RDF (Not Iran) Question
 

Do radio direction finders only "work" (i.e. locate the signal) when a radio is transmitting? If a radio is just turned on, can RDF equipment locate it?

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I think it depends on the system... I recall that each node in the Mobile Subscriber Equipment (MSE) division-level cell network, which was state of the art in the 80s and fielded US Army-wide in the mid-90s, transmitted at all times. When I was in a divisional signal battalion the officers seemed disconcertingly nonchalant about the EW vulnerability that presented - a captain explained to me that since there were so many transmitting in a division area the enemy wouldn't know which one to shoot at, unsure whether they were hitting the division CP or a battalion rear support HQ.

For a more conventional radio, I think it's more complicated/harder. I'm not sure if there's some sort of RF frequency energy given off by the electronics that a sensitive receiver can pick up. (The KGB/GRU supposedly had monitoring equipment that could detect and record the internal signals in electric typewriters, parking trucks with antennas outside NATO installations, so it's possible).

Generally speaking, only transmitting sets are locatable with military type gear. That’s one reason you’re taught to keep transmissions short, use terrain masking, minimize power and in some circumstances to move after talking if you can.

There are two exceptions-

Theoretically, a very precise directional antenna, signal strength meter/spectrum analyzer, and a vehicle powered or generator powered superheterodyne receiver can always find the receiver by localizing the drop in signal strength and the emissions of the receiver’s local oscillator as the carrier wave is separated out, but for practical purposes this is limited to close detection distances with a receiver due to the low power involved. (Holding two radios close while transmitting on a third can generate oscillator interference.) This is similar in principle to the technology used by radar detector detectors or by the tv tax van in the UK.

Equipment that maintains a low power data “handshake” with a network like cellular phones, Mobile Subscriber, and high power cordless are vulnerable to location of the affiliation signal by specialized equipment. As these operate outside the standard military bands, this equipment may be less common by 2000. Some of the countermeasures to this type of location are frequency agility, power management, and location/use of signal attenuation camouflage (corrugated metal works great).

That said, powering off saves batteries. I could see radio transmission schedules coming back into play as batteries become short. The original rechargeable whuch would have been around at the time weren’t a great piece of kit.

A heterodyne receiver has a local oscillator running at the frequency you're listening on. The radio compares this local frequency with a received signal to pull out the intermediate frequency, the signal that was transmitted. While the local oscillator isn't necessarily transmitting anything itself, wires/leads in the radio can end up broadcasting this beat frequency or a harmonic of it. Since those wired aren't a tuned antenna it won't be a strong signal and can be blocked by the body of the radio it's definitely possible for a sensitive detector to pick up such leakage.

For a military radio with their nice rugged metallic cases picking up these spurious signals would be pretty difficult and only effective at relatively short ranges. A damaged or scratch built radio would be easier to detect and at slightly better ranges.

Another possibility, even on unpowered equipment is detecting the P-N junctions of semiconductors. You broadcast a particular frequency (depending on the size of the junction) and listen for a harmonic response. Again this is not necessarily easy because it's an untuned antenna and the inducted power is minuscule. It requires a specially built detector and relatively close range. Good metallic cases with proper wiring/grounding will block most such signals. Many "bug detectors" work on this principal.

Passive "foxhole" radios don't have local oscillators and can't be detected when they're receiving. But besides that it's definitely a possibility if the PCs (or OPFOR) has good SIGINT gear and trained operators.

I believe the TV tax vans only ever used detectors that would pick up the spurious broadcasts from the TV's CRT or BFO. The earliest detectors picked up signals from a TV's horizontal scanning circuit (well a harmonic of it) while later vans listened to the BFOs of the VHF and UHF detectors. They had highly directional antenna arrays on the top of the van and using a registered turret could take a bearings on detected signals. With two bearings they could triangulate the house with a TV and comparebig to their list of paid and expired TV licenses.