Topband: The WD8DSB mini-flag antenna

Wed Feb 24 14:23:17 EST 2021

Hi Dave,

I hate to recommend anything and others have given you some good
recommendations, nevertheless I will make a few comments.

My workhorse radio for direction finding is the good old Realistic DX-440
(also sold as a Sangen ATS-803A) and you can still find nice clean used
ones on E-Bay for less than $100 (I suspect I drove the price of them up).
It is a very old and large radio, but I love this radio for numerous
reasons.  It has an external antenna jack as well as an excellent
continuously adjustable RF gain control that really comes in handy.  It
also has a BFO for receiving CW which sometimes is very helpful depending
on the signal you are tracking.  It uses 6 D cells and has excellent
battery life.  I use the radio in AM mode for tracking down power line
noise, but sometimes I need to use the BFO for signals that are just a
carrier as it helps to hear the signal (no need to look at a meter, etc).

I also use a laptop based SDR receiver which is just a cheap RTL-SDR dongle
along with a ham it up converter (both from NooElec) and I have both of
these stuck to the back of my laptop screen so I can go portable with it,
and this is a very handy tool especially if the signal is wandering all
over the place or if I am in a heavily populated area with lots of
different sources of RFI present.  If you go this route make sure you pay
the few extra bucks for a TCXO in both of them.

The IC-705 is also a radio that likely would work well, especially since it
has waterfall displays for both the RF and Audio Spectrum, and it also has
an Audio Oscilloscope and I'm anxious to see how helpful that might be.

I see there is now a very small handheld HF SDR receiver called the Malahit
(looks like the size of  nanoVNAs), but I don't know how good they really
work.  If they become a trusted device I certainly will be interested in
using one for DFing.

73 my friend,
Don (wd8dsb)

On Tue, Feb 23, 2021 at 9:18 PM David Raymond <daraymond at iowatelecom.net>
wrote:

