In a message dated 96-12-03 11:07:26 EST, you write:
>
>All I can say is -- don't waste your time and effort building low pass or
>band filters expecting to eliminate these "sigs". Some people evidently
>don't hear them because they operate on the same freqs. Are these the
>people with no towers--just wire antennas?
>
>One question for Ed. Are you sure that these "sigs" started up after your
>ice storm and were not there all the time? Like Ed--I would like to hear
>any plausible explanatrion for this "phenomenon".
>
>73 Dave N4SU
Hi Dave,
We can be sure these signals are mixing products. The only thing we can not
be sure of is where they mix. I think a simple answer is out of the question.
I have personally observed mixing products occuring in poor electrical
connections in cable TV (where aluminum cable was in contact with steel
support strand), telephone wires, fences, towers, and even my own antennas.
Of course, any pre-amplifier or receiver is subject to this effect. It could
never come from mixing in a BCB transmitter, because both the ingress signals
and the radiated mixing product are attenuated by the tuned circuits in the
system.
One way to tell if the product is occuring in your receiver is to disturb the
system by adding a high pass T network tuner, or an attenuator pad. If you
observe a much greater change in level of the garbage than desired signals
(when attenuation or high pass filtering is added) you can bet the garbage
comes from your own equipment being overloaded.
Changing directions on antennas nearly ALWAYS changes the levels of these
signals, regardless of the source location. That test proves nothing.
I had to build a BCB notch filter for my 751A to eliminate this problem,
adding a ~50 dB notch on three very strong local stations. A conventional
high pass was not enough. I would be amazed if conventional pre-amplifiers
using devices like MAR amplifiers are immune to overload, because it is
usually the total POWER the amplifier carries that is the problem, not the
level of any individual signal. I have to use high power (5 watt) CATV line
driver transistors here to avoid overload, small FETs and bipolar devices are
not intermod free unless the inputs are filtered by mono-band bandpass
filters.
73 Tom
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & KM9P
|