Topband: Feedpoint voltage

[K4SAV]

Gary Smith wrote:

>Another bugaboo for me to deal with; 
>
>I have the 160 inverted L set up pretty much ready to go. I have 
>about 150' of wire antenna set up somewhat similar to what you see 
>here http://users.erols.com/k3mt/inv_u/u_160.htm except I have far 
>more radials laid out. I will have the antenna resonant on the CW 
>portuion of the band and will not be needing more range than that.
>
>Problem is the capacitor. .....................
>

For a single series cap used for tuning an inverted L (or T), the amount 
of capacitance you need varies depending on the height of the vertical 
section and the length of the top wire.  The amount of voltage on that 
cap varies depending on the value of the cap and the power level.  In 
general the smaller the value of cap required to tune it to resonance, 
the higher the voltage on the cap.

Example: My inverted L requires 330 to 190 pf to tune it across the 
entire 160 band.  The voltage on that cap (at 1500 watts) varies from 
2200 to 2940 volts peak.  Peak voltage is important here, as well as a 
lot of margin to handle high humidity if the cap is has an air dielectric.

An easy way to determine the cap specs is to simulate it in EZNEC.  It 
will give you all the parameters.  I would also suggest that you model 
the antenna you are constructing and not rely on the info in that 
referenced link.  (Or find a better article.)  There are several 
significant errors in that article.  For example there is no way to get 
an antenna impedance of 150 ohms on a 150 ft inverted L after tuning to 
resonance on 160.  There is also no way to get an antenna efficiency of 
66% from the inverted L he described with two radials on the ground.  
His explanation of loss is flawed.  You also don't need the step-up 
transformer.  The feedpoint impedance of that antenna using about 30 
quarter wavelength radials should be about 35 ohms after tuning it to 
resonance with a cap.  That will be a reasonably good match for 50 ohm coax.

Jerry, K4SAV