[TowerTalk] Part 1:Soil Conductivity - Tutorial??

[Jim Reid]

Hi again,  I have been asked some questions about our antennas
and how they do/don't operate out here in Hawaii.  I have
tried to answer,  I hope accurately.  Should you care,
please have a look at the following to see if corrections
are needed before this stuff finds its way into several
of the various island's radio club newsletters!

This will be in two parts,  this the first,  and a second,  and
longer post because of the added questions this one
seems to have inspired!

This is what started it:

>Well we all know here on the lava rock of the Big Island, that the 
>ground has very little conductivity.  In fact GTE HI Tel and Helco, our 
>power co says "There is No ground."  Yes we know very bad for verticals.
>
>But what  about  beams?  Is the ground reflection below where your feet are?
>Does this mean the  tower is electrically higher than the base?  So is 
>poor ground good for beams?
>
>Still being a new HF ham, maybe I'm not understanding this correctly.

My attempt at some answers{

Well,  unfortunately for the beam,  the signal will still strike the surface
of our soil.   Since the soil is composed mostly of tiny glass volcano beads,
it is has pretty high dielectric characteristics;  that is,  the electric
field from our signals is perfectly happy to penetrate the soil,  as you
suggest,  just as an rf signal is happy to cross a capacitor.  However,  in
the process of penetrating until it finds somewhere to reflect,   the
electric field component of our signals is busy polarizing the molecules
of the volcanic glasses in the soil!!  This uses up energy from our signal
and is lost!!  The polarizing action actually moves the molecules which
causes friction which then produces heat,  and there goes,  who knows
how much of the energy we were hoping to radiate.  Not all of the
signal is lost this way,  of course.  In fact,  the reflection component
is the difference between what antennas do in free space,  and what
they due when ground mounted over excellent conductivity soil.

Note:  the total difference of a dipole's performance is, in theory,
something like  6 dB better when mounted down and over
perfectly  conducting  ground from  performance in free space.  The
first 3 dB comes from the elimination of half of the sphere of space
by cutting it away with the plane of the ground;  the 2nd 3 dB is
picked up from the far field reflection off the high conductivity
of excellent soil.  Even over lousy conductivity soil,  we get half
the sphere taken away by  the presence of the earth,  so we still
gain that 3 dB over free space antennas.

Over excellent soil,  the part of the signal which strikes the ground
out the first few wavelengths will be perfectly reflected and will
add up to 3 dB of signal in the direction you have your beam
directed.  To the extent some of that energy is lost,  and not
reflected and added to the direct signal,  you loose some of
the 3 dB difference between ground mounted and free space
antennas.  This is all described in the ARRL Antenna Book,  and
also, to a lesser degree in the ARRL Radio Handbook.

So,  you see,  the most we can be penalized for living on zero
conductivity soil,  when using a beam,  is about 3 dB.  

When using a vertical,   poor soil conductivity can cause two
problems:  you can loose up to 3 dB of your power,  trans-
mitted or received,  in ground current loss,  and you can loose
the advantage of a vertical,  which is to put a pretty good
signal out at very low elevation angles,  even to the horizon
if you happen to be over sea water. 

For beams to put much energy directly down to the horizon, they
must be mounted very,  very high,  a wavelength or so I suppose
to get energy down to 1 degree angle.  Of course all antennas
get some energy radiated at low angles,  else we would almost
never QSO with DX,  much of which is done with signals coming
from and going out to angles below 12 or so degrees from the
horizon.  Dipoles have their peak energy radiating away up around
30 degrees or so,  but there is some at very low angels,  just down
several dB from the main energy peak of the pattern.

I hope what I have written bears  somewhat on the truth of the
situation!  I think it is about correct,  but maybe.....?

73,  Jim,  KH7M


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