[TowerTalk] Top hat on mobile usefull?

[Tom Rauch]

> An antenna can be viewed as a transmission line. A quarter
wave antenna
> has the same properties as a quarter wave transmission
line. Just
> because one end of the line may be a high impedance
compared to the
> other doesn't mean that the high impedance end does not
see any power
> transferred to it.

Behind every EM field is an accelerating charge. Physics
texts descibe radiation as being produced by charge
acceleration.

> If power didn't get transferred to the high impedance
point then a half
> wave length vertical antenna would never work.

EM radiation primarily comes from the high current areas.

Even if we don't believe books that tell us EM radiation
primarily comes from high current areas of the antenna, we
can all plainly see this effect in models.

For example:
 1.) In collinear antennas it is the distance seperating
high current areas of each radiator that largely determines
gain. We can fold the low current open-ends over with
virtually no change in feedpoint impedance, gain, or pattern
so long as the high current areas maintain the same
seperation. If the entire antenna length shared equally in
gain, removing the contribution of a section of the antenna
by loading or folding would produce a significant change in
pattern.

2.) A vertical antenna has an elevation pattern largely
determined by the position of the current maxima. If your
proposal was true and the entire length was equally or
nearly equally responsible for EM radiation, current
distribution changes would produce no change in pattern. We
all know (or can easily see with a model) this is not true.

3.) An inverted V dipole's effective height above ground is
a large percentage of the apex or current maxima height
point. That's because amouint of EM radiation increases with
increased current.  If all areas contributed equally to
radiation the effective height would be height of the
halfway point of each leg, but it isn't.

4.) Moxon's wouldn't work well because low current areas are
close together.

It's pretty easy to manipulate current distribution and see
what I mean.

This is why a thin short monopole with uniform current has
about four times the radiation resistance as the very same
antenna with a linear taper. It's also why base current in a
short marconi antenna of given length with uniform current
has about half the current as one with a straight line
current taper to zero. It all comes back to current over
linear distance.