Topband: inverted L antenna match system

[k8gg at voyager.net]

Fellow Topbanders,

"sasas asasas" <tzitzikas_ee at yahoo.com>
wrote:

> Hi. I have a 500w linear amplifier at 160m band.
The output is 50ohm. My 
> antenna is an inverted L with lenght =
[5*(wave lenght)]/16. I read at 
> ARRL handbook that a variable
series capacitor 0-1000pf is enought to 
> match the transmitter
with my antenna. What do you think about this? Do 
> you have to
propose anything else? (maybe a variable coil with the one 
>
point grounded and the other point connected between capacitor and 
> linear's output)? 
> I have install 6 radials (2 are (wave
lenght)/4 and the other 4 (wave 
> lenght)/8) 
> 

>From both modelling and actual experience I have found that a 5/16
wavelength (wl) inverted-L antenna over a good ground has a feed point
impedance Zant of approximately:

Zant = 50 + 200j ohms.

The actual dimensions we used some years back on Curacao were 85
feet up and 75 feet out with about a 20 degree down slope on the
"horizontal" part of the wire.  There were 40 radials
each about 80 feet long in the ground system over sandy rocky soils. 
We were able to tune the antenna with a series capacitor to achieve Zant =
50 +/- 0j ohms.  The capacitor was set at approximately 450 pfd on a
1000 pfd 5000 V vacuum variable.

Using only 6 radials will not
achieve the desired results as ground losses will increase and Zant will
be greater than 50 ohms.  

If the ground system is
kept as described above, then either:
1.  An L-network to bring
the impedance down to 50 ohm can be used or
2.  The antenna
length can be shortened to bring the inpedance to Zant = 50 + XXj ohms
where the XX value can be tuned out by the series capacitor.  If one
has some sort of impedance bridge this is not hard to measure on a trial
and error basis.  Don't forget that the end wire folded back on
itself will act like the fold point is the end if it is clamped tightly to
the "live end" of the antenna wire:

      
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insulator

I hope this helps.  GL  
73    George   K8GG