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

To: Rfi List <rfi@contesting.com>
Subject: [RFI] Flag Pennant loop - added benefit of being broadband
From: Alan Higbie <alan.higbie@gmail.com>
Date: Mon, 12 Apr 2021 09:56:47 -0600
List-post: <mailto:rfi@contesting.com>
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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