Hello Don, Dave, Jim and others:
Thanks so much for all the wonderful information. I have started reading
the articles mentioned in order to build a DF loop. I think I am going
to try Don's design along with a pre-amp.
Apparently the next door neighbour (the houses are densely packed here,
so next door is "literally" 5-6 ft away) has a 4kVA inverter. Last night
when everyone is asleep, I stepped out with a shortwave receiver
(sangean radio which I borrowed from another friend) and could zero in
on the neighbour's house more or less. I initially suspected his fridge
and politely asked him to turn off the fridge for a few minutes while I
am looking at the spectrum. The birdies remained (assuming he actually
turned it off). He did mention that he has a 4kVA inverter powering
everything from refrigerator to other devices. That is the prime suspect
now because these birdies are present 24x7.
Going to do an on/off test of the inverter (if the neighbour is willing)
this weekend. Here in the India, there is very little one can do about
flagging violations. So, perhaps installing an x-phase based canceller
is probably going to be the only workable solution for my shack. :-( I
feel bad about letting a known source pollute the RF spectrum and I am
going to try my best to fix it somehow. May be buy toroids and wrap the
outgoing mains from his inverter or see if the chassis is properly
grounded on the outside.
73
Ram VU3RDD
On 21/07/2021 06:03, Don Kirk wrote:
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/
<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)
--
Ramakrishnan
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