Don,
Thanks for the update. It sounds like I have the right loop for
the job as even my main rig and antenna hear nothing down on the AM band (other
than many AM broadcast signals), and my interference gets really loud on 20
meters and higher frequencies.
I can just barely hear the noise down here in my basement using
the loop on 6 meters. I plan to start outside near my vertical when the rain
quits tomorrow. I will keep everyone posted.
73,
Joe kk0sd
From: Don Kirk <wd8dsb@gmail.com>
Sent: Friday, March 26, 2021 6:40 AM
To: Gary <gary_mayfield@hotmail.com>
Cc: Gary Johnson <gwj@wb9jps.com>; rfi@contesting.com
Subject: Re: [RFI] New Interference at kk0sd
Hi Gary,
While waiting for your upcoming weekend testing I did more investigation into
why the un-tuned shielded loop you use appears to have such low sensitivity on
MF and lower HF frequencies compared with a similar sized unshielded un-tuned
loop. I wound up building a conventional un-tuned shielded loop so I could
compare it with one based on the design you used (see attached sketch), and the
results were interesting (read below).
Also in search of some technical documentation I also contacted Ian (VK3BUF)
who demonstrates an antenna online based on the design you used but he was not
aware of any technical documents related to the design of this antenna (he was
very kind and responded quickly to my e-mail).
Here is the link to the Radio Amateur Society of Australia youtube video that
shows the construction of the loop you built based on their design for those
not familiar with it: https://youtu.be/Ivr9VbSgjUU
I compared a traditional un-tuned shielded loop with the design you used, and
both of my loops were approximately 14 inches in diameter. Bottom line is that
the antenna you use which is not based on any kind of traditional design that I
could find is very deaf down in the AM broadcast band (even with a preamp), and
it's performance definitely falls off below let's say about 10 MHz (kind of an
arbitrary cutoff point) . Both antennas had similar sensitivity around 15 MHz,
and above 15 MHz the design you use has a slight advantage (slightly better
sensitivity). All of my tests included no impedance matching, and I did not
test above 30 MHz.
The traditional un-tuned shielded loop on the other hand works well on MF and
HF bands and has a sensitivity that's approximately equivalent to an unshielded
un-tuned loop of equivalent size based on my simple tests.
The only difference in construction between a conventional design and the
design you used is the way the center conductor on one end of the coax that
forms the antenna is terminated. The design you used terminates the center
conductor into open air (connected to nothing), while a conventional design
connects this one end of the center conductor of the coax to the shield (see
attached sketch).
No doubt the Australian design is the easiest to construct and would be useful
in most cases from let's say 10 MHz and above, but it's performance below 10
MHz really degrades and certainly is of no use in the AM broadcast band in all
but the most unusual cases.
Can't wait for your weekend test results.
Just FYI and 73,
Don (wd8dsb)
On Tue, Mar 23, 2021 at 8:21 PM Gary
<gary_mayfield@hotmail.com<mailto:gary_mayfield@hotmail.com>> wrote:
My loop has served me well. It sounds like I need to add another loop to my
arsenal. If the VHF effort strikes out I will go that route next.
Thanks Folks and All the help is appreciated!
73,
Gary "Joe" kk0sd
-----Original Message-----
From: RFI
<rfi-bounces+gary_mayfield=hotmail.com@contesting.com<mailto:hotmail.com@contesting.com>>
On Behalf Of Gary Johnson
Sent: Tuesday, March 23, 2021 6:06 PM
To: rfi@contesting.com<mailto:rfi@contesting.com>
Subject: Re: [RFI] New Interference at kk0sd
Resonant loops are absolutely the way to go with portable radios. My simple
80-15m design, quite similar to that of W0IVJ, is included in the extra files
with the current ARRL Handbook. Many people have built this from their junk
boxes. http://wb9jps.com/Gary_Johnson/RFI_files/Handheld_DF_Antenna.pdf
Sensitivity is outstanding on this antenna. For instance, I have no problem
standing in my CA back yard copying all of NA on 20 or 40m. Bandwidth is
sufficient to see most of a single band with my portable spectrum analyzer
(Anritsu MS2711) though its sensitivity is not as high as a cheap shortwave
radio.
As others have reported, nulls are sharp. This little lollypop antenna, at
nearly zero cost, and with no batteries or other stuff to cart around, is the
mainstay of my RFI hunting. Every ham should have something like this as a
starting point.
-Gary NA6O
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