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Re: [TowerTalk] Ground mounting vs roof mounting verticals

To: "towertalk" <>
Subject: Re: [TowerTalk] Ground mounting vs roof mounting verticals
From: "Jim Brown" <>
Date: Thu, 22 Oct 2009 08:39:37 -0700
List-post: <">>
On Thu, 22 Oct 2009 05:17:37 -0700 (PDT), Dennis OConnor wrote:

>Bob, have you modeled it?
>I bet you will be surprised at what you find...

Good advice. But I would insert the beam into the model as being 
connected to the tower, and the tower connected to the bottom of 
the vertical on one end and to ground on the other, with the 
transmitter connected at the bottom of the vertical. Note, 
however, that the connection to ground doesn't do much for that 
vertical -- it's the metal below the antenna (and close to it) 
that matters most. 

Most verticals DO need some form of radials (counterpoise) to 
serve as the return for antenna currents AND the fields that the 
antenna produce. Those fields and the currents form a CIRCUIT, and 
if that circuit is not there or is poor, antenna performance will 
be poor. While it may be far from ideal, the beam could perform 
the function as a counterpoise sufficiently well to make the 
vertical work well enough to make you happy. 

Typical all-band verticals have the reputation of poor efficiency. 
There are at least two solid technical reasons for this.  First, 
the loading coils and other networks designed into them to make 
them work on a lot of bands (and on bands for which they are short 
as a fraction of a quarter wave) are lossy. Second, many verticals 
are installed without a sufficient radial system. Both of these 
failings burn some of the transmitter power. 

A few verticals are designed to work without radials, but most DO 
need radials (or some form of counterpoise). 

As to the antenna tuner -- yes, you are likely to need an antenna 
turner to get the transmitter to put power into the transmission 
line, and some of that power will be burned in the line if there 
is a mismatch at the antenna. BUT -- an antenna tuner will NOT 
make the antenna RADIATE, and it will NOT make up for losses in 
the antenna due to those loading coils and the lack of radials. 
What the tuner does is make the transmitter happy and allow it to 
put power into the coax.


Jim Brown K9YC


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