Polyakov mixer and 50Hz hum

There is something strange about my Polyakov mixer based direct conversion receiver for 28.1246MHz WSPR at the moment.

On the bench, when connected to a 50 ohm signal generator or sitting on the bench without an antenna connected, the background noise is a quiet hiss. With an earpiece in a quiet room I can just hear a -125dBm signal, even without an RF preamp: it is a good, sensitive and simple design.

However, the moment I connect my outside halo antenna via a coax to the (unscreened) breadboard RX the 50Hz hum level is quite high, certainly some 40dB higher audio than the MDS signal. This is with the RX run from a small sealed lead acid battery. With a mains supply I might expect some hum, but what is going on? Why the high hum pickup?

My coax antenna feed passes through the loft and then outside passing plenty of mains cables, so I must be inducing some 50Hz pick-up on the coax outer which is coupling into the copper laminate breadboard ground plane. A tidy rebuild will be needed before I could put the RX into a screened box.

I do not recall having a similar problem with other DC designs I have tried (single balance diode pair, NE602, SBL1 etc.) when used in a similar lash-up breadboard.

Anyone any suggestions?