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[aprssig] APRS DFing SUCCESS with only an HT!

Ray Wells vk2tv at exemail.com.au
Fri Jul 27 23:25:39 UTC 2007


Dave Baxter wrote:

>Nice one Bob...
>
>I've used exactly the same technique to track down many sources of urban
>QRM in the past, however it does get "interesting" when it's something
>connected to overhead telco' or power wires, and they are radiating it
>too.  The usual fix, is to get as close as reasonably possible with VHF,
>then change to UHF for the final phase of the hunt.  (3rd harmonic)
>Trouble with QRM, it's not always predictable of course, and it's not
>unusual to find there is no 3rd "Harmonic", but there is another UHF
>signal that is related to the VHF signal in some way.
>
>I've also done the same with a portable broadcast MF radio on one
>occasion.  The combination of the Ferrite rod's "dipole" response
>pattern, and relative signal strength did the job.  That turned out to
>be a next door neighbours portable TV, but only when it was in standby!
>
>You do get some funny looks from Joe Public though...
>
>In some parts of the UK, if you just turn up in a farmers yard, for
>whatever reason, you will probably have some "interested and potentially
>hungry" large 4 legged canine's coming for you, accompanied by a shout
>of "Gedorf or moy land yer townie!"   (Get off my land you town person)
>And if realy unlucky, two rounds of rocksalt coming at you too!...
>
>As always, take care, and stay aware of your surroundings, and events
>thereof!...
>
>Now, if I could only get the HF QRO thermostat to identify its location
>on APRS, a large area of North Bucks and South Northants in the UK would
>have some happy HF operators in the winter.
>
>I and others have failed for the last 10 years to find that particular
>bar steward!...  It can be heard on 70cm's some miles away from where I
>suspect it to be.  Close in (within a few miles) it's just too loud to
>get any bearing on, even on 70cms, and it's signal strength varies with
>each burst too, just to add insult.
>
>I think I mentioned APRS, so very vaguely on the edge of the topic...
>;-/
>
>73.
>
>Dave G0WBX.
>
>  
>

I spent a number of years locating and fixing sources of RFI/TVI for my 
employer at the time, an electricity supply authority. This work was on 
240/415v and, 11, 33 and 66kv systems, although one situation had me 
locate a faulty insulator on the railway's 1500V DC system. That one 
took two years to find because only certain trains would cause it to 
happen, and the next train would cause it to stop!

The tools I used were:
1. the eyes - to look for something obvious like a piece of wire 
dangling on a HV line. This was quite common in the Magpie nesting 
season when they'd stop for a rest and leave the wire behind.

2. the 80MHz mobile in the vehicle fitted with a limiter current meter 
to provide signal strength. In conjunction with that I used a crystal 
oscillator/diode multiplier to produce a weak, off-frequency signal to 
give the radio something to listen to.

3. a broadcast band radio.

4. A piece of commercial equipment that was essentially a crystal set 
driving a speaker and a meter. This mounted on the end of a live-line 
stick for probing around insulator strings.

5. an ultrasonic detector - ultrasonic microphone at the focal poiint of 
a small (about 14") parabolic dish. This was the most effective tool of all.

6. lots of leg work, and thousands of hours in a cherry picker. The 
latter served two purposes. One was to enable me to get real close to 
the potential source of the interference (it didn't always radiate 
downwards to be heard at ground level), and two, to apply a remedy which 
may have been conductive grease applied with a paintbrush on the end of 
a live-line stick or, stainless steel clips that were forced in the gap 
of the tongue and clevis between insulators in a string. The clip 
straddled the clevis pin, and was held in place with little "teeth" that 
dug in.

7. Lots of learned intuition.

Sometimes the interference came from a consumer's appliance which put me 
in a difficult postion because, mostly, we had no authority to make the 
owner fix the offending piece of equipment (unless it was causing the 
supply voltage to fluctuate). However, I could usually manage to get the 
customer to come to the party by telling them that although we can't 
make you fix it, we can tell all your neighbours where it's coming 
from!! Diplomacy?

That's enough of my ramblings.

Ray vk2tv






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