[TowerTalk] Inverted L Question

K4SAV RadioIR at charter.net
Sat Aug 30 12:00:52 EDT 2008


I'm not Dennis, but I think I can answer your questions.

The 175 feet number comes from the length required to get the resistive 
part of the feedpoint impedance of the L close to 75 ohms.  The 
capacitor is used to cancel the reactive portion of the impedance and 
you are left with a good match for 75 ohms.   That 75 ohm resistive part 
also varies with the length of the vertical section and how much the top 
wire slopes, so you have to analyze the exact configuration to figure it 
out.  This results in a single capacitor matching system, and that 
capacitor can tune the resonant frequency of the L across the whole band.

Why 75 ohm cable?  Often 75 ohms TV hardline is available for free (if 
you're lucky).

The best option for an L is to have the vertical section as tall as 
possible.  That minimizes ground loss.

Jerry, K4SAV

N7mal wrote:

>I have been reading all the answers to your questions. I have 2 questions. 
>Firstly I have had 3 160 inverted L's at different locations.
>My questions for you are: Why 175 feet.?. Where does that come from.?. Every 
>one I've used has been 66 feet vertical and 66 feet horizontal. 132 feet is 
>the common accepted starting point for 1/4 wave 160 meter antennas. Ideally 
>an inverted L is a 1/4 wave vertical with the voltage portion bent over 
>horizontally. That's where the 66 ft vertical and 66 ft horizontal comes 
>Why the 75 ohm cable.?. The input impedance of an inverted L with a 
>reasonable ground system is slightly less than 50 ohms.
>Everyone in the world is
>entitled to be burdened
>by my opinion

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