Topband: The WD8DSB mini-flag antenna

Wed Feb 24 16:55:55 EST 2021

Hello Don and John:

DXE has not measured the noise figure of the loop preamp yet, but we will
soon and it will be posted on the part detail for the loop amplifier.
https://www.dxengineering.com/parts/dxe-nl-pre-att-1

I suspect the noise figure is about 3 dB.

I am using the WD8DSB noise loop and the handheld preamp here at K3LR to
find several noise sources. I measured the pre amp gain at 32 dB on 160 and
10 meters.

73
Tim K3LR

-----Original Message-----
From: Topband [mailto:topband-bounces+k3lr=k3lr.com at contesting.com] On
Behalf Of Don Kirk
Sent: Wednesday, February 24, 2021 3:33 PM
To: john.kaufmann at verizon.net
Cc: TopBand List
Subject: Re: Topband: The WD8DSB mini-flag antenna

Hi John,

You started this mess (or maybe I did), and finally here is my response to
a few of the questions you had and thanks so much for waiting in line for
my response.

1)  As I have mentioned in a few other responses I suspect having the short
10 foot feedline helps to negate common mode noise and direct signal
ingress into the feedline that often haunts us.  I do have one
recommendation about pattern distortion as follows.  Check to make sure the
peak and the null of the antenna are in agreement on the signal you are
direction finding.  If you notice a slight skew (where they don't agree
with each other), then move away from existing objects and this will
correct that problem.  I sometimes notice a slight skew when in my backyard
near my house (looks like the null shifts slightly from where it should
be), and when I get out away from my house (I move to the sidewalk in front
of my house) the slight pattern skew goes away.

2)  I don't know what the DX Engineering preamp noise figure is, and there
definitely is no expectation from the designers standpoint that it's
spectacular.  Even though I did most of the field testing of this preamp I
have no idea what the amplifier part of the circuit is as DX Engineering
tightly controls their designs.

3)  My antenna does not have a long mast, and it easily fits in the back
seat of my very small 1996 Saturn Station Wagon.  It fits widthwise in my
backseat, and I have to imagine it will do the same in almost any car since
my car is about as small as they come.  Looks like the DX Engineering
version of my antenna has a longer mast, and that can easily be shortened
if it helps you fit the antenna into other areas of your vehicle.  I only
use the extension on my antenna if I'm doing some stationary tests in my
backyard, otherwise I use it as shown on the cover of QST.

I Really appreciate you jumping in so quickly on the antenna build and
reporting some of your results.  Sometimes when I have a few minutes I will
go outside and just play with the antenna and marvel on how simple it is,
but how well it works.

I hope I have now covered the majority of everyones questions and
comments.  If there are other comments or questions we probably should move
this discussion over to the RFI Reflector or e-mail me direct.

73,
Don (wd8sb)

On Tue, Feb 23, 2021 at 1:26 PM John Kaufmann via Topband <
topband at contesting.com> wrote:

> 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
>
>
>
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