parnass@ihu1h.UUCP (Bob Parnass, AJ9S) (10/25/84)
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TRICK FOR OUT OF BAND RECEPTION ON UHF RECEIVERS
Articles in "Monitoring Times" and "Popular Communications"
have described how one may "trick" various scanner radios
into receiving frequencies that they were not designed to
receive. These tricks fell into 2 categories:
1. Tricking the microprocessor-based control element by
making use of some bug in the firmware.
2. Listening to simple images (e.g. to listen to fre-
quency F, tune the radio to F + 2 * [the intermediate
frequency]).
This article describes a third technique, which I discovered
by accident.
My Radio Shack PRO30 scanner (10.7 MHz intermediate fre-
quency) can receive the audio from TV channel 44 (655.75
MHz) when the radio is programmed for 510.4875 MHz.
I think this is what's happening:
- When the radio is set to 510.4875 MHz, the local oscil-
lator is working at 510.4875 - 10.7 = 499.7875 MHz.
- This 499.7875 MHz signal is actually the output of a
frequency tripler, whose input frequency is 166.5958
MHz (approx).
- In addition to this 3rd harmonic, there seems to be a
4th harmonic component present (666.3833 MHz), which
when mixed with a signal on 655.6833 MHz, produces a
10.7 MHz IF (655.6833 MHz is close enough to ch 44's
655.75 MHz)!
Owners of the Bearcat BC100 handheld scanner (10.8 MHz IF)
have told me they can receive UHF TV signals on odd frequen-
cies also. After checking the arithmetic, I've concluded
that they are experiencing the same phenomena.
Here's the algorithm: to listen on an out-of-band UHF fre-
quency U (in MHz), try tuning your radio to:
[3*U - 7*(intermediate freq in MHz)] / 4
Conversely, if you hear a strange signal when your UHF radio
is tuned to T MHz, you may be simultaneously listening to
the frequency of:
[4*T - 7*(intermediate freq in MHz)] / 3
I would be interested in hearing from those that have tried
this on amateur 440 MHz receivers, or on other radios.
--
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Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass - (312)979-5414