On Sat, 2009-02-07 at 21:34 -0600, Bob McGraw - K4TAX wrote:
> Regarding the 238 tuner caps, I experienced some SWR drift with mine at high
> power and on certain bands. I one case I could use the same antenna and
> feedline on the lower end of 80M with no change in SWR. However, at the
> upper end with the same antenna and feed line on 75M there was SWR drift. I
> could always tune it so the SWR was low but then when the caps cooled the
> SWR had changed. Of course the SWR drift caused the tuning on the Centurion
> to change.
>
> I used a laser temperature meter to see what was heating. It was the ceramic
> caps located at the back of the Low Z / High Z switch. I replaced the 220
> pf caps units wired in parallel with some Russian made 470 pf 16KV RF caps
> purchased on E- Bay. There's a couple of websites that show how these can
> fit in. It takes drilling about 4 holes in the bottom for mounting and
> wiring them in place. Problem solved. As to a source of caps, should you
> not like Surplus Sales of Nebraska, try Fair Radio Sales. Or as I did,
> E-Bay.
Mouser shows a FEW NPO high voltage ceramic disks. 2KW at 50 ohms is 316
volts RMS, but using a ceramic rated at a few KV distributes the RF
current into a greater volume of ceramic which should reduce its
temperature rise and drift. Most any commercial material other than NPO
will drift more than you want with heat.
2 kw at 50 ohms is 316 volts RMS. A 220 pf padding loading capacitor will draw
1.8 amps at 4 MHz, less at 2 MHz.
Another possibility are the Cornell Dubilier metal cased mica capacitors
from Mouser. Such as the catalog number 5981-MCM01-EF220J-F (CD number
MCM01-009EF221J-F) rated at 1 KV and > 20 amps RF. Disc ceramics
generally do not have any RF current rating. These mica caps are
compact, about 7/16" x 5/8", it won't hurt to solder the case to a
copper strap.
>
> One other problem I found was the accumulation of paint, dirt and grime on
> the insulation of the window/ladder line that runs along the side the house
> and up the tower. Yes, the fellows that painted the house actually sprayed
> the ladder or window line section that runs under the eve to the tower.
> Again, this was a heating issue causing "what ever Z" change to take place.
> I cleaned the paint off of that section of line using lacquer thinner and
> the problem was solved.
>
> As some of you may or may not know, I use a couple of retired BC
> transmitters for 160M and 75M AM operation. In running these at a legal 375
> watts of carrier output with 125% positive modulation, I use a Tentec 238
> tuner for the matching chores. Trust me, that application will separate
> the good tuners from the poor tuners. I don't care what the PEP rating of
> the tuner is. I have one of the popular 3 KW PEP rated tuners that will
> flash over on modulation peaks and finally the "heavy duty" internal balun
> smoked, while under the exact same conditions, the Tentec 238 stays cool
> and performs exactly the same tasks as needed. Oh yes, the match values for
> the tuners are always set using a antenna bridge. I find that method
> absolutely necessary when using a tuner with a T network configuration.
Different tuner configurations, even in the same tuner can affect tuner
circulating current and tuner voltage rise. When there are several
tunings to select from, likely choosing the one with the greatest
bandwidth will be the most robust one for handling high power, because
its the one with the lowest loaded Q and so the least circulating
current which also is the one with the lowest voltage rise. Usually,
unless most of the impedance change is by toroidal transformer, the
further the load impedance is from 50 ohms the greater the loaded Q of
the tuner and the greater the tuner losses even with 3 or 5 KW parts.
>
> 73
> Bob, K4TAX
73, Jerry, K0CQ
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