Methods to Shunt Feed a Tower

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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

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