Mysterious Noise Burst (Solved!!!)

I had been chasing a mysterious noise burst on 2 meter EME for months.

Here’s what it looked like on the fast waterfall of wsjtx.

You can see a quiet noise floor for a fraction of a second after the tx period, then the noise burst kicks in for a few seconds in the graph.

This noise burst was only seen when using the large EME antennas.

If I connected a small antenna to the RX port and moved it around the station, I saw no noise.

If it had been a long time since I had transmitted, the noise could last up to 10 minutes. As I continued to TX, the burst duration would shortened down to just a second or so.

This was observed only when I pointed at certain antenna heading (right over the garage), and was very repeatable. It had all the characteristics of a response to the RF signal, just like a repeater.

I looked internal to the shack and one by one eliminated everything there, LNA, SSPA, K3/xvtr, power supply, and T/R relay.

I also turned off everything in the house which had a wireless connection.
I covered the power meter and gas meter with aluminum foil.

All to no avail….

In the process of testing, I replaced the SSPA power supply and the T/R relay both of which were needed improvements.

As a test, I completely removed and unpowered the LNA, but the noise was still there.
I added a relay to the FET bias on the SSPA SSPA, which completely turned off all current when not TXing, but the noise was still there.
I swapped out the T/R relay for one with a little better isolation, but the noise was still there.

I was stumped and just happened to post my problem on the EME-1 web page. Someone suggested I look for security lights. Well guess what there is one right on the back side of the garage.

Problem Solved!!!! It will be ceremoniously thrown in the trash!!!