[TowerTalk] Shunt-fed grounded towers

[Jim Brown]

On 6/30/2023 8:33 AM, Radio KH6O wrote:
> (*When the FCC enacted new repeater licensing regulations in the early
> 1970s, we were required to do this and plot our results on a
> topographic map; thankfully those regs were eventually relaxed.)
> 

As an EE student in the early '60s, I worked for a consultant who 
designed multi-tower MW AM broadcast arrays to fit new stations into a 
band that had been full for 20 years. The variables were the locations 
of the towers, their orientation with respect to each other, and the 
amplitude and phase of the current with which each was driven. A half 
dozen of us sat around a boardroom table with slide rules, math tables 
to look up Bessel function, and very wide accounting spreadsheets 
computing their patterns for every five degrees of azimuth and elevation.

Then I'd use FCC nomographs for ground wave attenuation vs FCC soil 
conductivity maps to plot field strength contours for interference to 
existing stations. The consultant would study the result, change a 
variable or two, and we'd repeat the computations.

These arrays were intended to produce deep nulls in the directions where 
stations must be protected. Once the array was built and every year it 
operated, stations were required to "run the radials," making 
measurements in increments of distance to verify that the pattern was 
still being maintained. A water tower in the wrong place could fill one 
of those nulls.

73, Jim K9YC