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 couldn’t 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 didn’t 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.
Here’s 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 didn’t take any pictures [sri]. The holes on the right
(on the left in the picture above) attach to two of the beam’s 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.
I’ve 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 .75” 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. It’s at 55’ with the tower extended.
It’s a length of PVC tubing with a 2x2 inside. The first time I tried this I
didn’t 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 – 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.