Topband: The WD8DSB mini-flag antenna

Wed Feb 24 09:32:43 EST 2021

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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