Alex wrote: In my reading about green radios I can't afford (but want anyway) and in various readings I've heard that detecting the 455-kHz LO of a superhet isn't all that hard, and that for instance in countries that require a radio license, they use such techniques to check if people in their houses have radios (or TVs) that they're not on record of having licenses for! And, in a lot of the military radios, the shielding is VERY good so that detection is much harder to do.
---------------- That's true for some tens of feet around for unshielded vacuum-tube radios that used fairly robust local oscillators. In countries where a license for a receiver is required, authorities sometimes located receivers by driving up and down streets with special listening vans. They could often hear the local oscillator if they were quite close to the receiver's location. Local oscillators don't radiate well. The antenna input to the receiver is tuned to the signal frequency, well off of the L.O. frequency, so it and any antenna doesn't radiate much. It's the same for TV's; There's virtually no radiation from the antennas. What receiver DOES radiate well enough to be heard for miles around - sometimes around the world - is a regenerative detector without an RF stage to isolate the detector from the antenna. In that case, the detector (when oscillating in autodyne mode for CW reception) is oscillating on the same frequency the antenna circuit is tuned to, and well-coupled to the antenna! Vacuum tube regenerative detectors might produce a watt or more of RF, and solid state detectors can produce 10s or 100s of milliwatts. In the late 1920's and early 1930's, before superheterodynes became popular for Ham use, the QRM from oscillating regenerative detectors was a very serious problem on many Ham bands. Armstrong (who invented both the regenerative detector and the superheterodyne circuit the Elecraft receivers use) wrote open letters to Hams that were published in journals like QST begging them to add RF amplifiers ahead of their regenerative detectors. That's still true today for people using vintage designs from that era, but they are relatively few in number and we're assaulted by so much other RFI (read the recent thread on digital RFI) that they're seldom noticed. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
