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Topband: wideband inv-L

To: <topband@contesting.com>
Subject: Topband: wideband inv-L
From: "DAVID CUTHBERT" <DAVIDNNAN@MSN.COM>
Date: Tue, 4 Jan 2005 21:29:32 -0700
List-post: <mailto:topband@contesting.com>
We are confining our design space to an inverted-L at this time. I did manage 
to achieve 200 kHz (2:1 VSWR) in simulation for a 45 foot tall inverted-L 
having a ground loss of 4 ohms. I can fire up a working model at a few hundred 
MHz this week (initially without the impedance transformation). I don't know 
that Dave wants to build this particular model as it is essentially a flat 
sheet (made of many wires) 50 feet across for 1.9 MHz. However, with a 12 foot 
wide inverted-L the 2:1 VSWR bandwidth is 100 kHz and going with a 16 foot wide 
gives 125 kHz. I have incorporated the impedance transformation into the basic 
design so we do not need any bandwidth robbing L-network. 

The antenna ends up quite similar to a couple I have designed for commercial 
use. The new part is the modification of the impedance transformation to allow 
one to select the integer step-up ratio without regard to the physical antenna 
width.

I did experiment with shunt T-lines and LC networks for further extending the 
bandwidth (using WinSmith). I do see the double VSWR hump and I do see a modest 
increase in bandwidth. However, the slightest loss really extends the bandwidth 
and I suspect this is what many find. I think we will avoid this route.

      Dave WX7G
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