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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