parnass@ihu1h.UUCP (Bob Parnass, AJ9S) (01/15/85)
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TRICK FOR OUT OF BAND RECEPTION ON UHF RECEIVERS
Bob Parnass, AJ9S
Articles in Monitoring Times and Popular Communications
have described how one may "trick" various scanner radios
into receiving frequencies they were not designed to
receive. These tricks fall into 2 categories:
1. Exploiting a bug in the receiver's microprocessor
firmware by entering a particular series of keys-
trokes.
2. Exploiting the receiver's finite image rejection by
listening to simple images (e.g. to listen to fre-
quency F, tune the receiver to F + 2 * [the inter-
mediate frequency]).
This article describes a third technique, which permits
listening in the 512 - 657 MHz range (UHF television
channels 21 through 44) using radios designed to receive
the 402 - 512 MHz range. The technique uses "multiple
injection frequencies", and requires no receiver modifi-
cation or additional hardware.
I discovered that my Radio Shack PRO30 scanner receives
the audio from TV channel 38 (619.75 MHz) when the radio
is programmed for 483.5375 MHz, and from channel 32
(583.75 MHz) when programmed to 456.5375 MHz. This is
also true for the Radio Shack PRO2020 scanner.
Bearcat BC100, BC210, and BC350 scanners can also receive
UHF TV signals on odd frequencies, but at different pro-
grammed settings: 483.7125 for TV channel 38, and
456.7125 for TV channel 32 (Bearcat and Radio Shack scan-
ners use different intermediate frequencies).
By checking the arithmetic, I've concluded that the same
mechanism permits out of band reception on all these
scanners.
The Method
In the formulas that follow, all frequencies are in units
of Megahertz. To listen on a frequency U between 512 and
657 MHz, tune your radio to:
3(U) - 7(IF)
------------
4
where IF is the intermediate frequency of the receiver.
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(IF)
------------
3
When using this technique to receive TV audio on a scan-
ner, the signal may be weak or distorted because:
- The bandwidth of a TV audio signal exceeds that of
the IF filtering in the typical scanner.
- A typical scanner radio is not as sensitive in the
512 - 657 MHz range as it is on the frequencies for
which it was designed, due to the inability of the
front end tuned circuits to resonate out of their
normal range.
- The TV signal format maybe unconventional.
Chicago's channel 44, a pay TV service, scrambles
the video and transmits the audio signal using an
unconventional format not known to this author.
PRO30 scanners receive Chicago channel 44 audio when
programmed for 510.4875 or 510.5875 MHz.
Technical Explanation
What's happening is that an injection signal with multi-
ple constituent frequencies is being being fed to the
mixer stage.
To hear the audio from TV channel 32 (583.75 MHz), the
PRO30 is set to 456.5375 MHz. At this setting, the local
oscillator/tripler chain is injecting a signal into the
mixer stage at:
456.5375 - 10.7
= 445.8375 MHz.
This 445.8375 MHz signal is the output of a frequency
tripler, whose input frequency is:
445.8375 / 3
= 148.6125 MHz.
But, in addition to this 3rd harmonic, there seems to be
a 4th harmonic component present:
4 (148.6125)
= 594.4500 MHz,
which when mixed with a signal on 583.75 MHz (channel
32), produces a 10.7 MHz IF:
594.4500 - 583.750
= 10.7 MHz
Other Frequencies Possible
Although the scheme described in this article requires no
scanner modifications or additional hardware, a multiple
injection scheme forms the basis for a product already
familiar to scanner enthusiasts, the Grove "Scanverter."
The Scanverter, a novel 216 - 420 MHz converter, makes
use of multiple injection frequencies to accomplish what
Bob Grove terms "band stacking". In the Scanverter, a
local oscillator produces a signal rich in harmonics.
This signal is fed into a broadband mixer along with sig-
nals from the antenna, causing several frequencies to be
received at each setting of the attached scanner.
The ability to receive TV signals on a scanner is usually
of little value, but there may be ways of using this mul-
tiple injection phenomenon to receive bands other than
512 - 657 MHz. I recall receiving the 787.75 MHz audio
from TV channel 66 somewhere on a BC100 once!
A simple scanner modification may be possible that pur-
posely distorts the injection signal in a way that produ-
ces both the fundamental frequency and its second har-
monic. Theoretically, this could cause a scanner with a
10.7 MHz IF and 420 - 512 MHz coverage, to be receptive
to signals in the 807.9 - 991.9 MHz band. Readers are
urged to experiment to find other frequencies to which
their scanners may be receptive. A well calibrated,
harmonic-free signal generator would be helpful.
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Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass - (312)979-5414