On Fri, 16 Jan 2009 17:05:45 +0000, Mike wrote:


Sounds like what I read many moons ago. Quite detailed info on the
operation and technology of the detector vans were published in the
Post Office Engineering Journal - that's when they had things inside
the vans rather than just a roof rack with a Yagi bolted on.

The issue was somewhere from the mid 70's to the mid 80's. Not sure
if a local reference library will still have them and I'll bet no one
has got round to scanning them and putting them online.

It was in "British Telecommunications Engineering", Oct 1984, entitled
"Television Detector Vans". I have that edition in front of me, but my
scanner's playing up at the mo... (Of course the article will be
copyrighted anyway!).

The van described looks for local oscillator radiation, using "two
connected and identical fixed aerials, adding in phase the received
signals, produce a polar diagram that is the result of multiplying
together trhe two expressions for their individual polar diagrams;
this gives a multi-lobed pattern, which is dependent upon the spacing
between the two aerials. Electronic adjustment maintains the lobe
pattern for changes of frequencies.
"The aerials used are broadband narrow-beam reflector type, each with
four broadband elements and a panel reflector. Four of these aerials
are mounted on the roof of the vehicle, two facing each side."

There's a panoramic display unit, and recording facilities linked to
take into account vehicle movement as it drives along the street.

Quite complex stuff really :-)

--
Frank Erskine