[RFI] New Interference at kk0sd

Fri Mar 26 22:29:23 EDT 2021

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 at gmail.com>
Sent: Friday, March 26, 2021 6:40 AM
To: Gary <gary_mayfield at hotmail.com>
Cc: Gary Johnson <gwj at wb9jps.com>; rfi at 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 at hotmail.com<mailto:gary_mayfield at 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 at contesting.com<mailto:hotmail.com at contesting.com>> On Behalf Of Gary Johnson
Sent: Tuesday, March 23, 2021 6:06 PM
To: rfi at contesting.com<mailto:rfi at 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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