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@Blomand.net>
To: <KnoxCol@aol.com>
Cc: <tentec@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@aol.com
> To: RMcGraw@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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