On Wed, 2008-01-23 at 20:33 -0600, Bob McGraw - K4TAX wrote:
> A 1KV disc ceramic will do just fine for the external cap. I have a couple
> of RF Doorknobs that I use externally depending on my needs. Voltage is not
> critical.
>
> Here is one thing that has shown to be an improvement with the 238 tuner.
> Replace the disc ceramics internally with RF rated doorknobs. These may be a
> challenge to get in the tight spaces physically but this settles things down
> completely if you are prone to run legal limit power in modes like RTTY .
> Otherwise, it is not a problem on SSB or low duty cycle modes or low power.
> You'll observe the SWR change on some bands and with certain antennas and
> configurations. These capacitors are the group of 220 PF caps. I found some
> 450 pF caps for those group of 8 x 220 pf units that are parallel off of the
> switch deck S1B and a single 220 for the other application off of S1A. See
> the manual/schematic for details. I also replaced the two 50 pf units in
> series with a 25 pf doorknob. All doorknobs are rated at 2500 volts. I
> ordered my caps from Fair Radio Sales.
>
RF doorknobs are uncommon in distributors. RF Parts has the best
selection.
Disk ceramics can be good. High voltage parts have greater area of the
plates and volume of the dielectric so don't heat as much. Many most
readily available ceramic dielectrics have very high temperature
coefficients. The most common ones (also highest dielectric constant and
so smallest size) can loose 3/4 of their capacitance when heated from
room temperature up to 75 degrees C.
Ceramic dielectrics COG or NPO are decent with little drift with
temperature. SL and anything else is not good for a tuned circuit.
In Mouser, their catalog number 75-561R10TCCT22 has a good chance of not
drifting too much with heat.
Metal clad mica capacitors are made for high current RF applications
(often used in RF power amplifiers right at the transistors). Mouser has
some from Cornell Dubilier. 220 pf at 1KV, 5981-MCM01-EF220J-F, nearly
$5 each. Mica capacitors have very good temperature stability.
> It all has to do with the impedance of the antenna and the type of reactive
> component be it Xc or Xl . I have a 75M coax fed antenna that show a natural
> resonance about 3925. SWR is about 1:1 at this frequency too. At 3980 the
> tuner will easily tune the load to 1:1 SWR but after about 15 sec of carrier
> at 1500 watts I see the SWR starting to rise. Let things cool for a couple
> of minutes and the SWR is again 1:1. Again, same antenna and frequency of
> 3600 with legal limit, SWR adjusted to be 1:1 the SWR does not rise.
> Checking components I find that the internal disc ceramic caps are getting
> hot on one freq and not the other. Thus they are changing value as they
> heat.
>
>
>
> 73
> Bob, K4TAX
>
73, Jerry, K0CQ
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