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Re: [TowerTalk] station to station interference

To: towertalk@contesting.com
Subject: Re: [TowerTalk] station to station interference
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 28 Sep 2016 10:07:25 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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