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
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|