Re: [Amps] Shunt Feed Tower Issues

[Dennis <pegasus@mho.net>]

Hello Javier...
  The rating of the capacitor is not defined by either frequency or 
power directly.   It is determined by the voltage that appears across 
the plates.  When using capacitors in such an antenna as yours voltages 
can go to extreme levels across the cap.  Traps, and especially parallel 
circuits, can produce voltages in the tens of thousands even with 
moderate power levels.  Depending on how it is used the voltages can be 
very tame, but I would start by using a cap with a very high voltage 
rating... perhaps 30kv or more, and even then increase your power 
carefully.  Also, be sure the external surface  of the capacitor is 
clean and MAKE sure that nothing is touching it while in use.  I have 
seen a doorknob capacitor throw a 1 inch arc to a nearby object then 
shatter in to a hundred pieces in a ball of flame.  The power level was 
about 800 watts. 
  I don't believe that tuner manufacturers should publish power ratings 
as agressively as they do... the voltages developed across the 
components are dependent on the quality of the load as much as the power 
going in to it.  I've seen a tuner throw a 3 inch arc at 200 watts.  
That same tuner can handle 1,000 watts with no trouble with a different 
load.   There is no way for me to tell you how large of a cap that you 
will need in your circumstance.  We'd have to know the size of the 
tower, where the shunt is, and lots of other things.  Then maybe someone 
smarter than me could calculate it very close.  But there is not enough 
information in your post to do that.
Start large, and go smaller if you can.  But 1,300 watts is a lot of 
power to go across an air variable in anything except nice 
low-impedence, low reactance, applications.
73,
Dennis
NØSP

Javier wrote:

> I was using an air variable capacitor (bought from Henry Radio) to adjust my tower on 160 mts, the swr were about 1.2.
>
> Last friday I installed a more powerfull amp (1300 watts) and after few hours of operation the capacitor arced so  along the weekend in installed a cpacitor bought as Kit from Ten-Tec but the swr it abt 2.
>
> I have stored an old Dentron GLT-1000 Tuner. Is my understanding that the power rating of this device is abt 1000 watts, but I would like to know if using this device could be suitable to be used in 1.8 Mhz at 1300 watts.
>
> Ideas welcome.
>
> 73's de Javier
> YV5MBX
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