Detection of RF local oscillators by governments - Page 1
Forum section: electronics / general

author: watchmaker
author_rank: Unknown
timestamp: February 17, 2025, 02:30:39 pm
content: I could not find e thread where this was discussed. Apologies.The discussion of crystal radios led to a wikipedia link (https://en.wikipedia.org/wiki/Crystal_radio) that included the statement below:When Allied troops were halted near Anzio, Italy during the spring of 1944, powered personal radio receivers were strictly prohibited as the Germans had equipment that could detect the local oscillator signal of superheterodyne receivers. Crystal sets lack power driven local oscillators, hence they could not be detected. Some resourceful soldiers constructed "crystal" sets from discarded materials to listen to news and music. One type used a blue steel razor blade and a pencil lead for a detector. The lead point touching the semiconducting oxide coating (magnetite) on the blade formed a crude point-contact diode. By carefully adjusting the pencil lead on the surface of the blade, they could find spots capable of rectification. The sets were dubbed "foxhole radios" by the popular press, and they became part of the folklore of World War II.In some German-occupied countries during WW2 there were widespread confiscations of radio sets from the civilian population. This led determined listeners to build their own clandestine receivers which often amounted to little more than a basic crystal set. Anyone doing so risked imprisonment or even death if caught, and in most of Europe the signals from the BBC (or other allied stations) were not strong enough to be received on such a set.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 17, 2025, 04:19:08 pm
content: I don't know how accurately the technology was depicted or not, but the movieSoldier of Orangewas quite interesting. There's a soldier wearing a wrist-watch with a meter, to detect radio transmissions (wired to more equipment on his bike I expect, but it's not shown).

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2025, 04:34:32 pm
content: A print ad from E H Scott (ca 1942) about their low-radiation receivers to avoid detection:https://repository.duke.edu/dc/adaccess/R0142

author: RoGeorge
author_rank: Super Contributor
timestamp: February 17, 2025, 08:36:23 pm
content: My guess is, back then many receivers were of type regenerative, or super-regenerative radio, which is a very clever topology that reuses the same tube as RF amplifier, oscillator, detector, and audio amplifier at the same time, but with the price of radiating a lot back, into the receiving antenna.Those receivers were also unintended transmitters.They were radiating back the frequency you were listening.https://en.wikipedia.org/wiki/Regenerative_circuit#Regenerative_receiverhttps://earlyradiohistory.us/1922sup.htmhttps://www.qsl.net/l/lu7did/docs/QRPp/Receptor%20Regenerativo.pdf

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2025, 11:40:14 pm
content: Quote from: RoGeorge on February 17, 2025, 08:36:23 pmBy 1942, almost all short-wave receivers, especially in military/merchant marine use, were superheterodyne. with LOs.

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2025, 12:55:41 am
content: Without an RF stage before the first mixer, the leakage from the local oscillator can be quite strong. Modern receivers require an RF stage just to block local oscillator leakage and meet regulatory requirements about unintentional transmitters.There were a couple of times on 2 meter transmitter hunts where at the sniffing stage, I hunted the local oscillator of the intermittent transmitter when it was in receive mode. It definitely does work. Once this became a known tactic, our hiders would set their transceiver to receive on a non-standard frequency to hide the local oscillator leakage.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 18, 2025, 01:08:29 am
content: Quote from: watchmaker on February 17, 2025, 02:30:39 pmThere actually wasquite an extensive thread(11 pages) in this very sub-forum on the subject.

author: ME
author_rank: Frequent Contributor
timestamp: February 18, 2025, 06:58:18 am
content: Fwiw, Years ago we used to have an old french guy come and play a piano accordian in our bar in france, He had both thumbs missing, I asked him what happened to his thumbs, Turns out he and his bro got deported to jersey by the boschduring the german occupation of france, His bro got caught by the germans with a radio,the guy with the radio got shot dead and the germans cut his brothers thumbs off(the accordian player) as punishment.

author: watchmaker
author_rank: Unknown
timestamp: February 18, 2025, 12:28:49 pm
content: Quote from: Analog Kid on February 18, 2025, 01:08:29 amYes, that is the one I was looking for. Someone claimed it was impossible to do such detection in the 1950s; yet it was well known during WWII.