Topband: The WD8DSB mini-flag antenna

[GEORGE WALLNER]

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)
> _________________
> Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
>
> _________________
> Searchable Archives: http://www.contesting.com/_topband - Topband Reflector