> Don. . .thank you for the very nice loop  article in QST.  And, thank
> you for your generosity of not taking compensation to help make it
> affordable.  The loop antenna would be a very useful tool to have in the
> arsenal for DFing interference sources of all kinds which continue to
> proliferate (even out in the country where I live).   I am currently
> experiencing an interference problem on 160m that is about 10-15 KHz
> broad with a couple of modest peaks. It often parks in the 1825-1835 KHz
> window on 160m.  The frequency range it occupies wanders some and varies
> inversely with the outside temperature.  I have DFd it fairly close with
> the HI-Z 8 circle array and know the direction of the source but need
> something portable to home in on it.  Do you or anyone here  any
> suggestions for a reasonably good portable receiver with an external
> antenna jack (might be hard to find?) that will cover the 160m band . .
> .and maybe up to 30 MHz?
>
> Thanks and 73. . . Dave, W0FLS
>
> On 2/23/2021 5:37 PM, John Kaufmann via Topband wrote:
> > As a follow-up to my original post, here are a few additional comments.
> >
> > Don, you mention that you designed the mini-flag for a deep null off the
> > back at low elevation angles, which is entirely understandable.  As I
> said
> > in my earlier post, the null is very pronounced in the AM BCB on local
> > groundwave signals.  However, I also see pretty significant nulls on
> > higher-angle signals, too.  Just a short time ago, I was listening to
> W1AW
> > on the low end of 160.  They are located only ~100 miles from me.  Their
> > signal has to be arriving at a pretty high angle, but the null is still
> > quite pronounced.
> >
> > My homebrew preamp, that I mentioned in my post, uses a cascade of UTO
> 511
> > and UTO 533 mini-amplifier modules.  I used this preamp, not necessarily
> > because it's optimal, but because I already happened to have it on hand.
> > The gain of the 511 is given as 16 dB typical while the 533 is 17 dB,
> which
> > should yield a net gain of ~33 dB for the cascade of the two.  The noise
> > figure on the 511 that serves as the input amplifier is specified as 2.3
> dB,
> > but its spec sheet gives an operational frequency range of 5-500 MHz, so
> I
> > can't be sure the noise figure (or the gain) holds up at lower
> frequencies.
> > Nonetheless I can hear the ambient noise in my receiver increase on 160m
> > when I connect the mini-flag to the preamp, which suggests the noise
> figure
> > for this preamp is at least adequate at my location.  I use a Yaesu
> FT-817ND
> > "backpack" radio as a portable radio with this antenna.
> >
> > In EZNEC I calculate the RDF of this mini-flag as 7.4 dB on 160m at a 20
> > degree elevation angle.  That's essentially the same as the K9AY loop or
> > other similar pennant/flag antennas.  For use as a receiving antenna, the
> > important thing is the noise figure of the preamp.  The DX Engineering
> Web
> > site does not give the noise figure of their preamp.  Don, perhaps you
> know?
> >
> > The other thing that might degrade the antenna is common-mode signal
> pickup,
> > which can be a problem for very low gain antennas where you are working
> with
> > very small signals.  However, based on what I observe in terms of antenna
> > pattern for this mini-flag, I can't say that I see any pattern effects
> that
> > might be attributable to common mode degradation.  Don, maybe you can
> > comment here as well on this aspect of the antenna.
> >
> > As I also mentioned in my earlier post, the dimensions of the DXE
> > implementation are somewhat smaller than what's given in the QST article.
> > For me, that works out well because the width of the DXE mini-flag just
> > manages to fit inside the trunk of my mid-size sedan.  A wider flag would
> > not fit.
> >
> > 73, John W1FV
> >
> > -----Original Message-----
> > From: Topband
> > [mailto:topband-bounces+john.kaufmann=verizon.net at contesting.com] On
> Behalf
> > Of Don Kirk
> > Sent: Tuesday, February 23, 2021 5:22 PM
> > To: wb6rse1 at mac.com
> > Cc: Top Band List List
> > Subject: Re: Topband: The WD8DSB mini-flag antenna
> >
> > HI Steve,
> >
> > Thanks for the nice implementation comment.
> >
> > The portable flag front to back ratio is highly related to the elevation
> > angle and frequency of operation (just like any terminated loop), and
> > therefore I did not want to overstate the front to back ratio in my
> > portable flag article.  I designed the portable flag for direction
> finding
> > local RFI (ground wave based signals) and therefore made sure I selected
> an
> > appropriate termination resistor to provide a very deep null at low
> > elevation angles on 160, 80 and 40 meters where I often deal with RFI
> (the
> > portable flag has a very high front to back ratio at low elevation
> angles),
> > and because of this it also has exceptional front to back ratio at low
> > elevation angles down in the AM Broadcast Band.  Very small flags have
> just
> > as good front to back ratio and RDF as a full size flag as long as the
> > appropriate termination resistor is used.  The problem is when the flag
> > becomes too large for the frequency of operation which causes the
> > directional properties to degrade.  You can see some front to back ratio
> > vs. elevation plots for my portable flag on my simple portable flag
> website
> > and here is the URL to that site:
> > https://sites.google.com/site/portableflagantenna/home
> >
> > Problem with very small flags is that the noise figure of the preamp
> > becomes a critical parameter, and because of this I don't recommend
> > attenuators be placed before the preamp as this causes degradation in the
> > signal to noise ratio.  I stumbled upon this issue when doing field tests
> > on one of the DX Engineering prototype preamps, and had them change the
> > design so the attenuators now come after the actual amplifier stage which
> > solved the problem.
> >
> > Everything I said above about the performance of very small terminated
> > loops assumes no interaction with surrounding objects, and ignores issues
> > related with feedlines since the feedline is very short on the portable
> > flag.
> >
> > P.S. I make no money from DX Engineering as I agreed to not be paid in
> > order to keep the price of the portable flag as low as possible.
> >
> > 73,
> > Don (wd8dsb)
> > _________________
> > Searchable Archives: http://www.contesting.com/_topband - Topband
> Reflector
> >
> > _________________
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> Reflector
>