I think using radio broadcasts as a means to communicate information to a
global 'spy' network has become passe. Modern spy and terrorist networks
increasingly use a method called 'Steganography' to hide messages and data
inside picture files or other media files that are easily posted through the
internet.
Lot's of free software to do this can be found on the net. Here's a link to
a tutorial:

http://elonka.com/stego/StegoLR.htm


Joe
W3JDR



"Michael Black" wrote in message
...
Dave Platt ) writes:
In article ,
Clark Magnuson wrote:

Could a spy modify an AM radio by adding a few capacitors?

Possibly, but I doubt that it would work well if you started from a
standard US-type AM broadcast-band receiver. The frequencies that
this article speaks of are up in the HF band, roughly 10 times higher
than the AM broadcast-band frequencies, and these radios' built-in
loopstick antennas are not well suited to receive these sorts of
signals.

A spy in most Western countries could simply buy an off-the-shelf
AM/FM/shortwave receiver (very widely available).

Simple but very effective direct-conversion or superheterodyne
receivers (capable of receiving CW, SSB, and AM) can be built out of
"junk box" parts by anyone with a modest amount of training and
experience. Many of the popular QRP receiver designs could be tweaked
to receive on these sorts of frequencies with very little difficulty.

Now, if somebody had an old "7-transistor" AM radio (circa 1960
or so), it would likely be possible to salvage enough parts from it
to build a simple direct-conversion shortwave receiver.

I doubt that this would be possible (or at least not easy) for today's
highly-integrated IC-based radios.

First, a "spy" is not going to need to build a radio. They will be
able to get their hands on something locally, or (as in WWII), they'll
be parachuted in with the needed equipment. And in this day and age,
with so many means of communication (and at least for the moment, no
all-encompassing wars), there are all kinds of mainstream means of
contacting spies without being tracked. Traditionally, spies were not
radio experts, unless their job was to accumulate information on
radio matters.

But, if someone really did need to assemble a shortwave radio, I
suspect it's far easier today than in the past. At the very least,
no more difficult than in the past.

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.

And of course, many would add a converter to an existing AM broadcast
radio. Sacrifice one radio to build a converter for another. Then
you get double conversion, which would be a good thing given the
common 455KHz IF for most AM broadcast radios. For best results, use a
car
radio for the radio, since they often have better selectivity and of
course
are well shielded compared to portable AM radios with their loopstick
antennas. Need a BFO? Those can be tossed together easily with a
transistor off some scrap board, and an IF transformer from a scrap radio
(and I seem to recall some using ceramic filters as the frequency
determining element in a BFO, and if those work, they are easily available
out of scrap radios, compared to ceramic resonators).

ICs may offer better performance. Since transistors in an IC are
cheap, they may be more likely to use a balanced mixer. Grab an IC
out of a radio, and you've got your mixer (and you have a choice of
the mixer from the AM or the mixer from the FM section). Some have
put together SSB receivers out of narrow-band FM receiver ICs (they
simply ignore the FM detector), and someone used merely the mixer
out of a more complicated receiver IC for a direct conversion receiver.
A lot of FM broadcast radios use IF strip ICs that don't allow for
separate use of the IF amplifiers from the limiters, but I've stripped
at least one car radio that offered up an MC1350 IF amplifier.
For that matter, use the FM IF strip as a quasi-synchronous detector
for AM, or if it's of the right design, feed a BFO into the limiter
input and you've got your product detector.

Find a CB set, and you get better IF selectivity, and many will be
double conversion. Retune the front end, and replace the synthesizer
with a tuneable oscillator. Get really lucky, and the garage sale
will offer up an SSB CB set, for even narrower selectivity and of
course CW/SSB reception with no mods.

Circa 1942, you'd have far more trouble getting parts. Oh, I
suppose there were more local electronic stores, but there was
hardly any consumer electronic equipment. Nowadays it's really really
common. And that supplies so many useful parts.

Michael VE2BVW