Topband: Antennas

[Larry Molitor]

At 11:55 AM 8/27/01 +0200, i4jmy@iol.it wrote:
>In the case you slope those wires down near to the ground, it's no
>wonder the system efficiency drops because also the Rr becomes probably
>very comparable with the unavoidable ohmic losses you have (basically
>the ground resistance).


As I said, Mauri, my experience and modeling efforts agree with your 
assessment. I think I need to define "short". Some folks seem to think a 
short vertical is a couple feet less than a quarter wave. I consider an 
short vertical to be less than 1/8 wavelength. Both my modeling exorcises 
and my practical "portable" vertical antennas are almost always between 
0.093 and 0.065 wavelength in the vertical portion.


>Keeping in mind that with short verticals the current is almost
>constant below the loading device, it's useless to have a longer
>antenna portion below the loading coil if the total current is
>consistently decreased by coil losses.

I have seen this to be true with very short antennas such as used in 
mobile-in-motion applications. However, at lengths approaching 1/8 wave, 
both modeling and experience have shown center inductor loading to have a 
clear advantage over top load and a small advantage over base load.

Thanks & 73,

Larry - W7IUV