Topband: modeling question

[Tom Rauch]

> >By changing height to 100 feet, no other changes, voltage 
> >becomes 2576v RMS or 3642 volts peak between the element and 
> >boom. Still high for the insulators I see on most antennas.
> 
> Thanks, Tom - I was wondering how you could quantify the voltage. 
> Sounds fairly safe with a 100-foot tower and 100 watts -- maybe ~900
> volts peak? The Force 12 antennas use PVC pipe with ~1/8" wall as the
> insulation.

My point is the voltage varies all over the place with the electrical  
characteristics of the structure. Power can NOT be used to 
quantify the problem. (100 watts might be excessive power in some 
cases) 

With that in mind......every system needs to be modeled to see 
what the voltage is, and you need lots of headroom for safety.

Even if you don't manage to burn through the insulator, it can still 
carbon track or arc causing distortion and bandwidth problems on 
CW or SSB.

IMO it probably isn't worth risking problems since the solution of 
using an minimal reactance inductor to ground the element to the 
boom is simple, cheap, and fast. Insulated element antennas are 
not a good idea on towers used as radiators, or used as 
counterpoises (like feeding slopers against the tower) for antennas.

73, Tom W8JI
W8JI@contesting.com 


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