parnass@ihu1h.UUCP (Bob Parnass, AJ9S) (02/02/85)
x
LISTEN TO 850 MHZ ON A 400 MHZ RECEIVER
Bob Parnass, AJ9S
A recent article1 described the technique of using "multiple
injection frequencies," and introduced an equation which
permits listening in the 512 - 657 MHz range (UHF television
channels 21 through 44) using radios designed to receive 402
- 512 MHz.
This article introduces another equation, permitting 851 -
1158 MHz reception using a Radio Shack PRO30 scanner desig-
ned to receive 380 - 512 MHz. The equation may be applica-
ble to other scanners.
Using this equation, my Radio Shack PRO30 scanner receives
the Illinois State Police District 15 (855.4875) when the
radio is programmed for 381.9250 MHz. Mr. James Cavanagh,
of Palatine, Illinois, reports that his PRO30 receives the
Palatine Police (856.7125 MHz) when programmed for 382.4500
MHz.
The Method
In the equations that follow, all frequencies are in units
of Megahertz. To listen on a frequency Y between 851 and
1158 MHz, tune your radio to:
3(Y) + 10(IF)
-------------
7
where IF is the intermediate frequency of the receiver.
Conversely, if you hear a strange signal when your UHF radio
is tuned to X MHz, you may be simultaneously listening to
the frequency of:
7(X) - 10(IF)
-------------
3
When using this technique to receive microwave signals on a
scanner, the signal may be very weak, because a typical
scanner radio is not nearly as sensitive in the 851 - 1158
MHz range as it is on the frequencies for which it was
designed.
Technical Explanation
What's happening is that an injection signal with multiple
constituent frequencies is being being fed to the mixer
stage.
To hear the Palatine Police signal (856.7125 MHz), the PRO30
is set to 382.45 MHz. At this setting, the local
oscillator/tripler chain is injecting a signal into the
mixer stage at:
382.450 - 10.7
= 371.750 MHz.
This 371.750 MHz signal is the output of a frequency tri-
pler, whose input frequency is:
371.750 / 3
= 123.9167 MHz.
But, in addition to this 3rd harmonic, there is a 7th har-
monic component present:
7 (123.9167)
= 867.4167 MHz,
which when mixed with the police signal on 856.7125 MHz,
produces a 10.7 MHz IF:
867.4167 - 856.7125
= 10.7042 MHz
One may think of this 7th harmonic as being the sum of the
3rd harmonic, the signal the circuit was designed to pro-
duce, and the unexpected 4th harmonic, described in the pre-
vious article.
Poor Sensitivity
During reception of both police transmitters described
above, the signal came directly into the mixer stage through
the PRO30 case, bypassing the antenna and RF amplifier
stages! Tests were made using the supplied flexible antenna
and an external 146 MHz antenna. The connection of either
antenna made no difference in signal strength. Disconnec-
ting the antenna altogether made no difference either.
Apparently, the RF amplifier/antenna combination attenuated
the microwave signals more than the plastic case, which is
painted with a conductive substance.
Don't expect very good range on these frequencies. In both
cases, the microwave transmitters were within a few blocks
of the scanner and line of sight.
A Regency HX1000 scanner (IF of 21.6 MHz), was tested for
microwave reception using this equation, but proved unsuc-
cessful. Once again, readers are urged to experiment to
find other frequencies to which their scanners may be recep-
tive.
__________
1. Parnass, Bob, "512 - 657 MHz Reception on Your Scanner",
Monitoring Times. February 1984, page 19.
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Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass - (312)979-5414