Wimpie wrote:

snip

I ran some simulation also (old version IE3D, now part of Mentor
Graphics). Of course this software has limitations also (for example
surface wave effects), but it accepts conductors on and inside lossy
dielectric layers.

When I saw your gain figure for the capacitive head case, some red
flags appeared in my mind.

For the free space case, loss of the wires is acceptable (that is 4
radials, 1 radiator and 4 capacitive head radials, all about 4.5m
(15') ) I get an impedance around 0.6-j1300 Ohms for: 4 radials, 1
vertical wire, 4 capacitive head wires, all 4.5 m long (about 15').

Some numbers:
Using 100W input and assuming very high Q for the series coil (that is
2000), feed current is about 13Arms. Voltage between top and bottom
radials will be about 24 kVp (24000 Vp, nice corona display). Because
of same size of radials and head, this 24kVp distributes as 12kVp at
the top, and 12 kVp at the bottom radials.

Yes, but this was an exercise on the effect of loading, load placement,
and the effects of a top hat and nothing more than that.

I also neglected to, but should have stated, that the resistance of the
coils is zero.

When approaching a PEC ground, radiation resistance rises with factor
2 (as expected). With a lossy dielectric layer (good ground), things
are different. Directivity is 4.4 dBi @ 19 degr elevation, but real
part of input impedance rises significantly, radiation efficiency and
gain reduces significantly.

Ehh?

For a fixed ohmic loss, as the real part of the input impedance rises,
the radiation efficiency should increase as radiation efficiency is
directly proportional to the two.

The quality of the ground does have a large effect on the numbers
and the ground I used was "average". Changing the ground changes
the numbers but not the general effects of loading coil placement
and the effects of top hats, which was the only point.

Based on comparison between measurements and simulations for similar
structures (but other frequencies), your radiation efficiency will be
in the 1 percent range. I would not classify that as a good radiator.

For such small structures, I would not expect to get numerical accuracy
any better than a single digit and correct order of magnitude.

And again, this was an exercise in getting the real part of the impedance
greater than the ohmic losses and showing the effects of load placement
and top hats, not an exercise in practial antenna design.



--
Jim Pennino