A Lowband Antenna that is Almost Free

The Shunt-fed Tower, An Effective Low Band Antenna, uses your beam as a capacitive top-hat and only needs a simple feed network and a good ground system to work DX on 80M and 160M.

Do you have a beam on a tower? If the answer is yes, the tower can serve as a vertical on the low bands (80M and 160M). I put up a 50 foot Glen Martin tower last year with a Mosley Pro 67-C beam (44 foot elements on a 24 foot boom) at 55 feet. It has worked very well on the bands it was designed for (10M-40M) and also on 80M and 160M by shunt-feeding the tower with the beam acting as a top- hat.

I have had no trouble achieving WAC on 80 and 160 with this antenna and am adding to my country total in pursuit of 160M DXCC. Shunt-feeding a tower involves running some copper wire parallel to the tower which is connected to the tower at the feed point near the top and insulated at the bottom. The simplest method of feeding the shunt wire is with a series capacitor (Gamma match) as shown above. High voltages may exist across the capacitor so a vacuum variable is recommended. In some cases, it may be necessary to use an Omega match which is simply a two capacitor network with the second capacitor connected from the shunt wire to ground.

One cautionary note. If the tower is short, say 30 feet, some inductance will probably be required to resonate it on 160M. This can complicate things because if the inductance is added at the bottom, the tower has to be insulated from ground and the inductor connected from the bottom of the tower to ground. If the inductance is added between the top of the tower and the top hat (beam), the beam has to be insulated. While this is feasible for a small beam, I wouldn't want to try it with a big one.

A vertical with the combination of capacitive top loading and shunt feeding has two desirable attributes: 1. The tower can be grounded, minimizing possible lightning problems. 2. The capacitively loaded vertical has a desirable antenna current distribution. Compared to inductive loading, capacitive loading increases the radiation resistance of the antenna which is very important since the radiated power (PR) is given by PR = I2RR where I is the antenna current and RR is the radiation resistance. Of course, there is no substitute for height and a 1/4 wave Vertical remains the standard by which other antennas are judged.

The radiation pattern of a short vertical in the elevation plane is nearly the same as that from a quarter wave vertical. This translates into long skip distances when working DX. The efficiency of the antenna is given by E = RR /(RR +RL) where RL represents the losses, primarily ground losses. It is obviously desirable to maximize RR and minimize RL . This is where the radials come in. More radials lower RL and result in higher efficiency. You can't install too many radials although after 100 you are definitely into the law of diminishing returns. The following table compares the radiation resistance and efficiency of three vertical antennas on 160M, assuming RL = 5 Ohms.

1/4 wave Vertical (130 feet high) 1/8 wave Top (Capacitively) loaded vertical (65 feet high) with 1/8 wave top hat 1/8 wave Bottom (Inductively) loaded vertical (65 feet high) with 1/8 wave inductance
RR =36.6 Ohms RR =18.3 Ohms RR =6.3 Ohms
E = 88 % E =78.5 % E = 56 %

For higher RL, things only get worse. References 1 and 2 give some very useful information for someone planning such an antenna, including extensive computer modeling in reference 1.

My beam is at 55 feet and has an effective length of 76o on 160M, assuming that the top hat roughly doubles the physical length. The measured SWR bandwidth is given in the following table. If this isn't adequate, it may easily be adjusted with the motor-driven vacuum capacitor.

2.0 @1800 Hz 1.0 @ 1825 Hz 2.0 @ 1852 Hz
2.0 @ 3480 Hz 1.5 @ 3520 Hz 2.0 @ 3555 Hz

Construction Details:

The tower sections need to have good electrical contact between them. I used a short braided jumper between each section as a precaution. I also ran a braided jumper between the connection point at the top of the tower and the mast. This needs to be long enough to allow the mast to rotate. The coax to the beam and rotator wiring should be routed inside the tower or at least, on the side opposite to that with the shunt feed wires. Using a RF and transient suppressor box on the rotor wires is a good idea, as is burying the coax for the shunt feed between the tower and the shack and coiling 10 turns of the coax where it enters the shack.


Radials are the key to getting out with a vertical. My homesite is heavily wooded with only a couple of open spaces with no trees. In fact, I had to remove 4 trees to put up my tower. I presently have 48 buried radials of various lengths. The radials are 14 gauge insulated solid copper wire and are only about 35 feet long for roughly 180o because of the proximity of the house to the tower while the remainder are 130 feet in length. A gas powered lawn edger is an easy way to cut slots in the grass a couple inches deep. If you don't have one, they are available at most rental shops. I made a burying tool by cutting an 8 inch diameter disk out of 1/4 inch plywood and grinding a C-shaped groove around the periphery. Then, a bolt and nut act as an axle to let the disk rotate on the handle. Burying the radials was just a matter of my XYL feeding the wire from the spool while I pushed the wire in. I didn't bury the radials in the wooded areas. Instead, I used J-hooks (shown lying on handle of burying tool in photo) to hold them to the ground until nature had time to bury them. The J-hooks are available from any building supply store and are intended to be used for attaching chain link fencing to its supports. The radials should be connected to the tower base and to several ground rods. Menards has some heavily plated ground rods which are reasonably priced. Don't bother with the types which only have a flash coat of copper, they will be covered with rust and useless in a short time.

Shunt Wires:

The spacing of the shunt wires from the antenna affects the impedance seen at the feed point. Mine are 15 inches from the tower at the top and 60 inches at the bottom. An easy way to fine tune the antenna is to vary the spacing at the bottom until a SWR of 1.0 is achieved. I should mention that an impedance/SWR analyzer is almost a must in tuning and makes the job a quick one. The connection point of the feed wires near the top of the tower also affects the impedance. Depending on your patience, this is another variable to experiment with. A cage made with several wires will increase the bandwidth somewhat. I'm using three 14 gauge stranded copper wires in a triangle configuration with one of the flat sides of the triangle parallel to one of the tower faces. The wires are spaced 4 inches apart.Turnbuckles at the bottom are used to adjust the wire tension.

Tuning Network:

Tuning Network in Protective Enclosure

Tuning the Antenna:

Initial tuning is best done at the antenna so changes may easily be made. Start with the relay contacts open and tune C1 for lowest SWR using an RF analyzer. Then switch in C2 and see if the SWR can be lowered further. If not, try varying the distance from the tower of the shunt wires at the bottom. A compromise shunt wire spacing may be needed to accommodate both 80 and 160M.

Parts Sources:

Vacuum variable capacitors are available (attractively priced) from Allen Bond, WB4GNT, 770 973 625, www.msg4u.com and from Surplus Sales of Nebraska, 402 346 3750, www.surplussales.com.

The relay is from Surplus Sales and is P/N (KO) RNF100-DP.

The drive motors are from Digi-Key, 800 344 4539, www.digikey.com, P/N CRA201-ND.

The enclosure is from Newark, 800 463 9275 www.newark.com, P/N 95F 2774.

The Rotor RF and Transient Bypass unit is from ICE, 800 423 2666, model 348.


1. Antennas and Techniques for Low Band DXing, John Devoldere, ARRL, 1994.

2. DXING on the Edge, Jeff Briggs, ARRL, 1997.

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Last Updated April 3, 2000 by Robert Bicking