77 lines
3.1 KiB
Plaintext
77 lines
3.1 KiB
Plaintext
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EAVESDROPPING ON SUBCARRIER TRANSMISSIONS
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A few weeks ago, I posed a question about listening to
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SCA transmissions on the FM commercial broadcast band.
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An article in Monitoring_Times claimed that connecting
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a vlf receiver to an FM broadcast receiver would permit
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SCA detection.
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Necessity being the mother of invention, a way was
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found to confuse the ICOM R71A into tuning below 100
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kHz1, and after receiving substantial inspiration from
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Will Martin, Phil Karn, and others I now can report
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success!
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With the R71A in the FM mode, tuned to 67 kHz, I con-
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nected the ICOM's vlf antenna input through a 0.1 ufd
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capacitor2 to the earphone jack of a $16 General Elec-
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tric AM/FM portable radio, and can now listen in on the
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world of SCA!
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In the first few minutes of tuning around, I've heard
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the Physicians' Network, Muzak, commodity reports, and
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several data transmissions.
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I built a simple SCA interface, consisting of a capaci-
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tor and resistor, into a plastic film canister.
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+-----------------------+
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| AM/FM radio |
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| earphone jack |
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| (ring) (tip) |
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+-----------------------+
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+--/|/|/|/--+
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| 100 ohm |
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| 1 watt |
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| -----
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| ----- 0.1 ufd
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+-----------------------+
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| (gnd) |
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| antenna jack |
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| ICOM R71A |
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+-----------------------+
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Most activity is heard with AM/FM radio tuned to the FM
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broadcast band, and the R71A set to 67 kHz FM. Aside
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from the Muzak, commodity reporting, and the Physici-
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ans' Network, I also heard an announcer reading from
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Popular_Communications on CRIS, the Chicago Radioland
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Information Service. This service carries programming
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of interest to the handicapped.
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With the AM/FM radio tuned to the AM broadcast band,
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and the R71A set to 60 kHz AM, I can hear what seems to
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be stereo subcarriers on AM broadcast stations claiming
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to transmit in AM stereo.
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__________
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1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
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published in Monitoring_Times.
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2. The capacitor is needed with the GE radio I used to
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block DC.
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