[Lowfer] Re:helically loaded verticals

[James Moritz]

Dear Lowfers,

At 12:02 PM 26/03/2002 -0500, you wrote:
>Rrad/Rrad+Rground remains basically
>unchanged no matter where the coil is located.
><<klip>>
>It works that way on 160 meters. It works that way on 1600 meters

It is true that, with a reasonably large top hat, the position of the 
loading coil won't make much difference to the current distribution - but 
the voltage distribution will be changed. The antenna between the loading 
coil and the ground will be at a low voltage, while the antenna connected 
to the hot end of the loading coil will be at a high voltage. Elevating the 
loading coil will move the high voltage parts of the antenna away from the 
ground, reducing dielectric losses in the ground and things near it, and so 
reducing Rloss. For a given antenna and top hat, the effect of dielectric 
loss will be greater at LF than at HF because the reactance of the antenna, 
and hence the voltage for a given antenna current, increases as the 
frequency is reduced. Rloss appears to be dominated by dielectric effects 
at LF, due to the high impedance nature of LF antennas. Measurements on 
several amateur LF antennas has shown that Rloss is very roughly inversely 
proportional to f, at least over the range where the antenna is small 
compared to a wavelength. So what happens at 1600m is not quite the same as 
on 160m, because of the difference in scale, so having helical loading, or 
an elevated loading coil could be an advantage on LF.

A couple of stations over here have tried putting the loading coil up at 
the top of the antenna, with limited success. There are considerable 
mechanical difficulties in doing it, especially if a large coil is needed - 
but some reduction in Rloss was obtained. Also, HB9ASB used a helically 
loaded vertical (with top loading), wound on plastic pipe, until it was 
destroyed in a storm.

Most of us have found over here that the losses in a reasonably constructed 
loading coil will normally be swamped by other losses in the antenna. The Q 
of a coil is only a weak function of it's shape. Big loading coils are 
still needed to avoid voltage breakdown or overheating when running high 
power - a long, narrow loading coil is an advantage in this respect, even 
if it has slightly lower Q.

Cheers, Jim Moritz
73 de M0BMU