Home Brew Tuning Capacitor
Irv Finkleman
capacitor for small transmitting loops. The voltage is very
high, and the capacitor must withstand voltages in
the 1 to 5 KV range. The maximum capacity should be in the
order of 50 to 100 pf. Thanks for any suggestions including
practical construction details or helpful hints.
--
Dx Type II Diabetes March 5 2001
Beating it with diet and exercise!
297/222/210 (to be revised lower)
58"/44"/44" (maybe lower too!)
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Visit my website at http://members.home.net/finkirv/
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Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada
eh...@bellatlantic.net
in some gamma matches? **VERY** easy to make.
Strip the shield and outer insulator off of some coax,
leaving the center conductor insulated. Slide the
insulated center conductor into a piece of metal
tubing that fits closely. The tube forms 1 plate
of teh capacitor, and the center conductor forms
the other. The amount of coax in the tube determine
the capacitance. Cut the tubing for the maximum
capacitance you want, and make the center conductor
a couple of inches longer.
Joe Bramblett, KD5NRH
wrote:
>Has anyone any good ideas on homebrewing a variable
>capacitor for small transmitting loops. The voltage is very
>high, and the capacitor must withstand voltages in
>the 1 to 5 KV range. The maximum capacity should be in the
>order of 50 to 100 pf. Thanks for any suggestions including
>practical construction details or helpful hints.
http://www.qsl.net/mnqrp/Loop/Mag_Loops.htm
Towards the bottom, he has details on some rather large air variable
caps that should be relatively easy to make with the proper tools.
Joe Bramblett, KD5NRH
kd5...@kd5nrh.net
Ivan
> Has anyone any good ideas on homebrewing a variable
> capacitor for small transmitting loops
Hi,
some ideas should be in the following articles - see
http://www.qsl.net/dj3tz/kondens.txt . I think the simplest
construction is the trombone-like one: two pairs of tubes going
tightly one into another, separated by a teflon foil. In fact it is a
pair of variable capacitors in series, so it should withstand fairly
high voltages.
BR from Ivan
Reg Edwards
magloops. The resistance of the THIN inner conductor is in series
with the capacitance. The Q of the whole loop is reduced.
So some of the advantages of using copper pipe for the main loop
are lost. The effect would be reduced if the coax was used only as
a trimmer in parallel with a large fixed-plate capacitor.
----
Regards, Reg, G4FGQ
http://www.g4fgq.com
===========================================
Bob Miller
>Has anyone any good ideas on homebrewing a variable
>capacitor for small transmitting loops. The voltage is very
>high, and the capacitor must withstand voltages in
>the 1 to 5 KV range. The maximum capacity should be in the
>order of 50 to 100 pf. Thanks for any suggestions including
>practical construction details or helpful hints.
I don't know if the values are right, but Ten Tec sells the variable capacitor
from its antenna tuner in kit form -- I think it runs about $49.
Bob
k5qwg
Rick Karlquist
it tends to arc at very high voltages like 10KV. This
is the biggest problem with "permittivity tuned" capacitors.
Gamma matches operate at low voltages, so they don't have this problem.
Maybe you're at a low enough voltage that you'll get away with it.
Rick N6RK
<eh...@bellatlantic.net> wrote in message
news:3BD64038...@bellatlantic.net...
Roger Leone
You can create very high voltage caps using glass as the dielectric. There
is an antenna tuner in the current issue of the ARRL Radio Amateurs Handbook
which uses inexensive picture frame glass. An estimated breakdown voltage
of about 12,000 volts was obtained this way.
I saw another article in which a regular air variable cap was used, with
labratory glass slides slipped between the rotor and stator plates. The
glass dielectric actually has 2 benefits; increased breakdown voltage and
higher capacitance due the the dielectric constant of glass vs. air.
The mechanical details of making a glass dielectric cap variable can be
overcome in a number of ways. An article in Electric Radio Magazine a few
years ago, also about an antenna tuner, used a screw mechanism to create
linear motion to move the "rotor" plate of the glass dielectric cap to
increase or decrease capacitance. If you wanted to get fancy, you could
motorize such a mechanism and control it remotely.
Hope this gives you some ideas.
Roger K6XQ
Tom Bruhns
> Irv:
>
> You can create very high voltage caps using glass as the dielectric. There
> is an antenna tuner in the current issue of the ARRL Radio Amateurs Handbook
> which uses inexensive picture frame glass. An estimated breakdown voltage
> of about 12,000 volts was obtained this way.
>
> I saw another article in which a regular air variable cap was used, with
> labratory glass slides slipped between the rotor and stator plates. The
> glass dielectric actually has 2 benefits; increased breakdown voltage and
> higher capacitance due the the dielectric constant of glass vs. air.
...
Something to be careful of is corona. If you replace say 90% of the
thickness of air by glass, but leave the remaining 10% as air, then
indeed you'll increase the capacitance. Effectively you'll have an
air dielectric cap of 10 times (1/0.1) the original, and a glass
dielectric cap of 1.11*k* the original, where k is the relative
dielectric constant of the glass you used. For example, if it was a
100pF air cap, and you use glass with k=5, you'd end up with 1000pF in
the 10% air and 555pF through the 90% glass part. The net is 357pF,
which is a nice gain. But...what about voltage rating? If the
original was rated at 1000V, assuming it's because of the field
between the plates and not high fields around any sharp edges on the
plates, then with all air, it was a uniform field between the plates.
