[RFI] Flag Pennant loop - added benefit of being broadband

Mon Apr 12 18:10:33 EDT 2021

Hi Alan,

Thanks for posting about my portable flag for direction finding.  I was
trying to stay silent on this post, but thought I would provide a link to a
simple website I have that provides more information on the portable flag
including a plot of gain versus frequency which will help answer the
question about how broadband the portable flag is.  The link to my website
is: https://sites.google.com/site/portableflagantenna/

The antenna has very negative gain and that is why a preamp is typically
needed unless the RFI is very strong.  Also as you go lower in frequency
the gain becomes more negative.  Basically the gain increases by
approximately 11 dB for a doubling in frequency (example: on 7 MHz the gain
is approximately -41 dBi, and -30 dBi on 14 MHz), so lets say approximately
1.5 dB increase in gain per 1 MHz increase in frequency when operating
between 7 and 14 MHz as an example (just an approximation to put things
into perspective).  The broadband nature of the antenna is a real benefit
as you have mentioned when looking at wide sections of spectrum.  And yes
it really is interesting to see that powerline RFI varies, and is often
even band specific.

Also note that the DX Engineering preamp that was designed for use with my
portable flag also works great with un-tuned loops and tuned loops.  A
simple un-tuned shielded loop made with coax works great with this preamp
and it's now part of my toolbox of direction finding antennas as it too is
broadband.  I find myself using my tuned direction finding loops much less
in favor of broadband directional antennas, especially my portable flag
which is unidirectional and a real game changer in my direction finding of
RFI.

Just FYI, and thanks again for your post about my antenna design.  I make
no money from anything DX Engineering sells including the portable flag as
I agreed to not be paid to keep the cost of the antenna as low as possible.

P.S. here is a link to a simple youtube video I made showing the portable
flag pointed North and then South when I was looking for RFI near downtown
Indianapolis and it's a great example showing just how valuable the
portable flag is for uncovering (sorting out) RFI coming from opposite
directions that's impossible with bidirectional loops due to their 180
degree ambiguity.  https://youtu.be/qFm_gLlfZgE

73,
Don (wd8dsb)

On Mon, Apr 12, 2021 at 4:44 PM Alan Higbie <alan.higbie at gmail.com> wrote:

> Dave ~
>
> Adding a spectrum analyzer should be interesting.
>
> Getting it all configured for easy use in the field is the next logical
> step.
>
> I hope it is broadbanded.  While the SWR measured 1.1 to 1 from 160 past 10
> meters . . . I'm not sure that means it is truly broadbanded.
> When viewing a power line source I noticed that its strength was not
> uniformly the same across the 9 MHz I was looking at.  However, that could
> be caused by resonance along the power lines.
> I don't know - but it's all getting easier than it used to be.
>
> ~ Alan K0AV
>
> On Mon, Apr 12, 2021 at 1:56 PM Dave Cole <dave at nk7z.net> wrote:
>
> > Hi,
> >
> > You sir are a mind reader, I just started working on a FLAG for just
> > this sort of setup-- portable use of a broadband antenna.
> >
> > I am happy to hear it is as broadband as you say...  I will be taking it
> > and my SDR on a few walking trips soon...
> >
> > Being able to see 10 or so MHz. of spectrum is enlightening as hell once
> > you start seeing your RFI on more than one band at a time...  When you
> > dip one source, you see many signals dip at the same time, and you
> > realize they are all related!  That and they all look the same, just
> > shifted in frequency.  :)
> >
> > That blog post you cited, (and thank you for that), is part II of a
> > three part blog entry...
> >
> > Part I covers setup and use of the SDR for this sort of thing, and is at:
> >
> > https://www.nk7z.net/sdr-rfi-survey-p1/
> >
> >
> > Dave (NK7Z)
> > https://www.nk7z.net
> > ARRL Volunteer Examiner
> > ARRL Technical Specialist, RFI
> > ARRL Asst. Director, NW Division, Technical Resources
> >
> > On 4/12/21 8:56 AM, Alan Higbie wrote:
> > > In the past few days I've started using a Flag Loop antenna (designed
> by
> > > WD8DSB).
> > >
> > > I supplement it by also using a National RF HF-DF bidirectional loop.
> > >
> > > The patterns on both have been confirmed by using a known signal source
> > (RF
> > > signal generator).
> > >
> > > Having a unidirectional antenna is very helpful.  The null of the
> > > cartioid pattern is quite pronounced.
> > >
> > > Example: for years now, I have had a noise source which seemed to be
> > coming
> > > from 40 deg. at times - and other times coming from 220 deg.  I believe
> > > that the results had been confounded by re-radiation along the power
> > lines. Now
> > > I have now definitely determined that it is from 220 deg. (i.e. a giant
> > > step closer to location and resolution).
> > >
> > > An ** interesting feature ** of the Flag Loop is that it is
> broadbanded -
> > > maintaining its directional properties over a big chunk of spectrum.
> > > Unlike a tuned loop (or tuned yagi), a broadband antenna allows you to
> > view
> > > a huge piece of spectrum at once.  And with an SDR this is quite easy -
> > and
> > > very interesting.
> > >
> > > Dave, NK7Z, has a blog where he describes using an SDR to make site RFI
> > > survey.  He was using a fixed broadband antenna.
> > >
> > > Look at Dave's work on this subject:
> > > https://www.nk7z.net/rfi-site-survey-part-ii-interpretation/
> > >
> > > Having a broadband and unidirectional ROTATABLE antenna allows us to
> > > quickly and more definitively see associations among the various
> sources.
> > > And, simultaneously begin to find the RFI source.
> > >
> > > Yesterday I set my SDR to receive a section of spectrum from 3.0 to
> 11.0
> > > MHz.  Then I rotated the Flag Loop  - - and the results were very
> > > interesting:  I could see some very strong and wide signals.  These
> > > appeared across the spectrum and which rose-and-fell together (meaning
> > they
> > > are from the same device).  I had not realized that junk was there.
> > >
> > > Still don't know if these wide signals are ACTUALLY AFFECTING MY
> > RECEIVER'S
> > > NOISE FLOOR on any band.  To be determined.  Interesting nonetheless.
> > >
> > > *Why this matters*: a broadband / unidirectional antenna with SDR now
> > > allows a view of what kind of RFI is really out there.
> > >
> > > 73, Alan K0AV
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