Topband: Top Fed Vertical

Tom Rauch
Tue, 31 Jul 2001 13:08:56 -0400

Hi Doug,

> top-feed my tower.  I like top feeding because the current is higher =
> above ground.  This gives a better shot over high, thick trees & I =
> understand that the further away from ground the current/radiation is, =
> the less loss to ground I will have, not having a good ground screen.  =

As Roy points out, you can move the feedpoint all you like and 
losses remain the same. The only way to minimize losses in a 
short antenna is to make current as uniform as possible in the 

That's true with ground systems also. You do not want to 
concentrate electric fields by using short radials, nor do you 
eliminate loss by moving voltage down near the earth. You simply 
move the loss around to different places, or worse yet concentrate 
the loss in a small area.

, a .68 X 1/4 WL length of coax coiled & shorted on =
> the far end , will provide a near lossless, non-radiating element for =
> the RF to push against.  

It is impossible to have a RF system with a non-radiating ground 
made from a single conductor...or even multiple conductors..if it is 
indeed acting as a "ground". The closest we get is a large perfect 
disc. As we move away from that, the problem always gets worse.

This is true for any antenna, even Beverages. That's why we can 
not shield a drop lead from a Beverage, and why "shielded loops" 
radiate just like non-shielded loops.

If you want to look at a good limited space system, use as big a 
hat as possible. Use the biggest ground system possible. 

It makes no difference where you feed it, as long as the feed 
system is designed correctly. There are no tricks, and no free 

73, Tom W8JI 

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