> The most common suggested method was the detection of a signal from the TV's local oscillator.
See: superheterodyne receiver. Most modern radio receivers generate high-frequency signal for tuning, which leaks as radio signal (receiver becomes in some way a transmitter). However, it's not confirmed if these vans had working detectors, or were just for intimidation.
There were also stories about:
- Radar detector detectors (in places where radar detection devices in cars were illegal)
- Broadcast radio receivers detectors installed near roads to measure popularity of radio stations among drivers
They had a couple of actual working vans, but these were purely for technical demonstrations.
The rest of the vans were labelled up and they'd drive around to places that didn't have a licence (For a while TV Licencing had the largest civilian database in the UK) and knock on doors.
Some of the "inspectors" used, and still use, pretty unpleasant techniques.
I'm generally in favour of the licence. I don't need one right now, and I've found it easy to get the letters to stop (I fill out an online form), but some friends get letters almost fortnightly. They have mental ill-health, and they find the letters distressing. It's annoying to me that TVL / Capita aren't better at supressing those letters.
You look for the IF from the tuner, almost all tuners are made using the exact same frequency building blocks, and thus the tuner can easily be detected and knowing which channel it is tuned to.
There's a reason all the BBC channels are on the same DTV mux with ITV & others on different muxes - it means the BBC channels that require a license are all on the same tuner frequency for a local area.
