I have just completed the design of a 151 MHz ASK transceiver.

On the bench the unit works fine with sensitivity to around -115 dBm
however, when I attach an antenna, the RSSI level increases dramatically
and it appears something may have become unstable.

I have tried a commercial ground independent antenna and a 1/4 wave whip
and the results are the same.

If I disconnect the antenna and leave the RF input floating, it behaves
properly. I can also attach a small wire (say 300mm) directly to the RF
input and it also works fine. It just plays up when I have a proper
antenna connected.

I am not sure if this may be some issue with the VSWR of the antenna
causing some instability (I tried a 3dB pad between RF input and antenna
and this did not fix the problem), or whether the input is so sensitive
that it is picking up the LO from the radio and causing regeneration ?

A Spectrum Analyser was used to determine if anything else is being
transmitted in that region and it appears to be clear to well outside
the bandwidth of my receiver.

Any ideas much appreciated for what might be happening or methods of
debugging my design.
(The circuit consists of an RF switch after the Antenna port (selects
TX/RX) then into a 50 Ohm RF Amp, followed by a tuned circuit into an
SA6505 RF IF. The LO is crystal based harmonic Butler emitter follower.

PS. I also tried bypassing the RF Amp but still no improvement.

Thanks in advance

Regards

David