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[TowerTalk] Matching 3 band Vertical

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Subject: [TowerTalk] Matching 3 band Vertical
From: "Robert Seymour" <>
Date: Mon, 3 Mar 2008 15:06:20 -0500
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I am planning improvements to my 160/80/40 TX vertical to improve efficiency 
and bandwidth.  Today it is mounted on a 4.5 ft post in a salt marsh and has a 
44 ft long radiator.  I remotely switch bands by shorting out parts of the base 
coil.  I want to increase height to 60 ft and top load it.  I can not get 
beyond 60 ft. and properly guy everything.

My main question is the following:  As antenna height is increased to improve 
efficiency and bandwidth, I get into the situation where feedpoint impedence on 
40M becomes very high.  As I pass over 1/2 wavelength, but stay under 5/8 
wavelength, the impedence decreases somewhat, so that on 40M EZNEC predicts  
1245 - j370. Most of what I read suggests feeding such an antenna with a tapped 
parallel tuned circuit, but when I calculate a standard L/C L network, the 
components seem practical -  80 pf shunt across the antenna and 5.8 micro-h in 
series with the feedline.  According to EZNEC the bandwidth should be OK.  

1. What determines the choice of a parallel curcuit over an L-network.  Is 
there any reason NOT to go the L network route?  Maybe the losses the deciding 

2. What can you suggest about the form factor and wire (copper tubing maybe) 
size for the L-network coils for all three bands - especially 160M where I need 
25 micro-h in the series element.  I want to handle 1500W max.

Thanks for any input - Bob  AF4OX

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