[TowerTalk] Omega Matches

[Kersus, Gerald J (Gerry), NNAD]

Inspired by N4KG's, W4UP's and K4RO's results with loading their tri-bander
booms on 40 meters, I decided to try loading my KT-34XA boom.  I've
duplicated W4UP's solid mechanical design for the Omega rod assembly.  I
used two 150 pF Hammerlund variable caps for testing purposes.  (I plan on
making the final caps out of RG-213.)  After shorting the reflector and
front director to the boom, the best SWR results were less than spectacular
(3:1 at 7.05 MHz).  So, I added about 4 feet of tubing to each end.  The
results were dramatically better (1.7:1 at 7.05 and 1:1 at 7.24 MHz).  The
"series" Omega cap is about fully meshed (reading is 162 pF) and the "shunt"
Omega cap is nearly fully open (reading is 23 pF).  While the SWR is just
below 2:1 at 7.00 MHz, I'd like to try to make it a little lower. The
matching rod is about 98 inches long and separated from the boom by 8
inches. 

N4KG just told me the rod is probably too long.  I'll make it shorter this
weekend.  I've been looking for some guidelines for the length.  ON4UN
recommends the Omega rod be 66% to 75% of the Gamma rod length.  I found
some gamma match software from K4VX (circa 1990) that indicates the Gamma
rod for my setup would be about 99 inches long. It would be convenient to
make it 72 inches long, but I don't know if that's now long enough.  ELNEC
indicates the source impedance to be about 40 -j10.  (ELNEC resonance and
2:1 SWR bandwidths are pretty close to those actually measured!)

Any ideas or pointers on the Omega match?

Tnx,
Gerry, W1GD

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