[amsat-bb] FT-991A Transmit Interference

[VA3ASE]

Hi AMSAT friends,

I'm having a technical problem and was wondering if anyone had any
suggestions. I've been experimenting with using an FT-991A transmitter and
SDR receiver to run full-duplex on the satellites. My problem is as soon as
I key the FT-991A transmitter, my SDR receiver waterfall goes completely
white and I don't hear anything but static. Classic overload symptoms.

I've tried several things:
- Upgrading my receiver from an RTL-SDR to an SDRplay RSP1A: no improvement.
- Adding a duplexer as a band pass filter: no improvement.
- Transmitting with a vertical antenna about 40' away from my yagis: YES it
did solve the problem but is not a suitable solution.

I'm using the same yagi antenna setup that I've used successfully for years
with an FT-847, and have also had the same problem with a handheld Arrow.

At last I tried transmitting with a different radio, and the problem
disappears. I figured the problem was in my FT-991A so I sent it for
warranty repair, but just got the call that they can't find anything wrong
with it. It has no measurable spurious or out of band emissions. I'm at a
loss here... I know people are using the same equipment (FT-991 + SDRplay)
successfully; Simon Brown comes to mind.

Here is a video I created demonstrating the problem with the FT-991 and
showing that it doesn't appear with an Icom ID-880H.
https://youtu.be/gXOIK5gLjgs

I still think the problem must be in the radio, but if they can't find
anything to fix then I'm out of luck. I'd love to hear any suggestions or
feedback.

Thanks,
Alex VA3ASE