Topband: CAPACITOR VALUE FOR INVERTED L ANTENNA

[K4SAV]

Commander John wrote:

>Hi All
>   
>  I am in the process of putting up a inverted L type antenna. 165' total length.  50' vertical, the rest horizon. 1.8 - 2.0 Mc.
>  What value variable capacitor should I use to tune this antenna , and the voltage rating of the capacitor, please.
>   
>  Thank you.
>   
>  john W9ZY
>  
>
Assuming the end of the top part of the L is also at 50 ft, and that you 
have about 16 radials of about 1/4 wavelength, and average ground, for 
resonance at 1.83 MHz you need about 400 pf.  The resistance of the 
antenna is 32 ohms (SWR 1.6).  The antenna is too short to get a 50 ohm 
match.  If you match the 32 ohm resistance and feed it 1500 watts, the 
voltage on the cap should be close to 1370 VRMS.  For resonance at 1.91 
MHz you need 300 pf, antenna resistance = 36 ohms (SWR = 1.4), cap 
voltage = 2060 VRMS.

If you want the antenna to have a better match for 50 ohm coax, the top 
part should be 140 ft in length.  With 200 pf cap this resonates at 
1.815 MHz with a resistance of 42 ohms (SWR = 1.2), 2 to 1 SWR bandwidth 
= 45 KHz,  cap voltage for 1500 W = 2680 VRMS.  With a cap of 150 pf it 
will resonate at 1.9 MHz with a resistance of 52 ohms (SWR = 1.06).  2 
to 1 SWR bandwidth = 40 KHz.  Cap voltage for 1500 W = 3560 VRMS.

If the top part of the antenna is not flat, it will significantly effect 
the answers.  Also number of radials, wire lengths used for the matching 
cap, and stray capacitance has a small effect.  For example if the end 
of the top is at 20 ft instead of 50 ft and the top is still 115 ft 
long, with 400 pf it will resonate at 1.84 MHz with a resistance of 22 
ohms (SWR = 2.3).

Jerry, K4SAV