Topband: FW: The WD8DSB mini-flag antenna (LONG!)

[Don Kirk]

Hi Rick,

This next piece of info is not really related, but here is another data
point for folks since you mentioned using an AM radio with built in bar
antenna tuned to 1710 KHz to track down powerline noise.  I just checked
the sensitivity of my portable flag versus built in bar antenna on a pretty
weak station on 1550 KHz.  With the built in bar antenna I can make out
(understand) the audio but not well, and with the portable flag and 20 dB
of preamp gain the station is loud and clear (using the same receiver which
is a DX-440).  Actually the portable flag with no preamp receives about the
same as the bar end antenna on 1550 KHz on the DX-440.  Also checked this
station with a Sony Walkman that must have a bar antenna and I can detect
the station is there but not readable.  Therefore based on my tests the
portable flag with just 20 dB of preamp gain will easily outperform an AM
radio with built in bar antenna at the top of the AM broadcast band.  I did
these tests at 1pm and the 1550 KHz station is located 49 miles away and is
running 250 watts, and I show up outside their daytime fringe zone by a
considerable amount.  YMMV.

Here is another datapoint from Craig (VK3OD): "I also have access to the
R&S HE200 HF DF antenna that our version of the FCC uses. Even with its
preamp it will not detect the majority of noise sources that we encounter.
The portable flag can easily hear  the  Ethernet powerline adaptor buckshot
noise from my distant neighbours which this professional DF antenna wont
even detect."
Note: Craig was using an Advanced Receiver Research preamp which I believe
provides 20 dB of gain.  I'm not sure what frequency (band) Craig was
using, and I'm following up on that.

P.S. In my original article submission I mentioned the portable flag may or
may not hear weak levels of RFI especially on 160 and 80 meters where the
gain is very negative but I also said it's not as bad as it might appear
because the normal noise floor often allows us to give away at least 20 or
30 dB of gain (due to ones typical noise floor on those bands).
Unfortunately this information did not make the cutting room floor so to
speak.  Again, this antenna was not intended for anything but direction
finding but it certainly will find other uses.  Besides its unidirectional
properties its broadband capabilities is often very handy.

I do appreciate everyone's comments and number crunching (John:).  It's a
really interesting topic, and the antenna system thermal noise limitation
is often a hard topic for folks to understand.  The portable flag is
definitely not intended to be a receive antenna for weak signal use (DX) on
160 meters to clear the air (so to speak).

73,
Don (wd8dsb)

On Fri, Feb 26, 2021 at 11:47 AM Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

>
> On 2/26/2021 7:31 AM, John Kaufmann via Topband wrote:
> > I think there may be some semantic confusion over the term "averaging"
> and
> > how averaging affects noise when making spectral measurements, so let me
> > clarify what I mean.  My comments are specific to the P3 but are fairly
>
> Averaging is a nice technique that mitigates the poor sensitivity
> of the mini-flag, but only for "bright lines" due to power
> supplies, etc.  I would be surprised if it would do anything
> to improve DF'ing power line noise, which is clearly a major
> application domain.
>
> 73
> Rick N6RK
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