[RFI] New Interference at kk0sd

[Don Kirk]

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> 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> On
> Behalf Of Gary Johnson
> Sent: Tuesday, March 23, 2021 6:06 PM
> To: 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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