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Re: [TowerTalk] Inverted L Question

To: "towertalk" <>
Subject: Re: [TowerTalk] Inverted L Question
From: "Jim Brown" <>
Date: Fri, 29 Aug 2008 14:38:40 -0700
List-post: <">>
On Fri, 29 Aug 2008 16:58:06 -0400 (EDT), DENNIS BRICKEY wrote:

>175' total wire length.....100' vertical 75' sloping horizontal 

>23 (qty) 130' radials. 

>Fed with 75 ohm cable hardline through a variable capacitor. 

One big reason you're having trouble matching it is the 75 ohm coax. 
Replacing the hard line with RG8 will get you closer. 

The radiation resistance (Rr) of a quarter wave antenna is going to be 
something like 25 ohms. Add 10 ohms for the ground system, and it's 35 
ohms. You're making it longer, which will raise Rr a bit and add some 
series L, which you're tuning out with the capacitor. That's good, and it's 
what I'm doing. My guess is that your Rr is about 35-40 ohms. When you 
switch to RG8, you'll be able to get the SWR below 2:1.  

2:1 is a fine match on 160 if the coax isn't very long, because coax has 
very little loss at that frequency, and any decent antenna tuner should be 
able to match it. 

Adding radials will INCREASE the VSWR, because you're lowering the 
resistance of the ground system. That's a GOOD thing -- it improves 
efficiency. This is yet another simple example of why VSWR is NOT a good 
indicator of antenna performance. 

If you want to improve your match (and bring it to 1:1), you'll need to 
make the antenna even longer. That will raise Rr and increase L. Rr is the 
only GOOD resistance in an antenna -- it's a theoretical resistor that 
represents the power that is radiated by the antenna. My guess is that you 
need something 30-40 ft more top-loading to get there if you use 50 ohm 
coax. You can also get that length by making the top a Tee rather than an 
inverted L. That is, keep the length you already have and add 30-40 ft at 
the point where the top turns horizontal. 


Jim Brown K9YC


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