[TowerTalk] vertical antennas

[Tom Rauch]

> The biggest advantage of a buried dense radial system over an elevated
> one, is that it's buried and you can grow grass on top of it. Up in
> the air gets ice, falls down, and you can see it, etc, etc.

Other problems are with small elevated systems, even at great 
height, you need to choke RF off the OUTSIDE of the feedline or 
efficiency suffers. 

You can NOT have a path to earth for RF, and that means you 
can't have a direct connection to earth for lightning. Not a good idea.

Also, no matter how well you "balance" currents, the radials will 
radiate quite strongly in the area around the radials. That means 
interaction and unwanted coupling.

Small resonant systems are also single band systems, and they 
even narrow the antenna bandwidth on the desired band!

All the above, even if we ignore the reduction in efficiency and 
mess of wire in the air, is enough that I won't use them!  

> In the ground screen you suggest, you need to pay attention to
> "dense". Depending on who you talk to, dense starts somewhere between
> 60 and 120 radials. 

RCA spent a fortune on studying this, and they concluded 50-60 
1/4 wl radials acted like a solid plate. The key is to not let the open 
spaces exceed about .025 wl between wires. That even works with 
a screen reflector antenna!

The 120 radial wires comes from the FCC's recommended ground 
of 120 1/2 wl radials for BC stations, where they want a LOT of 
headroom on the conservative side. 

73, Tom W8JI
W8JI@contesting.com 

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