Hi Jeff,
In a message dated 96-09-29 00:18:40 EDT, you write:
> Their mission
>is to maximize field strength in their service area usually less than 50
>miles. Radiation of sky wave is lost power and can cause interference to
>other stations assigned to the same frequency. In my case my goals are just
>the opposite.
A properly installed ground system does not affect high angle radiation. In
order for the radials to greatly affect high angle radiation, they have to
radiate. The only way a radial system will radiate is if we use very few
radials, and they are non-symmetrical in current or physical distribution.
>Generating a ground wave causes local interference and reduces
>the skywave energy.
Other than a large change in height or current distrubution of the actual
vertical element, the ground system does not control the ratio of radiaion of
sky wave to ground wave radiationl. The groiund system primarily changes
overall efficiency. Unless the radiator is substantially modified or the
radials are radiating like an antenna, a stronger groundwave always means a
stronger skywave.
> I'm sure if I were located on level lowland I would want
>to put out a thousand 300 foot radials but that isn't possible.
Once the radials are increased beyond 50-60 for 1/4 wl long wires, or 100 for
1/2 wl long wires, there is no real change. 100 1/2 wl radials are so close
to perfect anything beyond that is a waste of time. The exception is if you
are interested in very low wave angles and you can install many radials many
wavelengths long in the desired direction.
>One thing
>that Jeff said is true under all cases is that a ground screen in the
>vicinity of the antenna will reduce near field losses. This is true
>regardless of using ground mounted or elevated radials.
It would be a good idea to read a copy of the report from the years of
research by Lewis, Brown and Epstein. Their study conclusively proved unless
the radial field was very sparse, a ground screen does no good at all.
>This loss is not
>taken into account in many of the computer modeling programs. This accounts
>for some overly generous calculations of total gain for low horizontal
>arrays. When the ground is good it can deliver up to 6 db of ground gain
but
>if it is poor it can suck out 10 or 40 db.
Even saltwater won't approach 6 dB of "ground gain" with a low radiator (even
a vertical). What we really have is less loss until the ground becomes
perfect. The whole problem stems from the mental picture formed by drawings
(found a a general rule only in Ham magazines and articles) showing a
"capacitor" from the antenna to earth, and the thought that "capacitance"
causes displacement currents in the earth.
Charges in the earth are excited by both induction fields ( electric and
magnetic) and by the electromagnetic radiation field. Trying to ignore two
other large effects causes a lot of misundersatndings.
The long and short of all of this is put in as many long radials as you can,
and don't worry about all the recent gimmicks. With 60 1/4 wl radials above
the ground, on the ground, or slightly below the surface (and no ground
screen) the system will be within about a dB of theoretically perfect.
Otherwise we can take a chance on unconfirmed models and theoretical analysis
that uses a simple capacitor to replace complex induction and radiation field
effects.
If the radial system is small, or the antenna is very short, a large ground
screen will help. This stuff has been confirmed many times since the 30's in
real world measurements.
Thinking a few elevated radials will beat 60 radials is absolutely wrong. The
best we can hope for is the sparse elevated system might come close. From
what I've seen with field strength meters, I believe that's *very* wishful
thinking.
73 Tom
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