On 3/13/2014 10:05 AM, Kenneth G. Gordon wrote:
The NM-20B, which is a high-quality, 7 band, receiver, with switchable BFO,
tunes from 150 Khz through 25 MHz, and includes various "input devices"
including a shielded tuned loop, and a device to allow the input to be
connected directly to the AC line.
That's a lot of effort to resurrect a box that is probably obsolete and
may not be very portable. K1TTT's advice to use a RX with a ferrite bar
or shielded loop antenna is a good one. Likewise his advice on using a
shorter antenna, or no antenna at all as the noise gets stronger. The
Kenwood TH-F6A DOES have in internal ferrite loop antenna, and it is the
default antenna below 10 MHz. If I didn't have one of these talkies, I
would use a portable consumer radio that has AM RX capability at VHF.
The Tecsun PL660 tunes the AM Aircraft band around 120-130 MHz. It's a
VERY nice consumer radio, and costs about $130.
Remember this fundamental fact -- if the noise source is ELECTRONIC
(computer, switching power supply, battery charger, plasma TV, etc.) we
MUST chase the noise on the frequencies where we are hearing it -- that
is, the ham bands; but if the noise source is impulse noise generated
by arcing, mostly in the power system, we can chase it most effectively
by listening at the highest frequency where we can hear it. THAT'S why
we listen with an AM detector, why we start listening at VHF, and why we
try to listen at UHF when we get close to the noisy pole.
Why is UHF important (and useful)? Because arcing produces noise that
extends well into the UHF range. Lower frequency components are carried
along power lines, a very long line can be the radiator, and we even
hear standing waves along the line. But the highest frequency components
don't travel well along a line, so the wiring very close to the source
becomes the most effective antenna. Thus, when you hear impulse noise
will at UHF, you're very close to the source.
Why is AM detection important? Because the noise is AM, and FM
detectors inherently reject AM (although they do detect AM a bit by
slope detection -- the variation in signal strength with frequency).
73, Jim K9YC
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