I made some measurements of my homebrew amp output tank and RFC this
weekend that may be of passing interest (OTs can ignore!).
Following a suggestion by (I believe) Tom W8JI I made a phantom tube using
an RC combination and put it in place where the tubes normally would go (in
this case, 1800 ohms in parallel with 10pF...roughly 2 3-500z at 2500V
plate). I happen to own a full-blown HF network analyzer (GR 1710
system...but that's another story) so I put the RF into the goesouta and
measured S11 at the various bands. The objective being, of course, to be
able to tune to a good return loss, meaning that the tank/RFC combination
was presenting the proper impedance transformation and that the RFC didn't
suck...literally.
I had previously dipped the RFC, shorted, and found a nice dip around 23.5
MHz or so (oh, it's a B&W800 plate choke). So I was concerned that I
wouldn't get a proper return loss at 12meters, since that's only about 5%
away.
Well, happily I was able to tune the tank to > -40dB return loss on all
bands 30-10meters (I've given up on 40 and 80 on this amp because the
B&W850A tank assy can't do it and I don't want to hoark with it...besides,
with a different antenna in use it's probably better to use a separate amp
anyway). So I guess that means the RFC is dandy and so is the tank. BTW
it's great fun to twist the knobs and see the return loss null slide around!
Then I decided, why not measure the RFC with the network analyzer too.
It's got an SO-239 jack for HV so no problem. What I did was, disconnected
the bypass caps so the circuit was just the RFC. In series with the choke
I put a 50 ohm resistor, reasoning that if the choke worked, measured
return loss would be very low (because of the extremely high input
impedance due to the choke), but where it sucked out, the return loss would
be very low, since only the 50 ohm resistor was left in the circuit.
Lo and behold, look at that family of nulls! And all of them very close to
where the dipmeter said they were...23 MHz, 33 MHz, and 44 MHz. What a
deal. You could even use the old 'run a screwdriver up and down' trick and
see that the very center of the choke was the hot spot...the 23 MHz RL null
nearly disappeared.
And my XYL wonders why I collect all that old test equipment!
------------
Scott Townley
nx7u@primenet.com
------------
Collector of:
Stoddard Aircraft EMI/RFI receivers and accessories
Big Parts for that Big Linear Amp
70's era RF test equipment HP/GR/Tek
Radio-related technical reference material 1940+
...anything else that will keep me off the streets at night
--
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