Topband: Shortend Vertical model questions

[Tom Rauch]

> 
> Yet, isn't there a "watch out for" when using large differences of
> conductor diameters ?  (like a 6" vertical and #14 top loading as to
> the real world numbers)


Yep, that's the main reason Roy included average gain in the 
windows version, so you could watch for errors. You do have to 
carefully look for weird results.

Turns out average gain works especially for receiving antennas, as 
well as for looking for errors when modeling! What a nice feature it 
is!

In this particular case the model corresponds to past 
measurements and what theory predicts should happen. Nothing 
funny or odd seems to be going on.

1.) The hat is the primary "current sink", rather than distributed 
capacitance of the vertical section.

2.) Current is almost uniform in the vertical because most of it flows 
into the hat, that makes radiation resistance high. 

3.) The inductor corrects power factor equal current at each 
terminal (as it should have) and can be located anywhere in the 
vertical section without significant effects on performance.

4.) A thinner vertical section has less distributed capacitance, 
allowing more current to flow into the hat.

Radiation resistance and bandwidth don't change much with coil 
location, as expected from the above. 

My mobile antennas were constructed using the same principles 
above, and work very well. The low coil position allows me to use a 
large coil without mechanical difficulties, bandwidth is wide, and 
efficiency is high.
73, Tom W8JI
W8JI@contesting.com