Date: Mon, 14 Aug 2017 08:52:36 -0400
From: Art Greenberg <art@artg.tv>
To: TowerTalk@contesting.com
Subject: [TowerTalk] Need Capacitors for Matching Network
<I want to build a matching network for a 53-foot vertical for 80/75 and
40. I should be able to cover 80/75 with three LC pairs and 40 with a
single pair. So a set of fixed inductors and capacitors with a few
relays would work.
The design calls for capacitor values in the range 100pF to 300pF.
Assuming an unloaded Q of 1000, TLW says at legal limit the capacitor
will see worst-case peak voltage around 1500V and RMS current around
5.5A.
RF capacitors (doorknobs and plate mica) are built for this application
but they are way too expensive new, and not a whole lot more affordable
used. If they're the only real fixed-value choice, it would be less
expensive to use rotary components and motors, since I have a vacuum
variable and rotary inductor on hand. But I don't think the application
merits that complexity. I'd rather save those parts for a more deserving
application.
The MFJ legal limit tuner uses a bunch of 62pF high voltage disc ceramic
capacitors in series-parallel combinations to make up the values it
switches in. They are apparently sourced overseas and I haven't found a
local source for the small quantity I need. I can get some very
reasonably priced 1.5kV C0G disc ceramics. But I cannot find current or
dissipation specs for those parts.
Would silver mica capacitors in series-parallel be OK? I haven't found
current or dissipation specs for those either.
What should I expect the unloaded Q of a disk ceramic or a silver mica
capacitor made up that way to be?
Thanks.
--
Art Greenberg
WA2LLN
art@artg.tv
## Id use motor driven vac caps myself..and be done with it.
Plan B would be to use motor driven air variables. They will easily
handle the peak V and average currents involved. Use relays to
tap the single coil. Like a DPST, with all contacts in parallel..which
will double the current rating, and quadruple the power rating.
## use some BIG wire for the hb coil, like 8 ga. The entire mess has to
be in a waterproof box. Put a couple of metal finned resistors in there to
keep it warm, and dry. Then add some silica gel powder in there too, to absorb
any
moisture. The problem with air variables is the sliding contacts on the rotor
assy.
## A motor driven roller coil will probably give you grief....eventually. All
depends on the
type of contacts used.
## Vac caps typ have unloaded Qs of 5000. Coils are typ 200-2000
depending on construction.
8 gauge wire = .128 which is essentially the same as .125 tubing. .1875
tubing will also work.
How much inductance is required ?? Since this is just a 80 +40m affair, you
can get away with large diam
coils, which saves length. .1875 and .25 tubing typ comes in 15 + 25 + 50
foot rolls. Then it will freestand
in air.
Jim VE7RF
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