David Ryeburn wrote in
:

In article ,
Jet_Li wrote:

best is any vertical from Force 12 - I got one on 75 meters that
beats everything else I have put up.
got one on 40 meters from Force 12 - beats everything else I have put
up.

I went to the Force 12 website and clicked on the "Flagpole antennas"
link http://force12inc.com/F12-flagpole-ants-003.htm.

I read the "New 40 Meter Coil" portion of the page with interest --
sounded too good to be true. A moment's use of the calculator showed
that the VSWR figures and the quoted feed point impedances were very
inconsistent -- orders of magnitude wrong. The claimed SWR of 16 for
the unloaded antenna worked out really to be about 850 -- close enough
to infinity for me!

Your calc is about right (assuming Zo=50+j0).


If that's what the "useful A.R.R.L. TLW software" actually indicates,
then the League should withdraw the software. But somehow I suspect
that the software (which I do not have) doesn't predict what the web
page says it does.

TLW suffers from some of the same issues as tables in the ARRL handbooks,
but I haven't run Force12's numbers on TLW, I will use my own calculator.


I leave it to someone with a copy of EZNEC to see if an 1/8 wave
monople on 40 metres made out of 2 inch diameter tubing really has an
input impedance of about 5 - j460. That part, at least, sounds
reasonable to me

David, I ran an analysis of a 10m "flagpole" as a multiband antenna, and
the feedpoint impedance is around the 5-j460 if you ignore resistive loss
in the ground system. Force12 might not want to include such in the
analysis as it drives much lower efficiency.

If they did achieve a coil of 10uH with Q=600, the additional R is ~0.7
ohms (about 0.6dB loss in the stated scenario), so their new 6-j22 looks
a reasonable development(apart from the fact it also ignores ground
resistance), and I make the VSWR(50) to be 10 rather than their stated 6.

At the end of the day, the VSWR is not of itself the issue, the issue is
what is the line loss and load impedance presented to the ATU, then the
expected tuner loss.

Their estimate of 2dB line loss for 100' is probably realistic, I make
the loss on 100' of RG213 at 7MHz with load 6-j22 to be 2dB ( try it
yourself at http://www.vk1od.net/tl/tllc.php ), and the load seen by the
ATU is a manageable 8+j3.0. The Z into the line is highly dependent on
the line length, but since Force12 give it, lets look at likely tuner
efficiency. Using W9CF's tuner applet with default configuration, ATU
loss might be around 0.7dB.

This whole scenario predicts an antenna that might be quite acceptable to
people with space or covenant issues, 3.3dB of total loss.

But...

What if ground resistance was 10 ohms, efficiency of the antenna and
loading coil would be 5/(10+5+0.7) or 32% (5dB loss). That will help the
line loss though, now with a feed Z of 15.7-j22, loss in the same line is
1dB and input Z is 16+j2. ATU loss might be more like 0.4dB. Total loss
is 5+1+0.4 or 6.4dB.

Although there analysis ignores ground resistance, possibly so as to hide
the efficiency of the loaded radiator, the real situation might only be
3dB worse than they intimate.

Of course, if the coil wasn't as good as they state, the picture is a
little poorer. Similarly, achieving a low ground resistance isn't easy
and the story could be much worse.

Do they have credibility? Perhaps they should have stuck to the QSL card
count method as the quantitative support for the design!

The idea of a flagpole sings when an automatic ATU is placed at its base,
but that doesn't solve the efficiency issue that exists with any Marconi
with low radiation resistance. I wrote an article exploring an unloaded
vertical as a multiband antenna, it is at
http://www.vk1od.net/multibandunload...ical/index.htm and may be of
interest.

Owen