HomeBrew Capacitors?
John Ferrell
matching circuits at low power with common sizes of capacitors. Now I
want to move up to the 1 KW power level. Inductors are easy enough to
homebrew but capacitors are both expensive and scarce. Anyone have
ideas or maybe experience in this area?
DigiKey has a good selection of High Voltage units (3 KV Plus) at
modest cost but considering their size and the losses expected in the
capacitors I don't think they can dissipate the heat...
John Ferrell W8CCW
Gary
Hi John, Are you using L networks for matching? If so, then you want
a variable capacitor to be able to match a variety of loads. TenTec
sells a 500pf ~3kv variable capacitor kit for about $50 that I have
used with good results. It is the one that they use in the antenna
tuner that they sell. Takes about a hour or so to put together.
If you want some cheep low pf fixed caps use either RG-8 for real hv
and RG-58 for just hv. Consult the data books for the pf/foot rating
of the cable, or measure with Z meter or antenna analyzer.
Gary N4AST
Dave Platt
John Ferrell <jferr...@triad.rr.com> wrote:
People working on "small magnetic loop" transmitting antennas have
developed some homebrew approaches. These antennas present a highly
inductive feedpoint impedance, and must be tuned to a high-Q match
using a variable capacitor which can stand off very high voltages.
I've seen at least three types described:
(1) "Trombone" capacitors. These typically consist of a U-shaped
piece of copper pipe (adjustable element), whose ends slide into
two longer pieces of larger-diameter pipe that act as the
connection terminals for the capacitor. The sliding pipe is
insulated from the outer pipe - its ends are wrapped in sheets of
a suitable dielectric (Teflon or Kapton sheet is often used). The
cap can be adjusted by sliding the U in or out of the terminal
pieces... often done via a worm-gear motor drive, attached to the
center of the bottom of the U.
(2) "Butterfly" capacitors... two stator plates, one completely-
insulated rotor plate, and suitable dielectric insulation sheets
between them.
(3) "Sliding glass sheet" capacitors... plate glass dielectric, with a
fixed metal sheet sitting below (as one terminal) and a metal
sheet glued to the top of the glass (as the other terminal).
Slide the glass sheet back and forth on the bottom metal sheet to
vary the capacitance. Researching the dielectric and high-voltage
properties of leaded window glass, borosilicate glass (Pyrex and
similar), acrylic, polycarbonate, and Teflon sheets might be
worthwhile.
Other (non-homebrew, but available-surplus) choices are large-gap air
variable caps, and "doorknob" fixed-value transmitting capacitors. The
former are often known as "cheese slicers"... and I've actually seen
some people home-brew them, using either sheet aluminum or brass, or
copper-clad PCB material for the stator and rotor plates.
--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Jeff Liebermann
<jferr...@triad.rr.com> wrote:
>I have been successful at modeling and then constructing transmitter
>matching circuits at low power with common sizes of capacitors. Now I
>want to move up to the 1 KW power level. Inductors are easy enough to
>homebrew but capacitors are both expensive and scarce. Anyone have
>ideas or maybe experience in this area?
Lots of idea, but the only high power experience I've had is design of
a VMOS NMR power amp in the early 1980's. Reading between your lines,
my guess is that you're building a tube amplifier.
>DigiKey has a good selection of High Voltage units (3 KV Plus) at
>modest cost but considering their size and the losses expected in the
>capacitors I don't think they can dissipate the heat...
>
>John Ferrell W8CCW
For high voltage and high power, vacuum caps are a good way to go:
<http://en.wikipedia.org/wiki/Vacuum_variable_capacitor>
There are also porcelain capacitors:
<http://www.atceramics.com/products/power_capacitors.asp>
There are ceramic dielectic high voltage caps but they tend to be more
lossy than vacuum or porcelain.
Jennings vacuum caps. The voltages in the table are in Kv.
<http://www.jenningstech.com/ps/jen/caplist.cgi>
If you want to make your own, there are always Leyden Jars and window
glass with alternating layers of aluminum foil. I've used both for
Tesla coils.
I suggest you look at various existing KW amplifier designs and see
what they use.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Tim Shoppa
I'm guessing that you're talking about capacitors in the few pF to low
thousand pF range used in L matching etc.
(The other alternative is electrolytics in the power supply, where the
common solution since the dawn of time has been series connected 450V
units with equalizing resistors).
For fixed capacitors, a surplus source of "doorknob capacitors" is the
standard way. Not cheap or available at the corner store but they are
available. Try Ebay, where by far most of the parts you'll find are
Russian, or rfparts.com.
For variable capacitors, if you don't want to find some old
Hammarlunds or EF Johnsons or whatever, MFJ sells some new manufacture
large air variables, and other people homebrew their own out of sheet
metal, threaded rods, and nuts:
http://www.metaphoria.us/hamradio/DL5DBM_cap_plans.pdf
http://www.metaphoria.us/hamradio/transmatch.html
Tim.
JIMMIE
I have had some luck cutting out non ferrous parts like this using a
router with a carbide bit. Fasten the metal to a template and feed it
into the router. too easy.
Jimmie
John Ferrell
<jferr...@triad.rr.com> wrote:
>I have been successful at modeling and then constructing transmitter
>matching circuits at low power with common sizes of capacitors. Now I
>want to move up to the 1 KW power level. Inductors are easy enough to
>homebrew but capacitors are both expensive and scarce. Anyone have
>ideas or maybe experience in this area?
<-snip->
Thanks, all good info!
The application is to match individual antennas about 75 feet of coax
from the radio. Coax is way too lossy at high SWR's and open wire just
too cumbersome. I have a storage shed with a pretty good radial system
at the feed point. I suppose the best solution would be a high power
remote auto tuner but I am just not ready to commit that much to the
project!
A simple L match should seems the best alternative for the
experimenting. I will slow down until I fabricate/acquire the
necessary capacitors.
John Ferrell W8CCW
NOVICE
try this sites:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=10213
http://www.qsl.net/mnqrp/Loop/Mag_Loops.htm
Best regards,
73
Buco
S56LW
