[TowerTalk] Ground Screens, Radials, and Landscaping

[Tom Rauch]

> - A ground screen prevents losses in the ground for any antenna
> near the ground, vertical or horizontal.

Reduces or "helps prevent" would be better.
 
> - A ground screen need not be directly coupled to the antenna to be
> effective (e.g., a dipole over a screen).

Yes indeed.
 
> - A radial system provides the "current return" for a vertical antenna.

That can be a troublesome way to look at it. I like to consider it as 
providing something for the antenna to "push" against. Pushing and 
pulling charges into and out of the monopole antenna is like trying 
to push and pull a car while on roller skates. You can waste a lot of 
energy moving the wrong stuff around (like the feedline shield or a 
ground rod) if the ground doesn't have enough "electrical mass" or 
"electrical weight" to stay put when the antenna is pushed.

That's a different kind of loss than coupled losses from the 
induction and radiation fields around the antenna.
 
> - A radial system near ground with many radials can also act as a ground
> screen.

Yes. As a matter of fact with an reasonably adequate radial 
system a screen is unnecessary for loss considerations.
 
> - A radial system near ground with few radials will provide the current
> return for a vertical antenna but will not prevent ground losses.

It makes something lousy to "push against". It can "move around" 
more than the antenna, and hence creates fields (induction and 
radiation) all on it's own. If you have a single radial, it's like laying 
half your antenna against lossy dirt with that half having the higher 
impedance!

Not only that, you still have all the losses from the vertical sections 
induction and radiation fields acting on the dirt.

> - An elevated radial system of few radials over a good ground screen is at
> least as effective as a buried radial system of many radials.

Sure, as long as nothing is near the radials. Remember elevated 
radials radiate strongly in the near field. The more radials the less 
each one will radiate and the closer to the radials the field will 
cancel. The pictures that show the radiation from a balanced 
system not radiating are only valid at a distance where all the 
radials look like a single point in space. That distance where the 
radial's fields cancel is closer and closer the more radials you have.

That's why making the radials smaller and smaller, and less and 
less in number, is a problem. You have to treat small ground 
systems like the components in the system are each acting like 
an "antenna".



 

73, Tom W8JI
w8ji@contesting.com

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