On 12/26/22 22:31, tjschuessler--- via AMSAT-BB wrote:
> I do have a question for Yaesu FT847 users. With that radio, you can
> power a Preamp via the built in Bia- T power inserter, And assume
> that when this mode is turned on, it cuts the inserted DC to the
> preamp as the tranciever goes into transmit. Does this work fast
> enough to protect, even RF switched preamps? That might be a good
> safety measure as I have traditionally used external DC to power my
> preamps.
I have a homebrew UHF LNA that initially suffered the same fate as yours
before it was modified with protective PIN diodes on the output.
I am powering the relays which switch from "inline" (during RX) to
"bypass" (during TX) with the FT-847's bias-tee function. I found that
the relays might not be quite fast enough to avoid getting a significant
RF pulse back into the LNA output. This was most pronounced with CW,
while SSB was tolerated better -- probably because I waited a moment to
speak after pressing the PTT.
In a previous posting, I wrote:
>
> (The BAR64 PIN diodes were added to protect the MAR3-SM+ output gain
> block when transmitting. Without those, the MAR3 was dead within a
> few minutes of use at >10W TX.)
You might try putting some of those PIN diodes into your unit. It might
just keep it from going up in smoke.
--- Zach
N0ZGO
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