Topband: Fw: GAP VERTICAL QUESTION

[Tom W8JI]

> So here's a question. I have a vertical mounted on a cliff side that 
> performs incredibly. My amateur's approach to figuring out why is that I 
> modeled it in EZNEC as being elevated 400 feet. That shows it performing 
> nearly as well as if it were on a tiny island in the great ocean.
>
> Is it correct that an elevated feed point greatly reduces the ground 
> losses?
>

That makes sense because the VOLTAGE and CURRENT of the antenna, each of 
which cause loss problems, are remote from earth in the model. If BOTH the 
electric and magnetic fields do not have significant intensity in lossy 
earth, there isn't as much loss.

However, the idea behind the GAP antenna falls quickly apart because they 
did not move the antenna away from earth. They simply moved the feedpoint 
around. We can move the feedpoint around in a short vertical until we are 
bleeding from the fingers, and loss remains the same. The only way loss 
changes is if we move the voltage and current up away from earth.

I'm not sure what GAP's policy is now, but when questioned years ago about 
how the magic elevated feedpoint with the yellow lightning bolt worked, they 
handed out a paper about a very tall broadcast tower. They said the paper 
shows how an elevated feedpoint reduces loss.

The paper actually said nothing of the sort. The paper said a halfwave tall 
AM broadcast tower, operating on the low end of the AM broadcast band, had 
just very slightly less field strength when it was converted from an end fed 
half wave with 120 radials to a groundplane with eight radials 1/4 wave 
above ground.

Somehow they thought moving the feedpoint eliminated the need for radials 
with an electrically short antenna, when the real mechanism was a 1/2 wave 
vertical was converted to a 1/4 wave groundplane 1/4 wave above ground and 
it only got a tiny bit weaker. The groundplane still had 8 radials, but they 
were hundreds of feet in the air.

There was some more stuff about offsetting the feedpoint in that handout, 
but nothing that remotely applied to a fractional wavelength vertical just 
sitting on the dirt with a few radials laying directly on the lawn.

They got rid of lossy traps and loading coils by using even lossier coax and 
some folded wires for a loading system.

This is all why, as frequency increases and the current and voltage moves up 
the antenna, the GAP on most bands isn't terribly bad.  This also why it is 
a real dog of an antenna on 160 and 80, where it is very short electrically, 
has no ground system, has an exceptionally poor loading method, and where it 
folds the radiator back and forth which suppresses radiation resistance.

This is why a ten foot mobile antenna can tie it or beat it on 160, and why 
it is reasonably on par with anything else on most bands above 80 meters.

73 Tom