stub mast to the tower.

Shunt feeding the tower as folded monopole has a lot going for it. The feed impedance is higher than an insulated tower, which can help reduce losses, although the radiation resistance is not higher than the insulated tower. But note that the high feed impedance can mean lots of volts on the feed wire - like several kilovolts! Capacitors in the L network really need to be 5kV rating, and any fixed caps need to be rated for a lot of RF current.


Good luck with feeding the tower.

73

Peter G3RZP
 
Methods to Shunt Feed a Tower  
by WA2JJH on April 18, 2005 Mail this to a friend!
Nice technical piece. Wish I had the room on my Manhattan roof.
 
RE: Methods to Shunt Feed a Tower  
by KA4KOE on April 18, 2005 Mail this to a friend!
Easy to do mike. Cut a triangular hole on the roof and feed that Rohn 45 all the way to ground level.
 
RE: Methods to Shunt Feed a Tower  
by K5UJ on April 18, 2005 Mail this to a friend!
<<<At the base of the tower, bond by soldering them together and run a short lead from them to the bonding point on the tower.>>>

I would use copper strap here, at least 2" but preferably 3 or 4 inches wide.

The ARRL Antenna handbook has a CD in the back that contains TLW, a software program that among other things, helps model matching networks. Once you get your tower all set up with the shunt feeding, all you have to do is sweep it with a swr analyzer, noting the R, X and associated frequencies then plug these into TLW and it will give you an idea for different circuits, efficiencies, and values of C and L and currents they will be expected to handle for your expected power to the network.
 
RE: Methods to Shunt Feed a Tower  
by KA0GKT on April 18, 2005 Mail this to a friend!
At one time I had 40' of Rohn 25G guyed on the roof of a down town building. I tried a shunt feed with a few wire radials strung across the roof, but received poor results for my efforts.

Later, the building owner installed a standing seam metallic roof. The roofers were very nice and had a tower crew jack the tower and then reset it on the metal roof, they adjusted guys and plumbed the tower at the same time. I then tried the shunt feed again and voila! It loaded up beautifully and seemed to radiate reasonably.

I tried to figure a way to put an 18-HT up there but couldn't come up with a way that didn't void the roof's warranty.

So, a rooftop 25G is well within the realm of possibility...at least in small town Nebraska (pop. 30,000). Now I'm in metro Tucson and the CC&Rs won't allow me to have anything more than 8' above my rooftop. >>SIGH<< at least I still have the FM/TV transmitter site at 8550' AMSL

73 DE KA0GKT/7
 
RE: Methods to Shunt Feed a Tower  
by NB3O on April 18, 2005 Mail this to a friend!
I've shunt fed a few different towers and find using three or four elevated "resonant" radials to work reasonably well. In areas where the property line did not allow a radial length of 140 feet for 160 meters, I used a loading coil to tune the radial, similar to loading a dipole. While the tower system has little less bandwidth, it gives a noticeably lower take-off angle (better DX performance) than using the 160 meter dipole at 130 feet.
In some experiments, I used multiple radials cut for other bands to work with the shunt fed tower tuning system on 160, 80, and 40 meters (configured similar to a fan dipole). Using 14 gauge aluminum fence wire (with some de-ox grease) kept the sag out of the longer radials and required only a ten foot 4 by 4 post sunk in the ground 2 feet. A lot easier than burying a dozen or more radials in the ole' shale pit around here.
 
RE: Methods to Shunt Feed a Tower  
by K0BG on April 18, 2005 Mail this to a friend!
Peter G3RZP, adding radials one at a time and checking for intermediate results is an act of futility. Adding 20 at a time is more like it. How you're measuring the input impedance is also important.

There has been volumes written about the length, number, and configuration of radials, and some of it in the RSGB, as well as QST, and a myriad of technical papers. As Hal points out, it takes a lot of them. If you think your antenna works well now, add 100 radials and see what happens.

Alan, KØBG
www.k0bg.com
 
RE: Methods to Shunt Feed a Tower  
by KF4VGX on April 18, 2005 Mail this to a friend!
When I get more time I will look into this further and give her a try .
Now thats how to get a hams curiosity up .
Very interesting read.

73

 
RE: Methods to Shunt Feed a Tower  
by K9KJM on April 18, 2005 Mail this to a friend!
GREAT Article!
A few things I have learned over the years, The copper strap can be obtained from upscale roofers (Copper roof flashing) sometimes from the dumpster where they have made mistakes cutting for a few 807s.... Or usually even brand new strap for less than the usual sources.....
Do NOT use lead/tin solder outdoors/underground. Even plumbers solder is not the best. You can get "Silfoss"
a true Silver Solder from welding shops for about a buck a stick. This makes a joint almost as good as Cadwelding.
MAPP gas (Only about 5 bucks a cylinder) will provide enough heat to properly flow the silver solder. (Use in a standard propane type torch)
 
RE: Methods to Shunt Feed a Tower  
by G3RZP on April 19, 2005 Mail this to a friend!
I tried as many as ten radials - no change. Haven't got the room for hundreds of 'em. Measured the impedance with a professional Admittance Bridge.

Problem now is that the reduced top loading has the thing resonant in the middle of 80m. The range of adjustment on the matching components in the tuner isn't enough. New tuner to be built! There's a 30 microhenry roller coaster out of a wartime ET4336 that just needs a motor and position indicating pot fitting: not sure if I have a motorised 250pF 3kV variable spare, though.
 
