A small loop, although not very sensitive, when coupled with modern radios
(more than sensitive enough with a "real" antenna) is a good tool for
tracking down HF and even lower frequency interference sources. A few
years ago, I used a handheld loop from an old NF-105 EMC set and an AOR
AR-8000 receiver to track down Phonex modems on 3.52 MHz. (Ask me about
"the rest of the story.")
A light, one turn shielded loop may be made by simply bringing one end of a
coax near the other end's connector and connecting the center conductor to
the shield there. This is not the best balanced loop, but has a decent
null. A piece of semi rigid coax with an SMA connector makes a nice (also
cheap) loop and mounts easily on a handheld receiver.
One neat idea is to make a balanced loop from Radio Shack parts; a one foot
RG-58 jumper with PL-259's, a UHF series T adaptor, and a UHF female to BNC
male adaptor:
I cut off the crimped end of one center pin, and pull the center pin and
its plastic insert out of the connector shell. Then I strip the center
conductor back all the way to the shield, and push the center pin on its
insulator back into the connector, but this time, catching the center
conductor between the insulator and the coax shell. This grounds the center
conductor on that end and opens the center pin there. Then I plug each
PL-259 into one side of the UHF T adaptor, forming a loop. All I have to do
then is to support this loop at its balance point, so the T hangs down,
locating the physical and also electrical center, and remove a small strip
of insulation and shield from around the jumper at the center so the shield
is open at its midpoint.
Screwing the UHF/BNC adaptor into the T, I mount the resulting loop on a
radio like a VR-500 or other wideband radio. This is quite useful for HF
tracing and tracking, and is easily replaceable wherever a Radio Shack
store may be found.
Of course, you don't have to be that fancy. A BNC/banana adaptor
accomodates bare or insulated solid wire, and simply inserting a loop of
almost any wire -- even a coat hanger -- into the holes provided adn
tightening the posts, will also serve quite well. Another variation on teh
first coax loop above uses long piece of coax and simply makes a loop at
the end. Then plug the coax into your receiver or rig and use that as a
probe.
People pay a lot of money for probes you can make in ten minutes. But
apprearance counts; no one *believes* you when you've used coat hangers to
track down RFI.
Cortland
KA5S
> [Original Message]
> From: Frank N. Haas <kb4t@arrl.net>
>
> locate because it's pretty tough to lug around a 3 or 5 element Yagi for
2
> MHz. Since I have to do this several times a month, I need some
suggestions
> on how one can DF signals that are so low in frequency. How do you
> professionals out there DF signals at such low frequencies???
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