Topband: The WD8DSB mini-flag antenna

[Don Kirk]

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)