Topband: Resonating a 160 Meter Vertical

[k8gg at voyager.net]

Phil - K5PC wrote:

> I have a 160 meter vertical (ground
plane) that now resonates on 1530 KHz. 
> I want to put a
capacitor at the feed point to raise resonance to 1830 KHz. 
> Do
I put the capacitor in series with the vertical radiator, in series 
> with  the radials, or across the feed point? Is one method any
better than the 
> others? 
> (((73))) 
> 
> Phil. K5PC 

Phil, et. al.:

Do you have
the ability to measure the impedance of your antenna at 1830?  A
better answer could be given if the impedance both R and XL are known.

Depending on the geometry and ground conditions, a series
capacitor between the feed point at the bottom of the vertical and the
coaxial center conductor may do the job for you.  
The
inductive reactance will probably be around +150j to +190j.  If so
you can use a series capacitor of approximately 450 to 600 pfd to tune out
the reactance.  If your SWR is reasonable with the series capacitor,
then I wouldn't go any further.

However, the next question is
what is the resistive component of the impedance?
Theoretically, it
will be above 50 ohms, could be as high as 80 or 90 ohms, and an
L-network may be the best answer for antenna matching.

For
mobility up and down the band, it may be easiest to ignore the SWR in the
coaxial feed line and use an L-network tuner at the shack to get the best
match.  The losses will be insignificant if a good grade of RG-213 or
similar or decent 75 ohm hardline is used to feed the antenna.

73 & GL     George    K8GG