Topband: Dumb radial question

[Jim Brown]

On Wed, 25 Oct 2006 18:33:15 +0200, Vanderydt Theo wrote:

>Now i read following figures from the analyzer.
>SWR 1.5/1 X 0 and R 80 Ohms. Bandwith stayed the same at about 60 KHz. SWR 
>went up a bit, but what i'm puzzled with is the increase
>in impedance? 

Where are you making your measurement -- in the shack, or at the base of the 
antenna? When you added radials, you probably reduced the total impedance of 
the antenna a bit (that is, wire plus radial system), and increase the VSWR. 
if you measured at the transmitter, you'll see the same higher VSWR that you 
would at the antenna, BUT the length of transmission line will tranform the R 
and X values by some complex ratio that depends on the length of the line. In 
other words, if you're measuring at the transmitter end of a length of coax, 
what you've measured is quite normal. 

The math that lets you calculate the ratio is quite complex, but can be done 
on a Smith Chart rather easily. See the ARRL Antenna Book or other text books 
on Transmission Lines. 

Obviously, the best place to measure is right at the base of the antenna, 
because you don't have to go through those messy calcs. :) 

Yes, DO continue adding radials until you're tired of it (or have run out of 
wire). :) There's an excellent piece in the newest (20th) ARRL Antenna Book 
about the best compromises between number and length of radials. 

73,

Jim Brown K9YC