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

[Dave Cole]

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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