Topband: T - vertical improvement

[Herb Schoenbohm]

K4SAV wrote:
> WX5G wrote: At 1.5 kW the 370 pF cap will see 10 kV RMS. You are looking 
> at a vacuum variable capacitor.
>
> Mike wrote: Great discussion, is there a good source of vacuum variables 
> (at reasonable prices)
> for this application.  Everything I've found has been in the $300+ 
> dollar range?
>
>   

There is a lower cost approach to tune out inductive reactance then the 
more expensive vacuum variable especially in the 15KV range. Also bread 
slicers can be a problem with corrosion, pitting, and breakdown,  over 
time when mounted outside even in good enclosures.  What I recommend is 
an old broadcast ATU trick of making a fixed mica capacitor variable.  
Heres the trick..... if you need 370 pf as in the example quoted, put a 
fixed mica G2 of a higher value, lets say 500 pf.  then add an inductor 
in series.  (Preferably a silver pated flat wound) Move the taps until 
the combination gives the inductive reactance (you want to tune out) of 
the antenna equal to the C plus L that you have placed in series. 
Compared to what the originally cost AeroVox mica high current 
capacitors are very low cost on eBay.  Also if you want to start with 
500pf and can't find that value you can always buy two 1000pf units and 
bolt them in series for half the total capacitance..  In the later case 
two 5000 volt units will give you 10,000 volt rating. 


Good luck

Herb Schoenbohm, KV4FZ