But in the new 90% glass cap, you have 35.7 percent of the voltage
drop across the 1000pF air gap and the rest through the glass. Since
the air is only 10% at thick as it used to be, but has 35.7% of the
voltage drop, you better DERATE the capacitor voltage to
1000V*0.1/0.357 = 280 volts!
This is a somewhat simplified analysis, ignoring the high fields
around the edges of the plates, but it illustrates what can happen.
In fact, air in high voltage film caps and in high voltage
transformers can cause the same sort of trouble, leading to early
failure. One way around this is to find a liquid or gas which is a
good RF insulator and has high dielectric constant. Some oils are
pretty good that way, though can be really messy to deal with if
you're trying to seal up a variable cap. Note that the same thing
happens with Teflon-wrapped copper tubing or using the inner conductor
of coax. One advantage in a coaxial configuration if you have a solid
dielectric in intimate contact with the inner conductor, is that the
field in a coaxial cylinder capacitor decreases as you go out along a
radius from the center, though that's not a huge effect unless the
inner to outer conductor diameter ratio is large.
This effect generally isn't a problem in DC applications, because any
corona will quickly charge up surfaces and take away the field that
caused the corona. But in AC, from 50Hz on up, it's a serious
consideration.
Hope these thoughts help some!
Cheers,
Tom
Wes Stewart
[snip]
Tom makes some really good points, that are not well-understood in the amateur
(and sometimes the professional) world.
Some insulation (other than air) can be worse than no isulation.
|...
|Something to be careful of is corona. If you replace say 90% of the
|thickness of air by glass, but leave the remaining 10% as air, then
|indeed you'll increase the capacitance. Effectively you'll have an
|air dielectric cap of 10 times (1/0.1) the original, and a glass
|dielectric cap of 1.11*k* the original, where k is the relative
|dielectric constant of the glass you used. For example, if it was a
|100pF air cap, and you use glass with k=5, you'd end up with 1000pF in
|the 10% air and 555pF through the 90% glass part. The net is 357pF,
|which is a nice gain. But...what about voltage rating? If the
|original was rated at 1000V, assuming it's because of the field
|between the plates and not high fields around any sharp edges on the
|plates, then with all air, it was a uniform field between the plates.
|But in the new 90% glass cap, you have 35.7 percent of the voltage
|drop across the 1000pF air gap and the rest through the glass. Since
|the air is only 10% at thick as it used to be, but has 35.7% of the
|voltage drop, you better DERATE the capacitor voltage to
|1000V*0.1/0.357 = 280 volts!
|
|This is a somewhat simplified analysis, ignoring the high fields
|around the edges of the plates, but it illustrates what can happen.
|In fact, air in high voltage film caps and in high voltage
|transformers can cause the same sort of trouble, leading to early
|failure. One way around this is to find a liquid or gas which is a
|good RF insulator and has high dielectric constant.
Even liquid dielectrics sometimes have trapped gas. Because of the effects Tom
describes, the voltage gradient across the gas bubble can be higher that the
surrounding liquid. This little "spark gap" can break down, creating "partial
discharge" (commonly known as corona). When this happens, heat is generated,
sometimes generating more gas, that breaks down, creating more gas..... until
something pops.
|Some oils are
|pretty good that way, though can be really messy to deal with if
|you're trying to seal up a variable cap.
One of our missile programs used a klysron amp that, along with a pulse
transformer, was cooled and insulated with a silicone oil (Coolinal). If I would
have absorbed all of that stuff that was on my hands and clothes, I would have
grown boobs <g>.
|Note that the same thing
|happens with Teflon-wrapped copper tubing or using the inner conductor
|of coax. One advantage in a coaxial configuration if you have a solid
|dielectric in intimate contact with the inner conductor, is that the
|field in a coaxial cylinder capacitor decreases as you go out along a
|radius from the center, though that's not a huge effect unless the
|inner to outer conductor diameter ratio is large.
I (my company) once bought some corona testing equipment from Biddle
Instruments. As part of the deal, I took a one week short course there in HV
testing. One of the other 'students' was a guy who owned a cable making company
in Canada. One of his products was coax for HV power transmission. (If I
remember correctly he said it was the Bahamas that get power from the US
mainland via an underwater coax cable.)
In their cable, the dielectric was in intimate contact with the center conductor
because it was extruded over it, but at the outer conductor/dielectric interface
there were voids, or partial discharge sites. To overcome this, they coated the
outside of the dielectric with a semiconducting layer.
The fun part was at the end termination. If you think about the cut end of a
piece of RG-8, there are lots of little pointy ends of the braid wires that
create great discharge sites. They developed liquid-filled connectors to
overcome this.
As an aside, Biddle's shop looked like a cross between Frankenstein's and
Tesla's labs. The equipment I bought could generate 50 KVAC stacked on 10 KVDC.
The 100pF capacitor that coupled the test sample to the detector was about 10
inches in diameter and 3 feet long.
73,
Wes N7WS
Tom Bruhns
> Even liquid dielectrics sometimes have trapped gas. Because of the effects Tom
> describes, the voltage gradient across the gas bubble can be higher that the
> surrounding liquid. This little "spark gap" can break down, creating "partial
> discharge" (commonly known as corona). When this happens, heat is generated,
> sometimes generating more gas, that breaks down, creating more gas..... until
> something pops.
...
Yep, good point, Wes. And in fact, that corona is not just hot, it's
composed of a plasma of ("hot") ions, which are generally going to be
very reactive, chemically. That's bad around organic insulators, but
can cause problems even with things like aluminum surfaces.
Cheers,
Tom
(remembering back to my days in spark spectroscopy...)