RadioBanter - View Single Post - How hard for a spy to receive 6855 or 8010 kHz?

Paul Keinanen · January 1st 07, 12:17 PM posted to rec.radio.amateur.homebrew

On 1 Jan 2007 03:49:31 GMT, (Michael Black)
wrote:

Thirty years ago, magazines were full of simple superhets based
on standard AM radios. Some would rework the front end
circuitry to tune a shortwave band, and the fact that ICs are
used rather than transistors isn't likely to make that really difficult.
I can remember in 1971 when RCA introduced the CA3088 AM radio IC,
and QST ran an article pointing out how it could be used. Obviously
not the best choice, but an IC instead of transistors isn't likely to
make a worse receiver. The limitations would come from image rejection
and lousy selectivity (and maybe lack of IF gain), and those were
all there with transistor based radios.

Others would strip off the front end tuned circuits, and simply use
the AM broadcast receiver as a 455KHz IF, building a mixer and oscillator
to feed into it.

Using a standard IF is a bad idea if you are a spy, since the local
oscillator is going to be at a standard offset of known number
stations.

According to the "Spycatcher" book, MI5 used local oscillator leakage
to track down spies already in the 1950's. Thus, if standard IF is
used, at least shield the oscillator very well and use some RF stages
to isolate the antenna from the local oscillator.

The police in some countries use the local oscillator radiation to
detect illegal speed trap radar detectors.

Paul OH3LWR