Jim ...
This antenna is B&W's take off on the T2FD which has been
written up in CQ back in the 50's as I recall ... Countryman W3HH ???
It's in the same bag as the NVIS and other "beliefs" ...
>From my notes: for the T2FD ... the folded dipole length L = 164/F ...
feet
and wire spacing is S = 9.84/F ... feet with F in mhz. For 80M these
numbers
are L = 46 and S = 33 inches. For 160M L=90 and s = 66 inches.
The B&W 90 : L= 90 and S = 18 inches.
Just do this: Model this BW90 folded dipole in FREE SPACE without any
resistor R opposite
the feedpoint .. .then do it with R=600. If you look for
SWR measured against 600 ohms you find the antenna ranges 2-3 to1 from 6
meters
down to 160. They add a 12:1 "balun" to transform 600 to 50 a 12:1 ratio
... this
"transformation magic" adds more loss to the system so lowers SWR even more
and ...(heaven only knows what it does to the impedance .. it's no doubt a
voltage balun!)
then voila >> an allband no tuner ant!
LOSS cures all SWR problems!
Look at the impedances with and without R ... in FREE SPACE.
Next, put the B&W 90 up 40 feet, like a flat dipole over 0.006 and e=12
soil ... then look at what happens to the 600 ohm SWR! (with and without R
in place)
At the same time pick a few points in the antenna patterns on all the bands
you investigate and compare the variations .. picking the peak values is
an interesting exercise ... I have attached the table I came up with some
time
ago ... in plain text form ...
The whole thing all of sudden becomes very messy and SWR varies greatly
from band to band ... on 160meter you can work DXCC QRPp running
a KW!
I STRONGLY ADVISE anyone thinking about using this antenna to do their
homework ... then decide. what has to be OBVIOUS!
If someone gave me this thing, I wouldn't even GIVE IT away!
Finally, if you really want to get the skinny on this antenna and what B&W
claims to know about it .... go to their web page
http://www.bwantennas.com/ama/faq's.ama.htm
for a real HOOT! This is the link on their web page for the amateur side
of
things and the FAQ link under that. The whole story is there ...
especially
the very last paragraph ... The gain figures for the antenna is there as
well.
This antenna DISSIPATES LOSSES ....Their terminator (they don't say it's a
resistor)
does not CREATE losses, because to create losses it would have to be low
impedance
and theirs is high impedance .... AMEN!
And you see this antenna on top of tons of military/civil facilities ... too
bad they
don't have antenna engineers on their staff!
PRACTICAL ANTENNA THEORY ...
Theorem: 0.001 dBb (dB of BELIEF) astronomically exceeds 1293823912398.38
dBd !!!
Corollary: Looking at that dummy load hanging in his back yard, he sez - I
worked a ZL
on it! LEAVE IT ALONE -- END OF DISCUSSION! WALK AWAY!
... de dave w7fb
Attachment: Plain Text Table BW90.txt
..
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: <towertalk@contesting.com>
Sent: Saturday, September 03, 2005 7:52 PM
Subject: [TowerTalk] B&W folded dipoles
> I'm looking for some details on the dimensions and design of the Barker
and
> Williamson "folded dipoles", for the purposes of building an accurate NEC
> model.
>
> From some casual web searching (on TT, among other places), it looks like
> these things are basically a folded dipole with a resistor in the middle
of
> the fold.
>
> I'm looking for things like detailed dimensions (how far apart are the two
> wires?)
> What's in the "coax to antenna transition"? Is it a balun? Or, is it just
> two wires?
> What's in the "magic box" at the top of the antenna (a resistor? a tuning
> network?)... the photo on the B&W website shows a metal strap between the
> magic box and the coax, but it's not clear if that's an actual electrical
> connection (if so, to what)
>
> I'd also be interested in any measured performance data (who knows.. the
> gov't uses them, maybe someone actually tested one on an antenna range
> somewhere?)
>
> Thanks, all...
>
> Jim, W6RMK
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
BW90.txt
Description: Text document
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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