It could be that there are local signals in the band which are
overloading the receiver and the receiver lacks selectivity to
discriminate between on and off channel signals.Or an IF image response
is reacting to a specific interferer. Have you tried tesing inside of a
shielded screen room (faraday cage)? Secondly, the gain of the circuit
could exceed the shielding and grounding of the circuit layout, thus
feeding back from circuit to antenna. In this case, assuming a 50 ohm
circuitry, extend the antenna with a length of low loss coaxial cable
away from the circuitry, put the circuit in a shielded box with
appropriate feedthroughs, and check for interaction.

David wrote:

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


--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P