[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

On Mon,9/7/2015 6:07 PM, Mike Smith VE9AA wrote:
> This is about coax stubs.

Mike double-posted this to another reflector, where I referred him to a 
piece I wrote for the latest NCJ.

http://k9yc.com/LocatingStubs.pdf

Advice already posted here about looking for leakage and harmonic 
generation elsewhere in the station is also spot on. I have a 2-el 40M 
wire dipole with the driven element a few feet below my SteppIR Yagi. No 
surprise that the 2nd harmonic wiped out 20M. The excellent 5B4AGN 
filter set prevents overload, but can't suppress harmonics from the 
power amp. :)

I added a double stub to the 40M feedline as described in the NCJ piece, 
and while it made a big dent, it wasn't as much as the model showed, so 
I started poking around my antenna switching looking for leakage and 
crosstalk. I replaced the Array Solutions Six Pak with a 4O3A 6x2, which 
reduced the crosstalk significantly. On the bench, the Six Pak measured 
54 dB isolation on 20M, the 4O3A was about 74 dB depending on where you 
measure (the 4O3A box fails to carry the coax shield through the circuit 
board, depending on the chassis for return). While I didn't measure 
before/after at the 20M RX, I don't think I'm hearing that additional 20 
dB, so I've got some more work to do.

Note that I do NOT measure with other outputs terminated with 50 ohms, 
because monoband antennas are only a matched load on the band for which 
they are resonant.

73, Jim K9YC