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.