97 lines
5.6 KiB
Plaintext
97 lines
5.6 KiB
Plaintext
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PART XI. THE 40-50 MHZ CELLULAR SCANNER
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The cellular phone freqs. occupy the UHF spectrum previously assign
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to commercial TV stations. Since a TV channel occupies a Bandwidth(BW) of 6
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MHZ and each cellular channel requires 24 KZ (for a plus and minus 12 KHZ
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signal) and a 3KHZ guard band for each audio signal (thus the 30KHZ spacing).
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200 cellular channels can fit into one UHF TV channel. Thus, with a little
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fine tuning, and old TV set with variable VHF tuning can tune in all
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cellular freqs. between 824 and 890 MHZ.
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UHF TV tuners ares designed to convert these UHF freqa. to intermediate
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(difference) freqs. between 41 and 47 MHZ. Thus, by purchasing a commonly
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available UHF tuner (check sith a TV shop and try to get a copy of the
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SAMS for powering the AFC info, and avoid those with tubes). Tuner voltage
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is usually 8-24 volts, and must be correctly connected up. You should also
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remove with the tuner, the TV's channel select and fine tuning controls
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for ease of use - and they should be in good condition.
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Once you have the tuner, you can then wire it between a 30-50 MHZ scanner and
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a UHF antenna (highly directional yagi type is preferred). Since the tuner
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will probably have a 300 ohm input impedance, a twin antenna cable is
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preferred (Yes Radio Shack has 'em). If you are into directional-finding,
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the UHF antenna should NOT have AGC (automatic Gain Control) as those with
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active AGC will amplify reflections, resulting in readings from so many
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directions that the target will be lost. Because of the use of inconspicuous,
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commonly available, inexpensive, high-gain UHF antennas, using a good UHF
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tuner to scan cellular channels is a good method of doing it.
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Tuner output is usually through an RCA-type plug. CAUTION: BE SURE TO COUPLE
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YOUR SCANNER TO THE UHF TUNER WITH A 0.01-0.1 mf (50 V min.) CAPACITOR FOR
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DC BLOCKING. AND DO NOT TRY TO OPERATE THE TUNER THROUGH ITS TV SET AS THE
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DANGER OF HIGH-VOLTAGE DISCHARGE IS HIGH. Also, connect a ground wire
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between the tuner and the scanner.
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The table below describes how cellular freqs. can be downconverted by a
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commonly available UHF TV tuner (all freqs. are in MHZ)
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CELLULAR MOBILE FREQS & SCANNER EQUIVALENTS
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TV
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BAND CELL. CHAN. SCAN TV OSCIL
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CHAN. # and FREQ. FREQ. FREQ. LIMIT
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----------- ------------ ----- -------- -----
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73 (first) 0001-825.03 45.97 871 824-830
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73 (last) 0166-829.98 41.02 871 824-830
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74 (first) 0167-830.01 46.99 877 830-836
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74 (last) 0366-835.98 41.02 877 830-836
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75 (first) 0367-836.01 46.99 883 836-842
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75 (last) 0566-841.98 41.02 883 836-842
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76 (first) 0567-842.01 46.99 889 842-848
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76 (last) 0766-847.98 41.02 889 842-848
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77 (first) 0767-848.01 46.99 895 848-854
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77 (last) 0799-848.97 46.03 895 848-854
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CELLULAR PHONE FREQS. HAVE NOT BEEN ASSIGNED
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FOR CHANNELS 800-990
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73 (first) 0991-824.04 46.96 871 824-830
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73 (last) 1023-825.00 46.00 871 824-830
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80 (first) 0001-870.03 42.97 913 866-872
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80 (last) 0066-871.98 41.02 913 866-972
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81 (first) 0067-872.01 46.99 919 872-878
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81 (last) 0266-877.98 41.02 919 872-878
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82 (first) 0267-878.01 46.99 925 878-884
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82 (last) 0466-883.98 41.02 925 878-884
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83 (first) 0467-884.01 46.99 931 884-890
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83 (last) 0666-889.98 41.02 931 884-890
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83 (**) 0667-890.01 46.99 931 884-890
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83 (**) 0799-893.97 37.03 931 884-890
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CELLULAR PHONE FREQS. HAVE NOT BEEN ASSIGNED
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FOR CHANNELS 800-990
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80 (first) 0991-869.04 43.96 913 866-872
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80 (last) 1023-870.00 43.00 913 866-872
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(**) These freqs. are outside of the normal Channel 83 BW. However, most UHF
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tuners have a fine tuner that can be adjusted up to about another 6 MHZ.
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Note that the term "first" and "last" refers to the first and last cellular
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channels receivable by the UHF tuner for the given TV channel. Base voice
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channels are monitored when both sides of the conversation is required.
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Mobile voice channels or base control channels are monitored to locate a
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cellular phone. Tuning is simple:
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(1) Decide which cellular channel or freq. you wish to monitor.
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(2) Find what UHF channel includes that freq. and switch the TV
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to that channel.
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(3) Using the table, look up the corresponding TV oscillator freq.
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(ex: 919 MHZ for TV channel 81). Subtract the cellular channel
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freq. from the TV oscillator freq.
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(4) Tune your scanner to the difference freq.
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When you select a scanner, you should pick one that will scan in 30 MHZ
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increments to efficiently receive cellular transmissions. If you can't get
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one like that, then get one that will scan in 15,10 or 5 KHZ increments.
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*****************************************************************************
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