Topband: The WD8DSB mini-flag antenna

[John Kaufmann]

Some of you may have seen the article by WD8DSB in the latest issue of QST.
I believe WD8DSB is on this reflector.  His article describes a mini-flag
antenna that can be used for direction-finding.  The neat thing about this
antenna, besides its compact size, is that it is unidirectional and is very
broadband.  It works from the AM BCB through 10m.  It produces a sharp null
off the back which allows you to determine signal direction without the
direction ambiguity you get with a conventional unterminated loop.

 

DX Engineering is producing this antenna as a kit, along with a companion
preamp.  (Disclaimer:  I have no affiliation or commercial interest in DX
Engineering).  See:  https://www.dxengineering.com/parts/dxe-noiseloop.  I
just bought the flag kit last week and finished assembling it this past
weekend.  I see today that the kit is now back-ordered until April so it was
good that I ordered it as soon as I saw the QST article.  

 

It took me about 3 hours to assemble the mini-flag even though the DXE Web
site says it can be done in 1-2 hours.  There is a bit of fussy mechanical
assembly involved in getting the symmetry and dimensions just right,
although it's not hard work.  The flag is 42 inches wide and 21 inches tall.
The DXE version of the antenna has slightly smaller dimensions than those
given in the QST article, which results in a small reduction in gain, which
doesn't really matter, but the pattern is the same.

 

I did some testing of the mini-flag in the AM BCB.  The gain is very
low--about -65 dBi on 160m--so it needs a good preamp.  I used a homebrew
preamp made up of a couple of MMIC's that produce about 35 dB of gain.  The
DXE preamp for this antenna won't be available until April.  On the higher
frequencies, less preamp gain is needed because the gain of the mini-flag
increases with frequency.

 

My initial tests indicate this antenna clearly works.  By rotating the flag
for the deepest null, I could nail the heading an AM BCB station to a few
degrees.  

 

This antenna could also be used as directional receiving antenna on its own.
Although it is not hugely directive, it can be rotated easily to peak or
null signals or noise, and it is better than a conventional unterminated
loop.  It has essentially the same RDF as other larger flag or pennant
antennas but is obviously far more compact.

 

This is a nice contribution by WD8DSB.  Now I have to go off with the
mini-flag and chase some local noise sources that have been plaguing me this
winter on the low bands.

 

73, John W1FV