TV detector van
[1] These vans have been used by the General Post Office and later by contractors working for the BBC to enforce the television licensing system in the UK, the Channel Islands and on the Isle of Man.
When first introduced on 1 June 1946, the licence covering the monochrome-only single-channel BBC television service cost £2 (equivalent to £104.79 as of 2023).
[1][3] In the 1950s, the Post Office, which then administered the TV licensing system, ran converted Hillman Minx and Morris Oxford estate cars, which had large aerials attached to their roofs.
[8] In 2013, the Radio Times obtained a leaked internal document from the BBC giving a breakdown of prosecutions for TV licence evasion.
[10] However, no mention was made of TV detector vans being used to catch such people, prompting media speculation over the truth of their existence.
This bandwidth was chosen to allow reception of a varying frequency, yet also to exclude as much ignition noise from the van's engine as possible.
If two nearby TVs were each tuned to different channels,[i] they then created a beat frequency effect which could swamp the TV detector.
Because of the broadness of the TV transmission bands, and variations in the intermediate frequencies used, a detector receiver could have needed to be tunable between 29 and 240 MHz.
This was achieved with a dipole antenna which was tilted diagonally, placed in front of a metal mesh corner reflector, the whole assembly of which could be rotated.
An unusual feature of the vehicle was an optical periscope, synchronized with the rotation of the mast, which identified the house from which the signals were detected.
To avoid the operator needing to move their head with the periscope,[v] a pair of prisms were used to produce the rotation, with a fixed eyepiece.
The initial input stage was based on an existing TV 'turret tuner' design,[vi] with manual selection across 14 bands spanning 110–250 MHz.
As the number of broadcast channels in use locally was very small,[vii] only a couple of the mechanically-switched bands would be needed for each detection search.
Two new problems needed to be addressed for detection of the new sets: firstly the emissions were both higher in frequency and lower in signal strength than before.
Secondly, the propagation of signals at this wavelength and their tendency for confusing reflections from many nearby surfaces meant that the previous triangulation method from a couple of positions was no longer reliable.
[7] A suitable broadband aerial for the frequency range of 470–860 MHz was a log-periodic spiral, wound on a six-foot long conical former.
At low frequencies, the large spacing needed required an extension of the van's roof forwards in a prominent box.
A common misunderstanding was that the aerials moved in a scanning pattern when searching, as the VHF detectors had, but in fact they remained static.
The sets radiated from a number of sources, particularly the display controller and its low-voltage differential signaling link to the LCD panel.
[12] Modern sets follow a consistent internal structure of a separate front-end video processor and a display controller, each highly integrated to a single IC.
Signals within the video processor IC would be an obvious target for recognition, in a similar manner to older detectors, but these are far too low to be measurable outside.
[11] In the crudest manners, a discernible video signal can be recognised, but this would be hard to tie to a specific broadcast, or to an evidentiary standard.
According to the Comptroller and Auditor General of the National Audit Office, "where the BBC still suspects that an occupier is watching live television but not paying for a licence, it can send a detection van to check whether this is the case.
"[17] In July 2019, an article in the Oxford Mail recalled a two-week period from June 1960 when an olive-green TV detector van—one of a fleet of nine owned by the BBC—toured the city to investigate 500 addresses with no record of a television licence.
The van contained a team of three people, "including a radio expert and a local Post Office official" and was equipped with a number of aerials that could be adjusted to pinpoint a signal being given off by a television at a specific address.
The article quoted Ron Smith, who was in charge of the operation, and explained the process: "When a TV set is switched on it gives out a radio signal.
The detector van then used its adapted radio receiver which is tuned to the wavelength of the signal and then converts it to a whistle transmitted through the technical officer's headphones."
Correspondence between TV Licensing and the affected householder may be attached to the completed application forms, which pass through a quality-control "gatekeeper" to the authorising officers (AOs) at the BBC.
