Topband: modeling question

[Tom Rauch]

> When I changed out the old TH6 for a nice shiny C31XR I worried about
> the insulated elements. I asked Force 12 about it. I was told there
> has been only one reported instance of insulator failure in that
> application and that was using "excessive" power levels.

You'll see in my example at the end the statement above is a wild 
guess without any meaning. You were very lucky it worked Larry.
 
Voltage goes up only by the square root of the power increase. If 
you don't have enough headroom for 4 kW you certainly won't be 
completely safe at 1500 watts.

On the other hand impedances are all over the place. The 
difference in voltage between the insulated element and the boom 
can be staggering. Besides power, we must consider:

1.) The longer the boom and fewer the elements, the larger the 
voltage gradient between elements and boom.

2.) The higher the antenna on the tower, the larger the voltage.

3.) The shorter the tower (assuming losses remain low), the higher 
the voltage.

4.) The better the ground system, the higher the voltage.

You could do this with almost any modeling program: 

Model the antenna and tower. Connect the elements one at a time 
to the boom with a wire. Insert a very high resistance load in the 
wire, and find the current in that load. Use ohm's law to calculate 
the voltage across the load. Some programs will tell you the 
voltage across the load directly.

Eznec makes it simple. It will allow you to insert an additional 
current source in the wire between the element and boom, and set 
the current of that source to zero amperes. By looking at the 
source data table, you can directly read the voltage across the 
zero current source. That source voltage will be the exact element 
to boom RMS voltage, just multiply it times 1.414 to get peak 
voltage.

I just did that for a 50 foot mast with a 30 foot element, everything 2 
inch diameter. At 1500 watts, the element to mast voltage is 9221 
volts RMS, or 13kV peak. Ouch!!! 

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.

73, Tom W8JI
W8JI@contesting.com 


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