On Mon, 26 Feb 2007 06:10:45 GMT, David
wrote:

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

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 ?

If you suspect stability issues, you might try connecting a variable
resistor and variable capacitor to the input and try to check various
settings to see, if you can repeat the situation without the antenna.

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.

Depending on your location, the band noise level at 150 MHz may be
well above the thermal noise from a simple resistor. How well is the
spectrum analyzer calibrated ? Did you get the reading for a 50 ohm
terminator resistor at the input and then compare the band noise with
the 1/4 wavelength antenna ?

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.

"50 ohm RF amp" sounds like some MMIC, such as the MAR-x series. I
hope that you are not using MAR-8, which is not 50/50 ohms in/out :-).

Paul OH3LWR