Topband: The WD8DSB mini-flag antenna

[John Kaufmann]

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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