How to turn your boom into an effective antenna for 40 or 30 meters.

I have tried several different antennas for the 40 meter amateur band, usually with less-than-optimum results. After trying different dipoles, wire arrays, and verticals, I have found that loading the boom of my high tribander gives me by far the best bang for the buck. For the cost of a feedline and a few capacitors, I have a rotatable dipole at 99 feet which outperforms all other 40 meter antennas I've tried here.

Matching the boom of a large tribander for 40 meters was not my idea. I got the idea from AA4NU, who got it from N4KG. Tom's description of the theory can be found at the following link: http://lists.contesting.com/archives//html/Towertalk/2002-03/msg00731.html
My first attempt used capacitors which were too small. The enclosure was also not suited to the task.

For the new unit, I chose to stick with air variable capacitors for ease of tuning. Hopefully the new caps and better enclosure will last for a few contest seasons. The bandwidth is about 110 kHz. This is more than enough to cover the 40 meter CW and RTTY band.

Note that this technique can only be used for antennas which have their end elements (reflector and final director) grounded to the boom. Antennas with insulated elements would need to be modified by shorting the end elements to the boom somehow.

Following are a few pictures of my attempts at building an effective
omega match for my top PRO-57A antenna.

Give this a try. I think you'll be impressed with the results!


The old unit was poorly designed, and became corroded and weather-beaten quickly. The spacing of these capacitors was too small for operating at the 1 kW power level. The enclosure lost its lid after 6 months from UV rays. The mounting screw broke a hole in the left side of the enclosure after it also became brittle from UV and WX.


This old unit has seen better days...


The new unit has significantly larger capacitors.


A closeup of one of the capacitors fully meshed.
I've been collecting these at hamfests for the last few years.


The insulated knobs prevent hand capacitance and shocks at 100 feet while tuning.


The enclosure is a children's lunch box made by Aladin. I left it outside for 8 months to see if it became brittle from UV exposure. So far (3 years now) the box is holding up. The sturdy hinge on the back will also facilitate much easier maintanance than the old removable lid design. The box is mounted to the mast using a spare Mosley boom-to-mast clamp which was left over from an earlier project.


Here is the ATU mounted to the mast.


The finished product, flying high.

This is just one way to tune up your boom on 40 meters. I like the ease of tuning and flexibility of the air variables. It can be tuned for 30 or 40 meters, and can accomodate any possible future allocations. You can also make fixed capacitors out of coax, as N4KG recommends.

Tune your boom! I think you'll like the results.

A big thanks to N4KG for the concept, and to AA4NU for telling me about it.



Return to the K4RO Station Tour

Return to the Tennessee Contest Group Home Page