Topband: The WD8DSB mini-flag antenna

[Don Kirk]

Hi George and gang,

I really appreciate you as well as Steve WB6RSE making sure folks
understand there are performance limitations when using very small antennas
such as my portable flag as a dedicated receive antenna (especially on 160
meters) and that is indeed the case.  I definitely did not intend for my
portable flag to be a dedicated receive antenna for use on 160 meters
knowing there are limitations, but there still may be appropriate
applications for it on 160 meters besides being a portable direction
finding antenna depending how high ones noise floor is or as a
unidirectional sense antenna for a noise reduction system to combat very
strong local RFI, etc.  I suspect placement of the preamp at the antenna
would be wise if using the portable flag as a dedicated RX antenna to help
maintain the signal to noise ratio (deliver strongest received signal
possible to the preamp in order to offset the ill effects of the preamp
noise figure), but I'm certainly not promoting my portable flag for
anything but radio direction finding when used on 160 meters.

I designed the portable flag specifically for direction finding knowing
that I likely would not be able to detect signals using the portable flag
that I can hear just above my noise floor when using my half size pennant
or TX antenna on 160 meters and that is indeed the case, but to put things
into perspective it's probably not as bad as everyone imagines based on the
noise floor at my location which actually is not too bad (I really work
hard on keeping it in check by tracking down noise sources and working with
owners to reduce or eliminate it).

As a very crude test I have a local signal around 1.830 MHz that comes from
a Fire Control Panel 0.75 miles from my house that I can use as a signal
source for running some simple receive antenna tests.  I just checked on my
Kenwood TS-180s with a 500 Hz wide filter using my 68 foot base loaded
vertical and the signal is approximately 0.6 S units above my noise floor.
I then checked the same signal using my approximately half size pennant
(51.6% of a full size pennant) using the DX Engineering preamp and the
signal is running exactly 1 S unit above my noise floor.  I then went
outside and I can detect this same signal using my portable flag with 30 dB
of preamp gain but the signal is just barely detectable by ear but easily
observed on my SDR waterfall display.  This performance is actually better
than I thought possible, but we have to remember that the feedline on my
portable flag is only 10 feet long which likely negats a lot of issues
regarding common mode noise and signal ingress on the feedline.  I'm not
sure what the limiting factor I'm currently encountering on 160 meters is
with the portable flag (not enough preamp gain, too high of noise figure on
the preamp, antenna thermal noise limited, etc ????), but the current
system certainly meets my original design objectives and then some.  Note:
my noise floor measures S 5.5 during the day using my 68 foot base loaded
vertical on 160 meters (S meter not necessarily calibrated) and I'm just
able to detect signals that are 0.6 S units above this using my portable
flag when using the DX Engineering preamp that provides 30 dB of gain (just
to provide a rough picture).

Also it is often said that evertunally the antenna can get so small that
the feedline acts as the antenna.  I have used the portable flag on Non
Directional Beacons down at 338 KHz and the portable flag still exhibits
exceptional directivity but again we have to remember the feedline is only
10 feet long.  I just checked this NDB which is 4.5 miles from my house
with the portable flag and the front to back ratio measured 33 dB so the
portable flag itself is still performing exceptionally well as a
unidirectional antenna versus the feedline acting as the antenna down at
338 KHz.

I will try and answer some questions that others have asked in a few more
posts, and then I will probably ask that we move additional posts or
comments over to the RFI reflector since I designed this antenna for
direction finding of RFI and not as a dedicated receive antenna for 160
meters and I don't want to push the limits on what we should be posting on
the topband reflector.

Thanks again and always appreciate your comments.
73,
Don (wd8dsb)

On Wed, Feb 24, 2021 at 9:32 AM GEORGE WALLNER <aa7jv at atlanticbb.net> wrote:

> Don,
>
> I put that note out because friends were asking if with a
> "ultra-low-noise" pre-amp they could use it for DX. Unfortunately, that's
> not the case. (I have tried.)
>
> Of course, that does not diminish the usefulness of your antenna for DR.
> Also, a somewhat larger version (1.5 - 2 m per side) could be useful for
> very noisy locations.
>
> TKS for starting this discussion as we still have more to discover with
> resistor loaded loops.
>
>
>
> 73,
>
> George,
>
> AA7JV/C6AGU
>
>
>
>
>
>
>
> On Tue, 23 Feb 2021 21:46:15 -0500
>
> Don Kirk <wd8dsb at gmail.com> wrote:
>
> 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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>
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>
> >
>
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>
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>
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>
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