On Sun, 2009-06-07 at 16:35 -0700, Joe White wrote:
> What level of isolation or attenuation should the output diode T/R switch on
> a Corsair I Low Pass Filter Board have? I've been measuring the one on my
> Corsair by looking at the voltage out at the antenna connector and comparing
> that level to the voltage on the RX ANT jack with the RX/XCV switch in the
> XCV position. I'm seeing voltages at the RX ANT jack that are 1/1000 of the
> ANTENNA output voltage on 80 and 40 meters, and only 1/165 at 10 meters.
> These tests were made using a 50-ohm dummy load connected to the ANTENNA
> connector.
>
> The reason I ask:
>
> The Corsair I have had a close encounter with lightning before I bought it.
> Most of the damage was repaired, but the original owner missed the damaged
> diodes in the final T/R switch. Apparently after making the partial repair,
> he attempted to use the radio and, since the output T/R switch was
> non-functional, the output power from the final was fed back to the PIN diode
> T/R switch on the Low Level Driver Board, blowing the two diodes that make up
> that switch. That was the condition of the radio when I bought it. I've
> replaced the components that I found to be bad (mainly 4 T/R switch diodes
> and a shunting transistor). I have used the radio at QRP levels since then
> with no problems, but I've always been skeptical to run full power at the
> higher frequency bands, not knowing if I was getting sufficient isolation to
> protect the T/R circuitry on the Low Level Driver Board.
>
> Recently, I cranked up the power and the receiver went deaf, so I thought I
> had damaged things again, but it turned out there was no problem with the
> radio -- whatever caused the receiver to go silent was something else in my
> setup. The radio still seems to work fine, and I've been testing it on the
> bench at full power. However, I'm concerned that I have not totally repaired
> the output T/R switch and bad things could happen if I attempt to use it with
> a mismatched antenna. Can anyone tell me if the results I reported in the
> first paragraph are typical of a properly functioning Corsair? .. or is
> there a better way to confirm the operation of the output T/R switch?
>
> Thanks and 73,
>
> joe, WA4GIR
Mismatches in probe compensation won't affect taking ratios, but might
affect absolute voltages on the receiver port. And when you are at 40+
dB attenuation at 28 MHz stray coupling and imperfect shielding can lead
you astray too.
The attenuation of the PIN switches is dependent on low inductance and
resistance of the shunt elements and low capacitance and high resistance
of the open elements. Those two reactive elements will hurt the
attenuation at 28 vs 3.5 MHz. 44 dB down is putting +6 dBm into the
receiver. Probably not enough to hurt it, but probably ten dB above its
linear range.
If you left PIN diode leads long, you can have hurt the HF attenuation.
I've not checked that on my Corsair II.
73, Jerry, K0CQ
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