[Top] [All Lists]

Re: [TowerTalk] elevated radials vertical

To: "Al Williams" <>, <>
Subject: Re: [TowerTalk] elevated radials vertical
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Fri, 8 Jun 2007 05:59:15 -0400
List-post: <>
> However the elevation pattern and gain using real ground 
> shows virtually no difference with 0,2,4, and 8 radials. 
> The source impedance jumps from highly reactive with no 
> radials to 50 ohms mostly resistive using two radials and 
> lowers a bit using 4 and 8 radials.
> The pattern and gain being nearly identical no matter how 
> many radials are use (including none) has me wondering if 
> I have something wrong in my model?

Models don't do as well as people imagine for horizontal 
wires near ground. The model treats the earth like a big 
homogeneous mass with uniform characteristics.

Now if the radials are high enough, say 1/4 to 1/2 wave 
above ground, things are OK. But when we put a wire very 
close to earth things fall apart. If your model is showing 
no difference between zero and four radials or showing four 
radials no different than 30 or 60 radials something is 
seriously wrong.

You'll certainly make contacts with only two or four radials 
elevated  radials, but more than half the power will be lost 
in the fields concentrated in the lossy earth near each 
radial. You'll also have a hard time decoupling the feedline 
from the antenna, so you'll have significant common mode 
currents on the feeder to deal with.

None of this means the antenna won't work, and some people 
with vivid imaginations manage to convince themselves some 
very poor systems work well, but the fact is it won't be a 
good system unless you have ten or twenty radials minimum. 
And when you get the ten or twenty radials, they might as 
well be on the ground as elevated.

The sole exception to this is ground that is a near- perfect 
insulator or a near-perfect conductor, or if the radials are 
a large distance in fractions of a wavelength  above the 
lossy media. But even in those cases the feedline really 
needs decoupled because the antenna is neither perfectly 
balanced or perfectly unbalanced.

73 Tom 


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>