[TenTec] Extra Capacitance Voltage Rating

[Ron Castro]

I used some Russian RF doorknob caps I found on e-bay and mounted the ground 
sides to the chassis of the tuner under the switch and the performance is 
greatly enhanced.  Using a bent, quarter-wave sloper off a 55' tower on 160 
meters, I found one setting on the tuner that allows full coverage of the 
band with > 1.7:1 SWR.  Takes full power without 'drifting'.

         Ron  N6IE
      www.N6IE.com


----- Original Message ----- 
From: "Bob McGraw - K4TAX" <RMcGraw at Blomand.net>
To: <KnoxCol at aol.com>
Cc: <tentec at contesting.com>
Sent: Wednesday, January 23, 2008 6:33 PM
Subject: Re: [TenTec] Extra Capacitance Voltage Rating


>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.
>
> 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
>
>
>  ----- Original Message ----- 
>  From: KnoxCol at aol.com
>  To: RMcGraw at Blomand.net
>  Sent: Wednesday, January 23, 2008 3:54 PM
>  Subject: Extra Capacitance Voltage Rating
>
>
>  Hi Bob!
>
>    Just read your comment about using a 2700pf capacitor in lieu of the 
> 1000pf capacitor that Ten Tec supplies in order to extend the tuning range 
> of the 229/238 tuner series.    Can you tell me what the voltage rating of 
> this capacitor should be?
>
>  Thanks and 73!
>
>  de KD4BRL  Dick Knox
>
>
>
>
>
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