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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