On 13 Mar 2014 at 10:47, Jim Brown wrote:
> 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.
It certainly is both of those: however, I already own it.
I don't have the other stuff, and really shouldn't spend the money on it
either,
although, believe me, I wish I could..
. 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.
That is certainly all very good advice and that is good information. However,
at this point, I FIRST have to determine where the crap is coming from. At
this point, I have no clue.
> 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 --
Exactly. At this point, I am hearing it best in the 80 meter band. Therefore...
> 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.
Yes. Understood.
> 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.
Again, yes. I have understood that.
> 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).
In fact, even using a BFO makes the noise much less "clear". AM seems to
provide the best way to recognize it.
At this point, I do not (yet) have a VHF receiver (except one of the
Stoddart's) which has an AM detector.
The NM-30 covers from 20 MHz through 400 Mhz, but it ain't small, and it
sure as heck aint very portable either.
The NM-20 is at least "reasonable" in size, requires low power (25 watts or
less), can be battery powered (although I would have to "make" the
batteries), and was designed to be at least semi-portable. After all, it has a
handle on top. ;-)
One of the best things you can say about it is that it is EMP proof.
Ken W7EKB
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