
               EAVESDROPPING ON SUBCARRIER TRANSMISSIONS


         A few weeks ago, I posed a question about listening  to
         SCA  transmissions on the FM commercial broadcast band.
         An article in Monitoring_Times claimed that  connecting
         a vlf receiver to an FM broadcast receiver would permit
         SCA detection.

         Necessity being the mother  of  invention,  a  way  was
         found  to  confuse  the ICOM R71A into tuning below 100
         kHz1, and after receiving substantial inspiration  from
         Will  Martin,  Phil  Karn,  and others I now can report
         success!

         With the R71A in the FM mode, tuned to 67 kHz,  I  con-
         nected  the  ICOM's vlf antenna input through a 0.1 ufd
         capacitor2 to the earphone jack of a $16 General  Elec-
         tric AM/FM portable radio, and can now listen in on the
         world of SCA!

         In the first few minutes of tuning around,  I've  heard
         the  Physicians' Network, Muzak, commodity reports, and
         several data transmissions.

         I built a simple SCA interface, consisting of a capaci-
         tor and resistor, into a plastic film canister.


                        +-----------------------+
                        |      AM/FM radio      |
                        |                       |
                        |     earphone jack     |
                        |  (ring)       (tip)   |
                        +-----------------------+
                              |           |
                              +--/|/|/|/--+
                              |           |
                              |  100 ohm  |
                              |   1 watt  |
                              |           |
                              |           |
                              |         -----
                              |         -----  0.1 ufd
                              |           |
                        +-----------------------+
                        |   (gnd)               |
                        |     antenna jack      |
                        |                       |
                        |       ICOM R71A       |
                        +-----------------------+


         Most activity is heard with AM/FM radio tuned to the FM
         broadcast  band,  and the R71A set to 67 kHz FM.  Aside
         from the Muzak, commodity reporting, and  the  Physici-
         ans'  Network,  I  also heard an announcer reading from
         Popular_Communications on CRIS, the  Chicago  Radioland
         Information  Service.  This service carries programming
         of interest to the handicapped.

         With the AM/FM radio tuned to the  AM  broadcast  band,
         and the R71A set to 60 kHz AM, I can hear what seems to
         be stereo subcarriers on AM broadcast stations claiming
         to transmit in AM stereo.

    __________

     1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
        published in Monitoring_Times.

     2. The capacitor is needed with the GE radio I used to
        block DC.
    

