Hi Jim,
I wanted to follow up regarding the link that you provided to the NA6O
tuned loop. While it has some nice features, I'm concerned it still might
be a little too complicated (intimidating) for folks just starting out and
thought I should share an alternative design that I use to show how easy it
can be. Here is a link to one of my tuned loops that's just about as
simple as it gets for a tuned loop, and it covers 7 to 32 MHz with good
bi-directional properties throughout.
https://sites.google.com/site/smalltuneddfloop/home
I just used 2 single turn loops of wire (one turn for the actual antenna,
and one for the pickup loop). The antenna bi-directional properties are
just fine using the direct connection of a short section of coax to the
pickup loop (I used 42 inches which is plenty long) as long as you tune the
antenna to resonance (no transformer used besides the transformer action
between the main loop and the pickup loop, no common mode choke used, and
the pickup loop is as simple as it gets, etc). I suspect the transformer
action between the pickup loop with the main loop as well as the higher
signal strength from the tuned loop negates the need of a balanced pickup
loop like NA6O used or a choke on the feedline to maintain reasonably good
balance/symmetry. His use of a balanced pickup loop (balun based on its
design/geometry) might provide better symmetry (both nulls of equal depth,
etc.), but exactly equal null depth is not an absolute requirement for
simple radio direction finding of RFI . Nothing wrong with what NA6O used,
and he used some good robust concepts, but I'm just afraid it can be too
intimidating of a build for folks that need a simple/quick antenna for
tackling their immediate RFI issue.
Just thought I should share this simple design as an alternative that
others could try, as I really like to keep designs as simple as possible
while still providing good performance for radio direction finding so
others can quickly build something they can use to attack their immediate
RFI problem. While this antenna does not go below 7 mhz, a parallel
capacitor could be switched in similar to what NA6O does, but I really
don't think an enclosure is necessary to accomplish this. For specific
cases one could just solder in the parallel capacitor to drop below 7 MHz
if they needed to use the antenna below 7 MHz as an example, or they could
just use a mini toggle switch attached to the wooden mast.
I also have a tuned loop of similar design that uses multiple turns on the
main loop to provide coverage from 1.4 to 4.4 MHz but I suspect it looks
too intimidating for beginners (even though it really is the same concept
but uses PVC pipe construction and multiple turns on the main loop based on
a simple build by K2XT ). I think beginners should just go with the single
turn of wire for the main loop and single turn of wire for the pickup loop
for their first build of a tuned loop for radio direction finding to keep
things simple.
Just some thoughts and opinions from my end.
73,
Don (wd8dsb)
On Tue, Jul 20, 2021 at 1:23 PM Jim Brown <jim@audiosystemsgroup.com> wrote:
> The typical home in the US has dozens of electronic noise sources,
> mostly switch-mode power supplies (SMPS), but also devices that have
> microprocessors built in -- everything from refrigerators to home
> entertainment products to computers to battery chargers. Our antennas
> hear those sources and those of our neighbors. The clearly defined peaks
> and straight verticals (stable frequency) tell us that it's running on a
> clock, so most likely something with a microprocessor.
>
> RFI, however, is like peeling an onion -- eliminate the strongest noise
> sources, and we'll see many additional weaker ones. This app note ran in
> National Contest Journal several years ago, and the slide deck was for a
> talk at the Visalia DX convention.
> http://k9yc.com/KillingReceiveNoise.pdf
> http://k9yc.com/KillingRXNoiseVisalia.pdf
>
> NA6O has shared his design of a loop for direction finding here.
> http://wb9jps.com/Gary_Johnson/RFI_files/Handheld_DF_Antenna.pdf
>
> And DXEngineering is preparing to sell a much larger one designed by
> WD8DSB. https://www.dxengineering.com/parts/dxe-noiseloop
>
> 73, Jim K9YC
>
>
> On 7/20/2021 9:18 AM, Ramakrishnan Muthukrishnan wrote:
> > I have been experiencing QRM from some unknown source for the past
> > several months, a picture of it is here:
> > <http://rkrishnan.org/files/40m-qrm.png>
> >
> > This happens in every band and is repeats at 20khz. The signal is steady
> > and not wobbly like one sometimes see in SMPS created QRM. When
> > listening to SSB, I turn on the notch filter function on the SDR to get
> > rid of the "carrier" like signal. But that only does a bit.
>
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> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
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