[Elecraft] Question from a newbie > noise blanker

[Jack Smith]

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" <ny9h at arrl.net>
> To: "Tom Hammond" <n0ss at earthlink.net>; <elecraft at 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.
>>
>>
>
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