On Wed,9/28/2016 5:54 AM, Ed Sawyer wrote:
I would advocate "going clean" on the worst offender scenario and have pure
coax going from the amp to the antenna.
Yes. I had more crosstalk between stations than I thought I should, and
SWR was higher at different points in the station than made sense. I
bought a big spool of BuryFlex and 100 or so 83-1SP connectors and
replaced every piece of coax inside my shack, starting with my entry
panel. I also replaced a Daiwa power meter with N8LP's LP100A, and got
rid of sampling points to look at harmonics on a spectrum analyzer. And
I replaced a Six Pak with 4O3A's 6x2 (and a year ago,with his new 8x2).
I'm using several Top Ten relay boxes (some 6x2s to switch stub matching
networks for my 80 and 40 dipoles, and some 2x1s to trade several
antennas between the two radios). Taking apart one of the 2x1s, I
learned that they fail to provide a return path for RF current on the
circuit board -- the only return path is the chassis -- so SWR is a mess
at 10M and 6M, and while I didn't measure it, that sort of thing also
can increase crosstalk. I rebuilt those boxes to add a return path
paralleling the center conductor path, and the SWR through the box
dropped by more than half. The failure to provide a return path is all
too common in HF antenna switching systems and antennas. It's present in
the big Ten Tec tuners (229, 238), and in several expensive high power
antenna tuners I inspected in booths at Dayton a few years ago. When I
pointed it out to the engineer who designed them, he didn't understand
the problem. This can also be critical in filters. 5B4AGN has done an
excellent job with his design of the TXBPF filter sets for which he's
arranged several group purchases.
Ten Tec did it right in the power amps that I own (Titan 425 and Herc
II) -- there's coax from both input and output to the vacuum relay.
By the way, I don't need stubs - I have found out. Just Bandpass filters
and antenna separation. 1.5kW on all bands and SO2R.
You must have a LOT of separation. Where I think most of us run into
trouble is between 80, 40, and 20 CW, because the harmonics end up where
we're likely to want to operate on the higher band.
73, Jim K9YC
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