[BC] anti-fading radiators

[Richard Fry]

Clive Warner:
>The extra 2dB is not 'free' power, it comes from reduction of the skywave.
>I believe the overall effect (benefit) is to improve the 200mV/m groundwave
>contour, at the expense of the (fading prone, due to sky/groundwave mixing)
>intermediate zone where groundwave does not, in any case, provide a usable
>service.

True for distances where the skywave and groundwave fields have roughly the 
same magnitude.  The distance to that zone is a function of the 
conductivity/clutter/terrain profile of the groundwave path, vertical plane 
ERP, and the nature of the ionosphere.

But the discussion here was about parts of Pittsburgh losing KDKA's signal, 
and about their weak signal in the South Hills area.  The distance from 
KDKA's tower to the Pittsburgh Triangle is about 8-1/4 miles, and to the 
South Hills about 15.  The amount of skywave landing that close to the tx 
site will be extremely small.  Those distances are served by the groundwave, 
only, for all practical purposes.

>At the same time, the skywave radiation angle is lowered, resulting
>in a usable strong skywave signal at a greater distance than you would
>get from a simple quarter-wave.

Perhaps surprisingly, a 1/4-wave vertical has more relative field at all 
elevation angles between zero and 90 degrees than verticals of all greater 
heights to 5/8-wave.  The advantage of the higher (longer) radiators is in 
the shape of their elevation patterns, which at lower elevation angles have 
higher _absolute_ gains than the 1/4-wave.*   For a given tx power, this 
increases the groundwave, favors longer skip paths, and reduces the land 
area where the skywave and groundwave can mutually interfere.

  * except for heights of, and close to 5/8-wave, where the absolute gain
    exceeds that of a 1/4-wave radiator from roughly 60 to 80 degrees 
elevation.

RF