Simply trying a few SM's is a shotgun approach. Start off with a mica
compression trimmer to ground and adjust for the best match which should
be very close to 1.1:1. Then measure the value and substitute the
closest fixed cap.
Having a perfect match is rather meaningless in the long run if your
exciter delivers the proper power in bypass. Even a slight power drop
wont wiggle someones S meter any different.
Carl
KM1H
----- Original Message -----
From: "David Craig" <davidhcraig@verizon.net>
To: <amps@contesting.com>
Sent: Saturday, January 17, 2009 1:49 AM
Subject: [Amps] Bypass SWR RJ1As
> Hello All,
>
> Looking for input/advice regarding "tuning" the relays of my latest
> homebrew GS35b 50 MHz amp. Using a pair of RJ1As (used but hipotted
> good, I presume MRI pulls) for RF switching. In other amps I have
> been able to "tune" out slight "thru" SWR issues (driver thru, amp in
> standby) using silver micas to ground. Projects that worked fine for
> me were K5AND's Schrack relay board using a 10pf to ground, an SB220
> using ~68pf to ground...
> This is my first time trying vacuum relays & it "appears" I have
> capacititive reactance, I think? With just RG214 to the relays & RF
> in/out connectors, I get a whopping 3 to 1 SWR with amp in bypass. I
> tried various silver mica caps, and low value trimmers to ground from
> the NC to NC coax connection- lowest bypass SWR I can get is 1.3-1.4
> to 1 with a single 47pf silver mica to ground. This is usfficient to
> allow the driver to put out 100w with the amp in bypass mode but isn't
> "right." I'd much prefer a flat SWR.
> I am a bit fearful of trying a small inductor to ground, don't want
> to hit a resonance at 6m & burn out my exciter. I can't find any RF
> impedance data on the RJ1A but assume that it must be a fair bit off
> 50 ohms at RF, or maybe I have a bad relay? Anyone have this problem,
> and if so how did you solve it? Is is safe to use a parallel inductor
> to ground with my driver so long it tests OK at 50MHz on my MFJ 259?
> My relay commons are fed to RF in & out & NC leads connected to
> each other with about 2.25" of shielded/grounded RG214 between the
> relay posts. Both relays are in the cathode pressure compartment, not
> in a shielded box (which doesn't present a problem with amp RF out).
> Have also (of course) the cathode choke & separate bias choke in the
> compartment, along with a sefl-designed pi-input tuning network. The
> screwed-up planning I did on the cabinet makes it difficult to
> increase the NC to NC cable length, but could that help? I'd am
> curious if this is a common issue with RJ1As. Any input appreciated.
>
> Thanks & 73
> Dave N3DB
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