[TenTec] "Apparent" birdies in receivers
16 Jul 1997 09:37:48 -0500
The quotes are deliberate.
It is an improper test to just disconnect an antenna, and then say any stray
signals you hear are an unacceptable level.
Why? Because you have gone from a lower impedance condition of loading on the
input by the antenna, to a high impedance, and thus you have upset the circuit
parameters from normal design/operation assumptions.
If you carefully maintained the same load substitute for the antenna and then
had spurious response, you might have a concern. However, when you connect
the antenna in the presence of a "dead band," doesn't the noise floor still
rise? If so, that noise floor should be your reference for measuring spurious
responses, not an artificial noise floor of the receiver circuit alone. What
you really want, is to not hear spurious responses as high as the weak signals
you are trying to receive on your usual antennas, under usual band conditions.
No mixing scheme is perfect. Look at the math, you have products of mixing
that may fall into the bands of interest, unless prohibitively costly
different schemes are used on different bands. A number of our ham bands are
harmonically related which enters in to design issues.
All should read the excellent Ulrich Rohde papers on modern receiver design
that have appeared over the years, in "Ham Radio" and other magazines.
I would take Ten Tec receivers over the Yaecomwoods, (a few of which I own),
any day, to get the "weak one".
Just had to jump in and make sure apples are compared to apples.
Impedances must be considered and maintained in any testing.
73, Stuart K5KVH
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