dansawyeror wrote:
All,

A fundamental basic question, which is the primary purpose of radials:

1. is it to create a ground, that is a as close as possible to zero ohm
virtual reference for the 'real' vertical half of the dipole?

There is no such thing as "creating a ground". As for a "virtual
reference", you can declare any point on any conductor a "reference" and
for that matter "ground" that you wish.

2. Or are they to create a real resonant half of a dipole?

There's no need to try to make a dipole. If the radials are on or very
near the ground, their sole purpose is to reduce the amount of loss due
to current returning to the base of the antenna. The current entering
the antenna at the base equals the current flowing into the source
through the ground. This ground current results in I^2 * R loss; radials
reduce the R and therefore the loss.

In the case of a "ground plane" antenna with highly elevated radials,
the radials provide a path for the base current to flow (again, current
out of the source -- into the antenna -- has to equal the current into
it -- from the radials). Because of the physical configuration, it's
sometimes more convenient to build an antenna this way instead of making
a dipole. The radials radiate very little, and the vertical section
radiates twice as much per unit length as a dipole, resulting in the
same overall gain and pattern.

If it is the first then what does the 'efficiency' curve look like for a
shortened, loaded, vertical? That is if the vertical element is loaded
to resonate at 1/5 of a half wave length what does the ground resistance
profile look like for 120 radials at various lengths of 1/20 wave, 1/10
wave and 1/5 wave?

The answer to this depends on the ground conductivity and frequency. But
the radiation resistance of the shortened antenna will be less than that
of a full-height one. Therefore, if the ground resistance is fixed and
determined by the ground system (not completely true -- it does depend
some on the antenna height -- but close enough for discussion), the
efficiency of the short antenna will be less than for a full-height one.
I recommend finding and reading "The W2FMI Ground-Mounted Short
Vertical", by Jerry Sevick, W2FMI in March 1973 QST. He built several
antennas very much like you describe and made extensive measurements.


The question I am really driving at is if mesh is layed down at 100%
coverage about what fraction of a wave length needs to be covered to
create a 2.5, 5, and 10 ohm equivalent ground for the vertical above?

Sorry, I don't know the answer to that one right off the bat. It could
be determined with NEC-4 modeling, but I don't have time to do that. I
suggest that you locate a copy of Brown, Lewis, and Epstein's paper
"Ground Systems as a Factor in Antenna Efficiency", now posted on the
web. You should be able to get a fairly good idea from their
measurements of 113-radial systems.


In the paragraph above is the mesh simulating a ground or is it fact
operating as a ground.

I really don't know what "simulating a ground" and "operating as a
ground" means. But the radial field doesn't act like either real Earth
or a perfect infinite plane, if that's what you mean. If sufficiently
fine, a mesh will act like a solid conductor the size of the mesh.

Roy Lewallen, W7EL