RE: Methods to Shunt Feed a Tower  
by K3UOD on April 19, 2005 Mail this to a friend!
I've melted a standard propane tourch using MAPP gas. Spend the money for a real MAPP gas torch. It's worth it.
 
Methods to Shunt Feed a Tower  
by WA7NDD on April 19, 2005 Mail this to a friend!
Very worthwhile reading and educational. I have looked for a clear, practical, and successful how-to on shunt feeding a tower. I am installing a tower right now and will included this with all my other planed grounding. Thank you Hal, for your E-mail help. This type of information from all the articles presented here is the reason I am a paid member of eHam.net. You just don't see it in the ham magazines any more.
Thank You.
Jim, WA7NDD
 
RE: Methods to Shunt Feed a Tower  
by KG5JJ on April 19, 2005 Mail this to a friend!
Awww...heck! Just put a Step MonstIR beam at the top, and shunt feed the tower for 160-75!

Nice to have a tunable capacity hat at the top, no? ;-}

73 KG5JJ (Mike)
 
Methods to Shunt Feed a Tower  
by N6TZ on April 20, 2005 Mail this to a friend!
I have received several inquires re: more details on shunt feeding a tower. I will address the most common questions.

1. When opening the jacket of a coax cable to solder a ground lead to it, the re-sealing is a very critical step. Any water inside a coax will ruin the cable. My favorite way to seal it is as follows: first open as little of the jacket only on one side. Use a hot iron and some flux and get in and get out quickly. Then slip two pieces of about 4 and 6 inch long heat shrink over the jacket. The short piece is used first. Coat the opening and area around the opening with silicon seal and slip the heat shrink over it and the ground lead and shrink it down. Then coat the ends of that piece with silicon seal and slip the 6 inch over it and shrink down. When shrinking, start in the middle and work towards the ends.

2. When bonding to the tower base, use only a stainless steel bolt, nut and flat washers. DO NOT USE STAR washers, they are trouble in the long run. clean the tower galvanize with sandpaper or wire brush and use a little No-Ox. It is available at Home Depot.

3. The efficiency of a shunt unipole is maybe only slightly different than a regular vertical. This is only because of a possible easier feed point impedance, NOT because of improved radiation resistance. I refer you to this site for a very good discussion on that subject: http://www.w8ji.com/radiation_resistance.htm

4. Guy wires should be broken into lengths not exceeding about 1/8 wavelength of the highest frequence you are going to use the unipole. In my case, I use mine on 160 through 30 meters, so my lengths are about 12 or so feet. MFJ sells some really neat porcelin egg insulators.

5. Keep your feed point away from the base of the tower by about 15 to 20% of the height of the tower. My feed point is about 16 feet from the base. The feed wire slopes directly from the top of the tower to that feedpoint.

Hal, N6TZ
 
RE: Methods to Shunt Feed a Tower  
by G3RZP on April 21, 2005 Mail this to a friend!
Hal said:

Keep your feed point away from the base of the tower by about 15 to 20% of the height of the tower. My feed point is about 16 feet from the base. The feed wire slopes directly from the top of the tower to that feedpoint.


That's interesting - my feedpoint is only 3 feet away from the base of the tower. And it works.

Whatever, Hal's right about this being a good antenna. But as I said earlier, at least in my case, it's a DX antenna, not a local working one.

I haven't yet found out if the Steppir will have aproblem with the capacitive current. I have put lots of ferrite sleeves on the Steppir motor boxes, and on the control cable at the base of tower - the screen of the control cable is, of course, bonded back to the tower ground.

For connecting to coax, I've found that a wrap of plumbers Teflon tape, followed by Coax Seal and then overlay with heatshrink, works well. Coax seal is messy, which is why the Teflon tape.

73

Peter G3RZP
 
RE: Methods to Shunt Feed a Tower  
by WA6BFH on April 21, 2005 Mail this to a friend!
Uh guys, read some of the original posts from my artticle (between me and W6TH) that brought this up!

Think Gamma Match, -- analog ---- adjustible!
 
RE: Methods to Shunt Feed a Tower  
by N4BAA on April 22, 2005 Mail this to a friend!
Even better....

I have a motorized Vacuum Variable Cap that is switchable between two shunt wires....one for 80/75 and one for 160....

Tower is 60 feet tall with 10 feet of mast out the top...

Toploading is provided by:

FORCE 12 4BA at 70 feet
7 ele 6 meter yagi on 30 foot boom at 65 feet and
5 ele 20 meter yagi on 42 foot boom at 60 feet..

Put 60 radials out....good results....
But when 120 were burried...OMG...like something magic happened....and able to instantly tune it with the morotirzed Vaccum variable Cap..is a dream....

Have fun!..

Jose Castillo - N4BAA
 
Methods to Shunt Feed a Tower  
by KI4AGL on April 22, 2005 Mail this to a friend!
Any suggestions on how to feed a lighting
tower in a typical suburban shopping center from
a car with a HF rig parked next to the tower?
 
RE: Methods to Shunt Feed a Tower  
by N6TZ on April 23, 2005 Mail this to a friend!
For KI4AGL

I have given thought to the same idea of loading many tall metal objects from water tanks to flag poles. Some of these things would be great on 160 etc. However, I would not recommend loading a Light Post in a parking post. A couple of reasons:

1. On receive you may find the noise from the lighting to be so bad you cannot hear anything.

2. You would need a ground radial system on it, and may cause damage to the light fixture when attaching the ground system to the base.

3. Power levels of 100 watts or less would probably not harm anything, but you never know.

It may be better to string a dipole between two of the lighting posts.

Hal
 
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