I'll comment further on my 0.5-Meter shielded loop. What's different is
that I have five turns of conducter inside the copper tubing. I've found
very little to nothing online for multi-turn internal conductors but lots
of single-turn shielded loops. It works well for receiving and DFing.
Good response even down to the submarine frequencies around 20 kHz and
extremely well for DFing through 50+ MHz without a preamp into a battery
operated portable receiver. For the 20 kHz stuff, I do use the preamp I
previously posted. Using the HP 3585 as a 'receiver' and without a preamp,
I typically obtain 20 dB (S+N) / N ratio at the submarine frequencies.
Location is N. Colorado.
Dave - WØLEV
On Fri, Mar 26, 2021 at 11:40 AM Don Kirk <wd8dsb@gmail.com> wrote:
> 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> 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> On
> > Behalf Of Gary Johnson
> > Sent: Tuesday, March 23, 2021 6:06 PM
> > To: 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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> > RFI@contesting.com
> > http://lists.contesting.com/mailman/listinfo/rfi
> > _______________________________________________
> > RFI mailing list
> > RFI@contesting.com
> > http://lists.contesting.com/mailman/listinfo/rfi
> >
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--
*Dave - WØLEV*
*Just Let Darwin Work*
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