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Re: Topband: 1/8 wave vertical

To: "AD5VJ Bob" <rtnmi@sbcglobal.net>,"topband@contesting.com" <topband@contesting.com>
Subject: Re: Topband: 1/8 wave vertical
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 24 Dec 2007 19:18:45 -0800
List-post: <mailto:topband@contesting.com>
On Mon, 24 Dec 2007 11:42:03 -0600, AD5VJ  Bob wrote:

>Will an 1/8 wave vertical work for top band or do you have to start 
>with 1/4 wave? Perhaps top loading it would work??

The taller the antenna, the better it will work (up to at least 1/4 
wave). Top loading can make a short antenna more closely approach 
the performance of a quarter wave. 

Think of it this way, Bob. It's a series circuit that includes the 
Radiation Resistance (Rr) and R loss, where R loss = Rg + Rw   Rg = 
ground system  Rw is the wire, including skin effect.  Rr represents 
the power that is radiated, and becomes larger as the antenna gets 
longer. Rr is on the order of 30 ohms for a quarter wave. There are 
graphs of Rr vs antenna height in the ARRL Antenna Book (and 
probably in the Handbook). 

A loading coil adds loss resistance, while top loading adds very 
little. Both make the antenna "look" longer to the transmitter (that 
is, closer to 30 ohms). R loss adds to that, so an antenna with a 
lousy ground system will be closer to 50 ohms, and thus have less 
VSWR, but the ground system will burn more of the transmit power. 

My 160 vertical is currently 70 ft tall with enough top loading to 
make it look like a quarter wave to the transmitter. I'm working on 
making it taller, which will require less top loading. Obviously, 
you can make the height less important by reducing R loss - use 
bigger wire, parallel conductors, and a beefier radial system. 

Hope this helps.

73,

Jim K9YC


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