Frank,
I found an EMCO loop on ebay some years ago, a shielded passive untuned loop
that covers about 20Hz - 5 MHz. Realistically, one could build a similar
passive loop for the 2-30MHz range in the same size, varying only the number of
turns and have a very usable untuned DF setup. But its almost 3 feet across. A
smaller design on a monopod/tripod sounds usable. The 6511 seemed to give
decent nulls through 40, but at the time I only used it for 80. (The AT&T modem
affair.) I installed it on a tri-magnet mount and since the Bug Catcher mount
was on the roof, the loop went on the hood; I turned the whole car for a null.
Think of it as a giant gunsight along the signal vector. A real "poynting"
vector.
You've been doing this a long time, so you know the problems trying to DF
powerline noise; even when I got to a CW source I had to distinguish between RF
conducted elsewhere and the actual emitter; one was in a housing development
but the strongest radiator was a resonant advertising post/mast across the
street. It turned out the 6511 did not actually help much; putting up the
Bugcatcher got me close, then walking the block with a handheld receiver and
untuned loop nailed it.
I may have mentioned that "a funny thing happened on my way to the modem" one
Sunday; people would run out of their homes and into their back yards as I
approached. Turns out it was Super Bowl Sunday; they thought I was from the
Cable Company and were hurrying to remove evidence of unpaid connections.
Back in the 80's I set myself to track down an arcing source in North Andover
MA; I drove the town with my HF mobile antenna, then switched to a 2 meter
beam, then a 440 beam and found the problem atop a pole at the local airport.
When I reported that to the airport they got action fast. But since 9/11 I'd
likely be arrested -- or shot.
Cortland
KA5S
-----Original Message-----
>From: "Frank N. Haas KB4T" <kb4t@arrl.net>
>Sent: Jan 20, 2011 9:49 PM
>To: 'Paul Christensen' <w9ac@arrl.net>, rfi@contesting.com
>Subject: Re: [RFI] HF DF Antenna recommendations
>
>The National RF HF DF products do not cover the spectrum between 2 and 8
>MHz. I spoke with the engineer at National RF about the 80 meter band. He
>told me that they had started working on a loop for 80M but they never
>finished it. Other projects demanded their time and the 80M loop was set
>aside. They told me that there simply wasn't enough demand to justify
>investing the development time. He also cautioned me that the 80M loop would
>likely end up being rather tallish.
>
>I asked how they thought the 8 MHz loop would work at 4 MHz. They told me
>that the nulls would likely be skewed and not as deep. The vast majority of
>my "HF only" cases were copyable to at least 7 MHz. In the majority of those
>cases, as I got closer I could hear the RFI at least one more MHz higher.
>Those cases which are no higher than 4 MHz will just have to be worked with
>the G4TPH magloop. For all the other "HF only" cases, the National RF Model
>HFDF should do nicely and be a great deal easier with which to work.
>
>In any case, I elected to proceed with my order for the National RF Model
>HFDF active antenna device and all 4 available loops. The loops cover 8 to
>50 MHz. If the system works very well, I might commission an additional loop
>that covers 80 meters as well.
>
>As to popular 80M handheld DF units popular with ARDF competitors, there are
>some commercial unit available in Europe but none in the U.S. that I know
>of. I don't know the names of any of the European units and, frankly, I
>haven't tried to find out.
>
>One person who contacted me told me about Wellbrook, a UK firm that makes
>nice active loop antenna systems. (http://www.wellbrook.uk.com/antennashop/)
>The model I would like to try costs nearly $400 including shipping. While
>I'd be willing to spend the money, the form factor doesn't look like it
>would suit my need to be easily mobile and on foot.
>
>I've been told that many ARDF competitors build their own handheld 80M DF
>units. There isn't much published info about current designs. In fact, I
>have only found one elderly article about a homebrew unit. Parts for that
>design have become obsolete and unavailable. I'm not interested in building
>anything anyway. Perhaps others on this list know the brand names of popular
>commercial European 80M handheld DF units used by ARDF competitors?
>
>73,
>
>Frank N. Haas KB4T
>Utility Interference Investigator
>Florida
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