[TowerTalk] Radials tied to ground: good or bad?

[Jim Brown]

On Mon, 16 Mar 2009 09:38:50 -0700, Al Williams wrote:

>I suspect that the radial system doesn't "prevent" current flow but instead
>shunts a large portion of it.

That's a good way to look at it -- it's a question of degree. If you put a 1 ohm 
resistor in parallel with a 100 ohm resistor, the 1 ohm resistor prevents 99% of 
the current flow in the 100 ohm resistor. :)   

>It seems to me that if the radials are covered with earth, that current will 
>flow from the many earth contacts to the radials and then on to the source. Thus 
>there are many short earth currents which then flow in the radial shunt to the 
>source/.

The earth and/or the radial system interact with the E and M fields from the 
antenna, and those fields generate current in the radial system and/or the earth. 
It is a mistake to think of the earth as a good conductor. Indeed, it is a SEMI-
conductor, with characteristics that vary widely from place to place. The fields 
permeate the earth to varying degrees depending on the depth and characteristics 
of the earth. If you have only a few radials, most of the field finds the earth 
rather than the radials. As we add more, we are causing more of the field to see 
the radials than the earth. And if your antenna and radials are signficantly 
elevated above the earth (1/8 wavelength or more), it takes only a few resonant 
radials to intercept nearly all of the EM field.

>What happens if the radials in the earth  are insulated?  Is there 
>capacitive coupling and how does it compare to the direct contact?  
>What if the insulated radials are laying on the ground? 

Yes, there is capacitive coupling to the earth and to the E field. Many radial 
systems, including mine, use insulated wire and lay on the ground. Burying 
radials reduces the resistance to earth at DC, which is good for lightning 
protection, but it would benefit antenna performance only if the earth was a very 
good conductor (like sea water). 

Rudy Severns, N6LF, has done extensive work on studying the behavior of radial 
systems, much of which is on his website. So has Al Christman, K3LC. Google to 
find it. Rudy and Al have also written excellent material on the subject that is 
in the ARRL Handbook and ARRL Antenna Book. More detailed answers to your 
questions are there. 

73,

Jim K9YC