[Premium-Rx] Radios and High RF powers

[Richard Reich]

Hello list.

The problem of making receivers work in strong RF fields is not a new one. 
I would suggest that one of the prime criteria for the design of a 'military (or
commercial) application' receiver - our premium receivers - is that it should
work satisfactorily in very high RF fields (we are talking of Volts not
milliVolts). 
In the commercial world - where there is a receiver, there is usually a
transmitter in action very close-by. In reality, this design requirement may
actually make professional receivers a little insensitive (perhaps almost deaf)
compared to some ham equipment - but some of us know and accept that - you can't
compare the two radios at all. Putting a wideband active antenna at the input
un-does all the work the designer put in place with regard to the 'bullet proof'
front end. Professional users of these radios deal with the problem of
'blocking' and 'splatter' by fitting notch filters at the frequencies causing a
problem - usually a cavity type, so that a high Q is achieved. On-frequency
stability is maintained by careful selection of metals, and design, so that
temperature effects (the main cause of canned filter drift) are cancelled out. 
Unfortunately, it must be remembered, receiver specifications are written to
impress the less informed reader (usually an accountant) - to sell the radio.
The upshot being, don't believe everything you read. I like to think that I have
always approached radio engineering problems with an open mind, wearing a
'common sense hat'.
If your radio is being overwhelmed by something you don't want to hear - notch
it out before it eats your mixer or RF amp.
Notching is easy when dealing with broadcast transmissions because they don't
change frequency by very much (usually just a few 10s Hz).
I have worked at many professional broadcasting stations in the UK where there
are hundreds of transmitters (kWatts) and receivers in close proximity - and
there are few problems to speak of, because of thoughtful design. I can't think
of one single installation where an active antenna is used (apart from receiving
a LW time signal).

Regards to all....  Richard

***************************************************************
Richard Reich    
Principal Engineer Hardware
SAAT Technology Ltd
Web: http://www.saatt.co.uk

DDI: +44(0)1420 545388
Fax: +44(0)1420 87259

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