[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

On Tue,9/8/2015 8:23 PM, Jeff Clarke wrote:
> Wondering how those of you using a Six Pack antenna switch hookup your stubs? I have two coax cables from the Six Pack coming into the shack going to each radio. Each station has a multi band Dunestar switchable filter. For the most part the Dunestar works well but since I recently added an KPA500 to my run station, when I'm on 40 meters with the amp it gets into 20 meters on station #2 pretty bad.

Hi Jeff,

I have exactly that problem with a 40M wire Yagi whose feedpoint is a 
few feet below my 3-el SteppIR.  I don't switch stubs -- instead, I use 
stubs only on monoband antennas, installed between the 6x2 and the 
antenna, and carefully located according to this applications note

http://k9yc.com/LocatingStubs.pdf

The executive summary is that 1) stubs are most effective if placed at a 
high voltage point along the line at the harmonic frequency; 2) stubs 
can reduce the effectiveness of the harmonic filtering in the power amp 
if the line between them and the power amp is "wrong" for that power 
amp's output network; and 3) the app note gives a step by step method to 
get both #1 and #2 right.

Also, #1 applies to monoband antennas, where the antenna is severely 
mismatched to the line. It does NOT apply to antennas that are resonant 
on the harmonic, so for these antennas, stub placement matters only for #2.

> Are you all using a T connector and have the stubs for each band on a manual multi position antenna switch?

Remember that the switch plus the length of coax between the Tee and the 
switch are part of the stub, so they must be in place when the stub is 
tuned to length. Also, because Vf varies a bit with frequency, the 
harmonic suppression stubs should always be tuned at the harmonic 
frequency. This matters most below 20M.

> Also does anyone know if the Dunestar's are tuned for a specific part of the band. On 80 meters mine seems to have a very high SWR when I switch it in even though the SWR on the KPA500 looks good.

Several years ago, I measured all the bandpass filter sets I could 
borrow from NCCC members, as well as some I owned myself.  My report on 
that work was in NCJ.  Here's the internet version.

http://k9yc.com/BandpassFilterSurvey.pdf

The Dunestars are fairly narrow, and don't cover all of the wider (in 
percent bandwidth) ham bands.  Measured response curves for all the 
filters tested can be downloaded from a link in the pdf.

My measurements were done with the VNWA Vector Network Analyzer, 
designed by German prof DG8SAQ, which is built and sold by a ham in the 
UK. It's an excellent unit, and I paid just over $700 with shipping to 
my door.  Price in dollars varies a bit with currency. If you're going 
to work on your filter set you'll need one of these or something like 
it, and it also does the measurements needed to optimize stub performance.

I suspect that your 80M filter may be out of alignment or have a fried 
capacitor. I repaired a 10M Dunestar several years ago by replacing a 
fried cap. Dunestar gave the owner good service on the replacement part. 
Replacing any part in one of these filters is likely to require at least 
minor realignment.

73, Jim K9YC