Topband: Broadband Inverted L

[Jim Brown]

On Thu,11/20/2014 11:17 AM, Tom W8JI wrote:
> Joe's antenna virtually doesn't have a ground connection to radials at 
> all, and this has almost nothing to do with the number of radials or 
> type of radials.

Yes, but there's another important concept that is being missed here. 
The function of a radial system is NOT to couple the antenna to the 
earth, it is to SHIELD the antenna from the earth. Rather, the function 
of a radial system is to serve as a return for both the antenna current 
AND THE FIELD produced by the antenna. That is, a good radial system 
acts as a SHIELD between that field and the lossy earth. Joe's system 
has a counterpoise (a return for the current), albeit not a good one, 
but it doesn't perform the shielding function because it's not just 
below the base of the antenna.

K2AV's folded counterpoise is another example of a system that provides 
an effective return for antenna current, but provides much less of a 
shielding function.

 >

>Time to go out and connect more radials directly to the
>feed point ground.

NOT to the earth, to the coax shield.

>I can also elevate two of them.

I suggest that you study the link I posted. N6LF has done a lot of excellent work on radial systems, much of which is summarized in that link.

On 160M, radials must be at least 16 ft off the ground to work as "elevated radials," and they should be of equal lengths and heights. Radials on the ground can be of any length --the only more or less universal rules of thumb is that more copper on the ground is better, and more short radials is better than a few long ones.

Think of it this way -- the current in any radial must be minimum at the end, and if it's less than a quarter wave, will be maximum at the feedpoint. The antenna return current divides between the radials, and the loss is I squared R, where this R is coupled from the lossy earth. The more radials there are to divide that current, the less will be the lost power! That's because the current is divided by N (the number of radials) while the loss is divided by N squared.

The reason a fewer number of elevated radials can work as well as many more on the ground is that the fields from the elevated radials don't couple as closely to the earth, so there's less coupled R, and thus less lost power.

73, Jim K9YC