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[TowerTalk] vertical antennas

To: <>
Subject: [TowerTalk] vertical antennas
From: (Tom Rauch)
Date: Thu, 25 Oct 2001 11:39:00 -0400
> grounding the coax shield at the base of the tower. With a shunt fed
> tower and elevated radials, there's no grounding problem.

With elevated radials, ANY path to earth for RF is a problem that 
reduces efficiency. This was one of the things that was claimed in 
the original elevated radial article that I verified. I was actually 
surprised by that effect, since it runs contrary to intuition, but it is 
absolutely true and a measurable repeatable effect.

There very clearly is a loss in efficiency when any conductive path 
is provided to earth for RF in a sparse elevated system.

A shunt fed tower, or a tower stuck in the ground and fed directly 
from an elevated system, always has an additional loss in 
efficiency over the normal loss in efficiency associated with sparse 
elevated radial systems due to that additional current path through 
lossy earth.  

> interactive possibilities. At a quarter wave distance the cancellation
> is mostly formed.

It is a geometry problem.

Look at the problem off the end of a single radial. There is virtually 
NO opposing field that cancels radiation from that radial, because 
the fields generated in that wire are much much closer and 
stronger than any other field that may oppose the radiation.

It isn't until you move back to a distance where it looks like all 
fields originate at ONE single point in space or to a point where 
path distances can be consider approximately the same that 
opposing fields will cancel.

A dense multiwire system has fewer problems because the field 
from each radial is reduced by the large number of radials. Charge 
movement (magnetic field) and the difference in charge distribution 
(electric field) are spread over a very large area. 

In traditional large systems, fields are not highly concentrated 
except for current flow near the feedpoint. Even at that point, the 
conductors are physically closely spaced and opposing sides of 
the radial system cancel much of the effects of current flow.

The last thing we want to do is concentrate the charges in small 
areas (like two radials instead of 60 radial), especially where they 
can couple to other systems or lossy media like earth.

When I walked under a 30-40 foot high elevated radial at a 10kW 
BC station with six radials, dragging a few dozen feet of wire 
behind me that was tied to climbing belt, the wire actually arced 
through my jeans and burned a big RF burn on my backside!!

The electric field below the six radials was huge, and fluorescent 
tubes could be lit 75 feet from the ends of the radials! 

After a conventional ground was installed and the elevated system 
removed, any area was safe to hold the same wire barehanded! The 
only place a fluorescent tube would light was near the tower base. 
You could grab any radial at the outer end and not get fried, where 
as the elevated radials would light lightbulbs 75 feet away!

There is a big difference in RFI, as other people have observed and 
73, Tom W8JI 

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