[TowerTalk] Number of Radials?

[Tom Rauch]

> N6BV has postulated that elevated radials need to be 0.05 WL
> (wavelengths) or more above ground for good efficiency.  That
> corresponds to 7 ft on 40M, 14 ft on 80M, 28 ft on 160M.

That would not be a good assumption, because clearly what 
happens is as more radials are used the system becomes less 
dependent of height.

With eight radials at height of 1/4 wl, a system was measured as 
just slightly less than one with 100 radials on the ground.

It's well known at height of 1/2 wl above ground , four radials are 
acceptable. There is some pattern ripple and distortion, but 
efficiency is good if you choke current off the feedline. 

As height of the radials are lowered you need to use more radials, 
as height is increased you can use less depending on how much 
pattern distortion you can tolerate or accept and what is below and 
around the antenna. 

There is no magic "ground independence" height or magic number 
of radials at a height because the results depend on many things in 
the system and even what you expect to achieve.

With great system height, a few radials are enough for efficiency 
**BUT** the system becomes more critical and pattern is distorted. 
As a matter of fact, with a 1/4 wl groundplane and four radials you 
have to use a choke-balun at the feedpoint or the feedline greatly 
influences pattern and SWR!!  That's true at ANY height, yet you 
see many radial guru's not understanding that and describing 
systems that do not decouple the feedline from the sparse radial 
systems!

Results are influenced by many things. No way does a certain 
height mean "it works this way" and another height suddenly mean 
"it now works this way".

By the time you have enough radials to make the system non-
critical, you might as well just lay them on the ground and not have 
a rat's nest of wires making your antenna area a living hell of 
overhead wires. Not only will things be more predictable, you'll have 
added additional lightning protection at the same time. Something 
you don't get with elevated radials!

There is no free lunch.








73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com