[TowerTalk] Top hat on mobile usefull?

[Gary Schafer]

Dudley Chapman wrote:
> 
> Secondly, in terms of effective height, I am of the opinion that so little
> current exists in the whip above the coil of a heavily loaded short vertical
> that the effective height is not much higher than the coil itself.  So
> therefore, unless you have a very big tophat at the top of the whip, such
> that you could greatly reduce the inductive loading (thereby increasing the
> current above the coil significantly) the position of the tophat doesn't
> make much difference to effective height.  The mast below the coil, and the
> coil itself probably contribute to almost all the radiation.  Also, I think
> a few feet difference in effective height on 40m is not very important.
> Your reasoning might be a better argument for increased radiation
> efficiency, rather than increased effective height.

A lot of people are under the impression that the higher the current in 
a particular part of the antenna the greater the radiation from that 
part. That is not the case. In the instance of a quarter wave length 
antenna the current is highest at the feed end and lowest at the open 
end. That theory should then say that only the lower part of that 
antenna radiates and the top, high impedance part, does little 
radiating. But all of that antenna shares nearly equally in the 
radiation of the signal. Once the electric and magnetic fields are a 
short distance from the antenna they equalize, no matter which one 
started out stronger. So it does not matter whether you are radiating 
from a mostly high current antenna or a mostly high voltage antenna. The 
current distribution does affect the pattern somewhat but not the amount 
of power radiated.

Raising the the current point up on the mobile antenna raises the feed 
resistance and allows more power to be coupled to the antenna to be 
radiated.

73
Gary  K4FMX