Tribander 40 Meter Boom To Dipole Conversion by K4BEV

 

N4KG has presented an interesting method of converting a TH-7DX or other tribander with a 24 foot boom and grounded parasitic elements to a 40 meter rotatable dipole. Tom uses an Omega match network to resonate his boom to 40 meters and speculates that another Omega system could be added to the boom to cover 30 meters as well.

The Omega match looks like this schematically:

I built one of these, mounted it to the TH-7 and then tuned it with my tower cranked down and the antenna at about 28 feet. When cranking the tower back up and the antenna at 63 feet it would not resonate in the 40 meter band. I could go on either side of 40 meters, but couldnít hit it. Each adjustment meant cranking the tower down, attaching a ladder, making an adjustment, removing the ladder, and cranking the tower back up. It didnít take too long to abandon this tuning method, and the antenna too.

 

One day while working on something in the shed I noticed 2 old, dot matrix printers just sitting there. As soon as I saw them I knew the antenna could be fixed with their stepper motors.

I ripped the motors out of the printers and made one of these:

 

The capacitors are fairly small, but my max power is 150 Watts and no arcing has been observed as yet. Higher power would certainly demand larger capacitors.

 

I have an Allen-Bradley SLC-500 (an industrial controller with i/o). It took a few minutes to have the motors turning the capacitors through 200 feet of #22 wire. The stepper motors run on 6 VDC.

 

Hereís a shot of the motorized caps in their box. The box is a plastic food storage container totally covered with Scotch 33+ for UV protection.

The arm connects to the lug on the left of the picture, and the control wires connect to the terminal strip on the right. The couplings between the motors and the caps had to be changed out to a plastic material, but I didnít take any pictures [sri]. The holes on the right (on the left in the picture above) attach to two of the beamís mast attachment bolts.

The motors are mounted to some plastic pieces removed from the printers and secured to the board with glue and to the capacitors with aluminum wire.

The motors turn at 1.8 degree per step, which is more than fine enough resolution.

The antenna now covers both the 30 and 40 meter bands with a 1:1 SWR on either. The caps are tuned from the shack when I want to change bands. It also can be matched to 1:1 on 6 meters.

Iíve purchased a PIC module, but have not incorporated it as of yet. I plan to set up a controller with the PIC that will tune this antenna and a 160 meter inverted L as well. The choke is for the 40 meter antenna.

 

The arm is a .75Ē aluminum tube that was part of a motor home roof rack at one time. It was already bent and looks good up there. The arm is slanted so that water runs out of it. I used a stainless steel tube clamp to attach the arm to the boom, and stainless steel hardware throughout. The boom and TH-7 are fed with separate coaxes. I get severe RF in the shack when using this antenna at powers above about 20 Watts. I get very good signal reports on 40 at 10 Watts though!

The white plastic tube at the top of the tower is used for wire antennas. Itís at 55í with the tower extended. Itís a length of PVC tubing with a 2x2 inside. The first time I tried this I didnít reinforce the tube and it was shaped like a U in a few months. This has been in the air for 3 years now and is still solid. A pulley is mounted at either end to make getting the wires up pretty easy.

 

73,

Don Burke Ė K4BEV

Editor's Note:
Due to the spacing of the capacitors used, Don's remotely-tuned antenna is probably best used at lower power levels (100 watts.) K4RO has built a manually-tuned high-power version of the 40 meter boom dipole. To view it's construction, click here.