[TowerTalk] Top hat on mobile usefull?

[Gary Schafer]

Tom Rauch wrote:
> Hi Gary,
> 
> 
>>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.
> 
> 
> I'm under that impression, as are all the textbooks and
> modelling programs I have.
> 
> 
>>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.
> 
> 
> Sorry, but I strongly disagree.
> 
> It is a well established and accepted fact the open ended
> region of any linear conductor...be it a vertical or
> dipole....contributes very little to radiation. That's why
> we can replace the open end areas with hats or other forms
> of "current sink" and have very little change in pattern or
> radiation resistance.
> 
> Basic physics tells us EM radiation is directly tied to
> charge acceleration, not voltage. There are significantly
> more charges moving in high current areas of the antenna
> than at open ends.
> 
> This is why inverted L antennas are so effective as vertical
> radiators, and why there is very little pattern or radiation
> resistance change when the end of an antenna is bent.
> 
> It all comes down to ampere-feet.
> 
> 73 Tom
> 

Radiation resistance increases along the length of an antenna. Higher 
radiation resistance means less current for a given amount of power.

73
Gary  K4FMX