[Towertalk] Grounding an Elevated vertical

[Guy Olinger, K2AV]

See below. 73, Guy.

----- Original Message -----
From: <K4IA@aol.com>
To: <k2av@contesting.com>
...snip...

>If the ground screen is not connected to the antenna, what is the
mechanism
>by which reducing the ground current losses improves your signal?
Where
>would the ground current be going?  Seems to me if it does not go
back into
>the antenna system, it is heating worms no matter how conductive you
make the
>earth below the antenna.

It is not necessary for elements in an RF system to be literally
interconnected by wires. If it were, transformers would not work.
Yagi's would not work. No antenna with a parasitic element would work.

There is a shielding effect that occurs inside or behind a system of
conductors. For a given frequency, as the ground mesh or radial system
get increasingly dense, the system goes from being buried conductors
that are just passing current on to semiconducting (hence lossy) earth
underneath to being a shield which prevents penetration, effectively
behaving like a copper sheet. This occurs with 1/4 wave radials for
their extent of coverage as the number of radials runs up to 60.

I think I recall some stuff that W8JI did with 60 raised radials and
being shielded underneath, but I'll let him chime in with that if he
wants. Same effect.


>Point 3, I understand but on the first bounce you are way beyond the
radial
>field.  And, as you point out, ground clutter may have a substantial
impact.
>Perhaps the water and salt water verticals benefit from a clear line
of sight
>as much as from a conductive ground?

>And herein lies another source of confusion for my overheated brain.
If the
>ground is highly conductive, why isn't absorbing energy instead of
reflecting
>it?  Isn't that what conductive means?

Loss in most of these discussions is a shorthand for resistive loss
converted to heat, and not radiated. A perfect conductor in a yagi
would have a current limited by "loss", but the "loss" would be
entirely to RF radiation, which is desired. Hence the term radiation
resistance. Absent radiation or transfer of power due to a transformer
effect, increased current in a conductor is due to DECREASED resistive
loss. If it's not heating the ground due to a shielding effect it HAS
to go elsewhere. It doesn't have infinite current, so the only
destination is radiation, hence the bounce.


>Low resistance would equal high
>current in the ground.  Put another way, if I want to reflect light,
I use a
>mirror, which is highly resistive to light passing through.  I could
use a
>piece of clear glass but I lose a lot of reflection because the glass
is
>highly conductive.  Just another way of looking at it . . .

Radio K4IA
Craig Buck
Fredericksburg, Virginia 22401 USA

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