File:British Post Office interference finding truck 1927.jpg

English: A radio direction finding truck used by the British Post Office in 1927 to find illegal radio transmissions. The source says that in addition to finding unlicensed radio transmitters operated by radio amateurs who neglected to get a license, it was also used in London to track down "bloopers" - regenerative radio receivers that produced interfering radio signals. The regenerative radio was one of the most popular types of radio during the 20s. However it used a vacuum tube with positive feedback that operated very close to its oscillation point, so when improperly adjusted it could radiate a radio signal that could produce shrieks and howls in nearby receivers. The interference problem got so bad that the British Post Office, which regulates radio communication in Britain, prohibited improperly radiating radio sets, and used this lorry to track them down.

The back of the lorry contained a sensitive 3 tube radio receiver attached to the loop antenna on the roof, which could be rotated by a wheel in the lorry. The loop antenna is only sensitive to radio signals in the direction of the plane of the loop, so by rotating the antenna until the signal is strongest the direction of the interference could be determined. Thus with a crew of two, a driver and an operator for the radio direction finder, the lorry could track down the house the interference was coming from. They would give the owner a warning and information on how to prevent the problem. In Britain, radio receivers had to be licensed by the Post Office, so If he failed to fix the problem, his receiver license could be revoked, so he could not operate the receiver. The US eventually also prohibited receivers from radiating interference.