Actually, a swept spectrum analyzer is a very difficult machine to use
for tracking noise, unless the sweep is triggered with the power line
zero crossing.
If you look at a pulsed waveform from broadband noise with a spectrum
analyzer in free run mode (the normal operating mode) you may see some
noise spikes drifting around, perhaps moving to the left on the screen;
perhaps moving to the right. In the worst case, where the law of
perversity applies, the spectrum analyzer's sweep rate will be such that
no spikes are seen.
If the noise is power line generated and hence has a rep rate of 2X line
frequency, then switching the SA to line synch mode will cause the
spikes to stand still.
The reason for these effects is that a the spectrum of the gap-discharge
noise is a line spectrum.
If you hook an oscilloscope to a broadband receiver (such as the output
of a Z10000 buffer amplifier on a K2's post-mixer stage) and if the
interference is strong enough and the scope has enough gain (or an
auxiliary broadband amplifier is employed) and you set the scope trigger
to the power line, you can often see the RF pulse produced each multiple
arc and ringing. (The arcs are extinguished every half-cycle in the
ordinary case.)
If one were to go about an effective noise blanker, I believe you could
do much worse than building a new version of the old Collins approach --
a broadband receiver tuned to 30-35 MHz to detect noise and then a fast
gate (with suitable delay for synchronization) to clip the received signal.
The tunable subtraction units have, in my experience, highly variable
effectiveness. I have one here and there are some noise sources that it
will work with, but far more that it does not.
My experience is that most power companies will fix problems, but they
are often not well equipped or staffed to locate problems. The tools of
the trade include a wide band receiver in a vehicle, a hand held AM
receiver in the 200 MHz band with a built-in yagi, and an ultrasonic
receiver with a parabolic dish.
In the vehicle, drive around listening to the noise, and keep increasing
the frequency as the higher the frequency, the shorter the distance over
which it can be heard. If you are fortunate, this will localize it to a
few poles (in really severe cases, you can hear the noise up to 600 or
800 MHz). Then, the hand held 200 MHz AM receiver and ultrasonic
receiver are used to locate the specific pole or insulator.
Like most things, it takes a bit of practice and experience, but the
noise sources can be found. In my case, it took Dominion Resources 10
years to find and fix the problem that made it impossible for me to
operate below 10 MHz, unless it rained. It turned out to be an arcing
wavetrap on a 500 KV line, at a substation perhaps 5 miles from where I
live. The 500 KV line runs about 800 feet in back of my antenna and
Dominion's EMI techs kept looking for the problem on the transmission
line near my house. After the main EMI boss retired, one of his former
technicians was promoted and started the hunt from the beginning and
zeroed in on the substation.
Jack K8ZOA
www.cliftonlaboratories.com
Geoffrey Mackenzie-Kennedy wrote:
Perhaps something like the business end of an Evasive Noise Blanker?
Jack's PAN box would be a great addition for hunting the rubbish.
73,
Geoff
GM4ESD
----- Original Message ----- From: "Bill NY9H" <[EMAIL PROTECTED]>
To: "Tom Hammond" <[EMAIL PROTECTED]>; <elecraft@mailman.qth.net>
Sent: Monday, March 12, 2007 1:38 PM
Subject: Re: [Elecraft] Question from a newbie > noise blanker
with the FCC becoming more and more deaf to our pleas to fix broken
powerlines and the like ; wouldn't it be great for somebody ( hello
Larry LP & Jack PAN)
to make a NB widget hat was tunable width /depth and whatever
parameters are needed...maybe using his PAN box to examine the junk ???
I know that the scopes on icoms can tell a bunch abt the pulse junk.
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
