Topband: Multiple Ground Systems

[Tom Rauch]

> Now imagine radials at 45 degrees away from the other 
> vertical.  They are almost
> 3/32 wave long, but meet at the bus wire at a right angle. 
> In that case, a radial doesn't
> see an extension of itself, but rather an array of radials 
> at right angles that can't
> carry any current in the direction needed.

Why does the current need to be in any particular direction 
Rick? That buss, assuming you have 50 radials or more, has a 
pretty low termination impedance for the small currents in 
the radial. While it may not be the shortest path possible 
to a distant area because of the angle, it is a pretty large 
path with a low impedance.

The only way it would be a problem is if there weren't 
enough radials. With enough radials is still closely 
approximates a sheet.

> How does all this jibe with the canonical 1/2 wave 
> broadcast radials?


The rule at broadcast stations, if you want to avoid proving 
the system works through measurement, is 120 1/4 wave 
radials. The only reason ever explained to me by people in 
the broadcast division was that number is about twice what 
was really necessary as a safety margin. When I look at 
RCA's Ground Systems as a Factor in Antenna Efficiency 
study, that explanation makes sense. It is about twice what 
is really needed to get out on the flat part of the 
efficiency curve.

While the path provided by a buss termination isn't the 
shortest possible path, with all that wire (or even 40 or 50 
radials) it still has a good bit of area covered with 
multiple parallel paths.

Worrying about the path length with all those wires is 
almost as silly as worrying about going from 60 to 100 
radials. There just isn't much there to be gained, probably 
less than 2/10th dB.

It's a problem when a sparse radial field is used in a 
directional array, but not with a good ground system.

73 Tom