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[TowerTalk] vertical antennas

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Subject: [TowerTalk] vertical antennas
From: (Tom Rauch)
Date: Tue, 23 Oct 2001 20:41:18 -0400
> Even in this area I have found some different ideas.  Mainly, their is
> one train of thought  that ground radials need not be longer than the
> height of the vertical radiator (in my case, 60 ft.).    The other
> train, always use quarter wave radials (130 ft) regardless.

That is an unfounded old Ham's tale, just like the idea that two or 
four elevated radials is enough. (Four radials are enough if they are 
a hundred feet or more above ground on 160).

This was all measured out many years ago in a study 
commissioned by RCA. Prior to the study virtually all ground 
systems were sparse resonant counterpoise systems. The study 
drove a stake through that idea. Now some have taken a step 
backwards and claimed the opposite, but every case I have seen 
where a system was compared (I did two separate tests as well as 
measured one BC station) with ONLY a radial change there was 3 
to 6 dB advantage using a conventional large ground system.   

Shorter verticals actually require larger (and that means longer) 
ground systems, because the current is much higher. You need 
long dense radial fields to have a low ground resistance. As the 
vertical is made taller, the ground system becomes less critical.

The general rule is that when the largest wire spacing is about .025 
wavelength, the wires "look like" a solid sheet. Adding more wires 
does not good at that point. 1/4 wl long is normally long enough in 
any situation on 160 meters.

That would mean the open end of the radials should be about 14 
feet apart so you'd need about 50 or 60 1/4 wl radials to reach the 
knee of the efficiency curve if the radials were all evenly spaced.

What I do, in areas where I can't run a full 1/4 wl, is move a 14 foot 
rope along to mark the perimeter. I wind up with more radials where 
the radials are longer.  
> I wonder what the "thread" is on this.  In my situation, I can lay 130
> ft radials for 270 degrees.  In the other 90, radials are going to
> decrease in length to about 60 ft for the shortest (perpendicular to
> the property line). Is this going to noticable affect the performance.

The effect would almost certainly be nothing you would notice, as 
long as you go by the .025 wl maximum spacing rule.
73, Tom W8JI 

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