RECEIVER HUNTING USING THE 'I.F.' PRINCIPLE

              By Nigel Ballard
                 28 Maxwell Road, Winton, Bournemouth,
                    Dorset, BH9 1DL, England.
                            5 August 1990

Firstly, what is an 'I.F.'?  Well, incoming signals to any modern radio
are mixed with a fixed internal signal , these are produced by a circuit
known as a local oscillator. Your incoming signal mixes with the fixed
internal signal and produces an Intermediate Frequency, or I.F.

The I.F. frequency always operates above or below the incoming
frequency. If the incoming occurred at the exact same frequency as your
receivers I.F., then your receiver would find this an impossible signal
to detect. As an example, many cheaper receivers have the all important
first I.F. at 10.7MHz, if you had a bug operating in your room on that
exact frequency, then your average receiver would not aware of it's
existence. This is not a BIRDIE in the classical sense, more a
non-usable frequency. A normal Birdie is simply a dead channel caused by
internally generated noise in the rf circuits. This 10.7MHz frequency is
not blanked by internal noise, but simply dead because it falls on the
same frequency that the I.F. operates on.

The I.F. frequency is thus generated, not by adding them together, but
by taking one from the other. The resultant freq is known as the first
I.F. frequency. Dependent on the radio type, and where in the spectrum
you are monitoring, the Local Oscillator may be operating above or below
the received signal. Although we need to know the frequency of the
radio's first I.F., it is the Local Oscillator's output we are
interested in.

I'M RECEIVING, BUT I'M ALSO TRANSMITTING....SAY WHAT! You don't have to
have vast experience of TEMPEST and the like, to know that any piece of
equipment that is turned on and uses crystal controlled or ceramically
resonated circuits, generates spurious output. Put an antenna on to this
piece of supposedly dormant equipment, and you now have unwanted
radiations, in effect when your radio or scanner is switched on and
connected to an antenna, you are constantly transmitting a signal, small
it may be, but it is there! And if an amateur like me can receive them
at up to 50 feet, then how far can the pro's get! 'BULLSHIT' you say!

OKAY DISBELIEVERS
If I shoot the breeze in general terms for a while, just to convince you
that your Bearcat (example) scanner sat in your bedroom listening on one
specific frequency, COULD be a dead giveaway to the authorities.

THE MILITARY
You don't need to convince the forces of both east and west that this
principle of detection works, they have been using it and trying to
defeat it in their own radio's for years and years.

EXAMPLE TIME
In the UK, all handhelds used by the Police walking the beat are between
451.00 and 453.00MHz NFM, no ifs or buts, that's the band limits that
they all operate in (London is excluded from this). Suppose you knew
that the first I.F. of the latest Motorola radio's they used were 24MHz.
Now suppose you came across an officer who just refused to key his radio
up so that you could scan the 451 to 453 area with your scanner. Not
daunted by this, you set your scanner to scan 24MHz below this band,
i.e. 427.00 to 429.00MHz. Getting as close to your target as possible
with a reasonable scanner using an external antenna tuned to this band,
you proceed to tune over his L.O. output. If his radio is switched on,
and he is NOT currently transmitting, as soon as you tune over his L.O.
your scanner will stop on a weak but constant low tone. If your target
then transmits the tone will disappear, as the L.O. can only be picked
up in receive. Make a note of the L.O., say it was 428.500, add the
original I.F. shift of 24MHz and hey presto you now have the EXACT
frequency he is sat on. I make it 452.500. It is now a simple case of
sitting on that spot until he decides to talk.

STILL UNCONVINCED?
Well get a friend with a h/held to let you try it out.