Topband: The WD8DSB mini-flag antenna

[Don Kirk]

Hi George,

Thanks for mentioning that antenna system thermal noise eventually
establishes the limiting factor where additional preamp gain will not
provide any additional improvement in performance.  Earlier today I was
looking for a previous discussion by Tom (W8JI) that explained this very
well and I believe he provided some examples but I have not yet located it
(but still looking).

73,
Don (wd8dsb)

On Tue, Feb 23, 2021 at 8:20 PM GEORGE WALLNER <aa7jv at atlanticbb.net> wrote:

> The smaller the flag the lower its gain and its signal output. The limit
> of
> usefulness is reached when a weak signal is below the thermal (Johnson)
> noise of the system (~ loading resistor). Below this point a pre-amp will
> no
> longer help. That limits the usefulness of small flags for weak signals,
> regardless of RDF.
>
> 73,
> George
> AA7JV/C6AGU
>
>
> On Tue, 23 Feb 2021 18:37:03 -0500
>   John Kaufmann via Topband <topband at contesting.com> 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)
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