Hi Ram,
I recently built and tested side by side numerous direction finding tuned
loops, untuned unshielded loops, untuned non shielded loops, loops with
chokes, loops with transformers, terminated loops, etc.
Based on all my testing the simplest but still reliable bi-directional
antenna for MF and HF portable direction finding that you can go with would
be a simple single loop of wire similar in size that Dave (W0LEV) mentioned
(12 to 17 inches in diameter) fed directly with coax, but you need to use a
choke right at the antenna feedpoint consisting of 14 turns of coax (RG58
or RG174, etc.) wound on an FT240-31 toroid core (this choke is based on
the K9YC choke design). I would use enough coax to get the 14 turns of
coax wound on the toroid core plus another 36 inches for running to your
preamp or receiver input. The output impedance of this single turn loop is
reasonably low, and you can therefore connect it direct to the 50 ohm input
on your preamp or receiver, etc (no transformer required). If you omit the
choke, the antenna balance is not reliable depending on what frequency you
use the antenna on (the feedline becomes part of the antenna which then
alters the pattern and you will find big differences in the depth of the
nulls and even experience only one null depending on how high in frequency
you go). This antenna will be bi-directional if built properly.
The other simple option that I prefer is a untuned shielded loop like Dave
mentioned made with a piece of coax. I tested numerous designs, and the
most reliable and easy to build untuned shielded loop for full MF and HF
coverage is what I call a traditional untuned shielded loop in which the
shield of the coax that makes up the antenna has its shield exposed (jacket
removed) near the antennas connector and the far end of the coax (coax that
makes up the antenna) shield and center conductor connect to each other and
they are then soldered to the shield that was exposed near the antennas
coax connector. Then about 1 inch of shield on the coax that makes up the
antenna is removed at the center of the loop which was formed by the coax.
You can then feed this simple untuned shielded loop directly into your
preamp or receiver since once again the impedance of this untuned shielded
loop is reasonably low (no transformer needed). A choke is not needed
because the shielded loop has a built in balun based on the geometry of the
design. This antenna is really my favorite portable untuned bi-directional
loop. Once again I would make it about 12 to 17 inches in diameter (the
larger it is, the more sensitive it will be but you need to stay 0.1
wavelengths in circumference or smaller).
The signal strength of the interference will dictate if you need a preamp
or not regardless of what portable MF/HF direction finding antenna you
use. A 20 dB preamp will normally be adequate in all but the weakest
levels of RFI. My favorite preamp is the DX Engineering preamp that was
designed for use with my portable flag, but I also like the W7IUV preamp
which you can power with a 9 volt battery for portable use.
My favorite antenna for direction finding is the portable flag I designed
for radio direction finding that Jim (K9YC) mentioned since it's
unidirectional and this antenna saves me an enormous amount of time since I
instantly know what direction the RFI is coming from (no need to
triangulate like you have to do with a bi-directional antenna in order to
determine what direction the signal is coming from). Here is my simple
website that describes the portable flag, and it really is an easy antenna
to build, but once again you will likely need a preamp.
https://sites.google.com/site/portableflagantenna/
You could also just go walking around with a portable shortwave receiver
and whip antenna looking for the strongest signal, but that's really a
process that can cause a lot of indecision and guessing.
P.S. it does you no good guessing what device is generating the RFI as long
as you understand its behavior (example: on 24 hours a day, not drifting,
what frequencies you hear it on, etc). You first must locate the property
where the RFI is coming from using radio direction finding gear, and then
you can figure out what the device is.
73, and let me know if you need more details on the antennas I mentioned
above. I have lots of pictures and diagrams I can share with you, and they
really are simple to build.
Don (wd8dsb)
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