[Elecraft] OT antenna question

[Bob McGraw K4TAX]

When I updated my 2 tuners, I used some doorknob style capacitors, of 
Russian source I presume,  which I purchased via E-Bay. Something like 5 
for $20.    These were 470pf units rated at 16KV.    Don't be mislead 
with the voltage rating, as it the current handling ability being the 
key for RF usage.

In most all cases one will need to provide a somewhat different mounting 
as they have screw terminals on either side and are larger physically.  
I just happen to have one in the desk drawer. Typically 1.0" dia x 0.75" 
tall with, guessing, #6 or #8 screw terminals on either side.  Could be 
Metric screws as I don't recall and can't tell without my thread gauge.

Another approach is to divide the total current  by using 3 or 4 caps in 
parallel,  if you wish to use disk ceramic caps.  Thus if your circuit 
requires 330 pf then use 3 pcs of 100 pf or 3 pcs of 120 pf in parallel 
and keep the voltage rating the same as the originals.  This way the 
total current is divided between 3 devices as opposed to one device.  
This also works.

In various transmitter updates, I have used dipped silver mica caps.  
These work well.  Just keep the voltage rating the same as the 
originals.  And it won't hurt to use 2 or 3 units parallel by taking the 
original capacity value and dividing it  by 2 or 3 for the pf value.    
This approach was used in restoration of some of my old boat anchor AM 
transmitters.  These caps are more stable, meaning less tuning drift, 
thus work much better than the original disk ceramics units.

73
Bob, K4TAX



On 1/14/2016 8:27 AM, Al Gulseth wrote:
> Bob,
>
> Are silver(ed)/dipped micas (CDE etc.) the preferred type for a tuner, or is
> there a superior newer technology available?
>
> TNX/73, Al
>
> On Wed January 13 2016 2:52:52 pm Bob McGraw K4TAX wrote:
>> If your tuner uses disk ceramic caps, as many do, these can be heating thus
>> not being able to handle the RF current.  They heat and cool and change
>> value which in turn changes tuning.
>>
>> Replacing them with suitable RF current rated units is the solution.
>>
>> Bob, K4TAX
>>
>>
>> Sent from my iPhone
>>