Topband: Spiderbeam pole

[Jim Brown]

On Fri, 09 Jan 2009 08:04:23 +0000, Paul Ormandy wrote:

>Would be interested to hear any experiences or dimensions, 

The literature is pretty clear on this, and NEC is pretty good 
at doing the math. The goal of the top loading is to get the 
ELECTRICAL length close to a quarter wave. Essentially the top 
loading can take any form, but the more symmetrical it is, the 
more the radiation from it cancels and you're left with the 
vertical portion. The vertical portion is low angle, the 
horizontal portion is high angle.

A VERY rough approximation to get you to resonance is to view 
the sum of whatever wire is on top as being added to the 
vertical length. This will NOT be exact, and you should model 
it in NEC to get a better answer. NEC is not at all costly, and 
not very hard to learn to use. You plug x, y, and z coordinates 
of the conductors into the software, push some buttons, and you 
get plots of SWR and the radiation patterns. You can also move 
the cursor to any point on the SWR curve and read the feedpoint 
impedance. One very nice version was developed and is sold by 
W7EL. Google to find it. 

The good news is that you don't necessarily have to hit that 
quarter wave resonance, you only need to get the impedance at 
the feedpoint within the range of your antenna tuner. Loss in 
coax is rather minimal at 1.8 MHz, so don't lose a lot of sleep 
about matching as long as the line is not VERY long and you can 
make your transmitter happy. 

What's FAR more important is a lot of copper on the ground 
under it. 

73,

Jim K9YC