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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