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Text File  |  1996-01-28  |  3KB  |  77 lines
  1.  
  2.                EAVESDROPPING ON SUBCARRIER TRANSMISSIONS
  3.  
  4.  
  5.          A few weeks ago, I posed a question about listening  to
  6.          SCA  transmissions on the FM commercial broadcast band.
  7.          An article in Monitoring_Times claimed that  connecting
  8.          a vlf receiver to an FM broadcast receiver would permit
  9.          SCA detection.
  10.  
  11.          Necessity being the mother  of  invention,  a  way  was
  12.          found  to  confuse  the ICOM R71A into tuning below 100
  13.          kHz1, and after receiving substantial inspiration  from
  14.          Will  Martin,  Phil  Karn,  and others I now can report
  15.          success!
  16.  
  17.          With the R71A in the FM mode, tuned to 67 kHz,  I  con-
  18.          nected  the  ICOM's vlf antenna input through a 0.1 ufd
  19.          capacitor2 to the earphone jack of a $16 General  Elec-
  20.          tric AM/FM portable radio, and can now listen in on the
  21.          world of SCA!
  22.  
  23.          In the first few minutes of tuning around,  I've  heard
  24.          the  Physicians' Network, Muzak, commodity reports, and
  25.          several data transmissions.
  26.  
  27.          I built a simple SCA interface, consisting of a capaci-
  28.          tor and resistor, into a plastic film canister.
  29.  
  30.  
  31.                         +-----------------------+
  32.                         |      AM/FM radio      |
  33.                         |                       |
  34.                         |     earphone jack     |
  35.                         |  (ring)       (tip)   |
  36.                         +-----------------------+
  37.                               |           |
  38.                               +--/|/|/|/--+
  39.                               |           |
  40.                               |  100 ohm  |
  41.                               |   1 watt  |
  42.                               |           |
  43.                               |           |
  44.                               |         -----
  45.                               |         -----  0.1 ufd
  46.                               |           |
  47.                         +-----------------------+
  48.                         |   (gnd)               |
  49.                         |     antenna jack      |
  50.                         |                       |
  51.                         |       ICOM R71A       |
  52.                         +-----------------------+
  53.  
  54.  
  55.          Most activity is heard with AM/FM radio tuned to the FM
  56.          broadcast  band,  and the R71A set to 67 kHz FM.  Aside
  57.          from the Muzak, commodity reporting, and  the  Physici-
  58.          ans'  Network,  I  also heard an announcer reading from
  59.          Popular_Communications on CRIS, the  Chicago  Radioland
  60.          Information  Service.  This service carries programming
  61.          of interest to the handicapped.
  62.  
  63.          With the AM/FM radio tuned to the  AM  broadcast  band,
  64.          and the R71A set to 60 kHz AM, I can hear what seems to
  65.          be stereo subcarriers on AM broadcast stations claiming
  66.          to transmit in AM stereo.
  67.  
  68.     __________
  69.  
  70.      1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
  71.         published in Monitoring_Times.
  72.  
  73.      2. The capacitor is needed with the GE radio I used to
  74.         block DC.
  75.     
  76.  
  77.