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:
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.
References:
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