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Author Topic: ethylene glycol as a dielectric  (Read 14551 times)
KE7BZI
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Posts: 14




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« Reply #15 on: June 03, 2011, 04:26:50 PM »

Ok, see if this makes sense or if I'm missing something (else).
According to a little calculator I found at http://www.standpipe.com/w2bri/software.htm

If you have an butterfly air capacitor (0.5 the pF, 2x the voltage) with:
10 plates of 4 in sq. meshed surface area.
0.03" spacing

You get:
135 pF good for 4500 volts.

It seems like more capacititance gives you more tunable bandwidth and 500 pF is not too much to want.  So we'ed like a cap 3 of 4 times that value.

Now if you use something with a higher dielectric constant and a larger break down voltage things would seem to get better.
The same cap with 10 X 4" sq. is 5000 pF and has a voltage rating of 40 kV.

There may be reasons why it won't work, like resistive loss,  but it seems absolutely worthy of investigation. Tongue

Marc 
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KE3WD
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Posts: 5689




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« Reply #16 on: June 03, 2011, 06:15:31 PM »

At "only" the 100W level you have plenty of good and inexpensive options.

*Many hams devise a Trombone Capacitor.  Screwdriver on threaded rod for remote control.  Google, there are plenty of good examples about.

*Some use flat glass as their dielectric and slide one copper-sided piece of PCB over two others beneath the glass.  Coolbeans if the antenna is a permanent installation indoors, such as in an attic.  Again, the screwdriver motor and threaded rod are implemented to obtain remote control.

*MFJ sells two butterfly caps, the ones used in their magloops. 

#1 - BUTTERFLY LOOP AIR VAR CAP, 18-136 PF, $89.95

Quote
This is our MFJ-23, a larger low loss butterfly loop tuning capacitor. The capacitance is variable from 18-136 pF.

This low loss butterfly loop tuning capacitor has a voltage rating of 4200 Volts rms. Built-in insulator feet give 1-inch of space under the bottom of the capacitor allowing convenient mounting and high voltage insulation. Solder lugs are provided on the stators and rotor for easy wiring.

Longer than the MFJ-19, this butterfly capacitor measures 10 inches in length, 3 inches wide and 3½ inches tall. Included is a 2½ inch long ¼ inch diameter insulated nylon shaft so a knob can be easily added to aid in turning the capacitor rotor.

http://www.mfjenterprises.com/Product.php?productid=MFJ-23

#2 - BUTTERY LOOP AIR VAR CAP,12-67PF, $69.95

Finally, as a lifelong homebrewer, take a look at this guy's brilliant approach and use of sliding coca cola cans: 

http://www.youtube.com/watch?v=gaJlrN3cElo

If you don't appreciate the elegant answer of the readily available coated aluminum cans as trombone cap and his use of two ordinary plastic medical syringes plus a length of plastic tubing to make a *hydraulic* remote capacitor controller that even has CC markings as to position, what can I say aboutcha?  In my book, this kinda thing is what ham radio is *supposed* to be about. 

Outside the box,

73



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KE7BZI
Member

Posts: 14




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« Reply #17 on: June 05, 2011, 01:43:04 PM »

Thanks for the link to the video.  That is a very elegant solution.  And the good news is that you could make it out of bear cans Smiley.  With the proliferation of odd can sizes I'll bet you could find several that would nest.  I'll have to see what I can find on the self.  If you used a metering pump you could "gear" it down to have more precision.

Marc
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WB6BYU
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Posts: 17195




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« Reply #18 on: June 05, 2011, 04:43:16 PM »

Quote from: KE7BZI
...And the good news is that you could make it out of bear cans...


A can would have to be pretty big to hold a bear, even a young cub.  And I don't think I'd want to
deal with getting live bears out of their cans, unless I'd had too much beer.
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KE7BZI
Member

Posts: 14




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« Reply #19 on: June 06, 2011, 07:49:26 PM »

Oh sure, make fun of the hieroglyphical challenged guy.  Wink
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W6RMK
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Posts: 680




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« Reply #20 on: June 06, 2011, 08:26:01 PM »

liquid dielectrics always seem interesting at first, but ones with high dielectric constants (e.g. glycols) tend to be hygroscopic, and with water comes loss and radically lowered breakdown voltage.

If you must try oil, then castor oil is probably your best bet. It's around 4-5 dielectric constant, has the usual several hundred kV/inch breakdown, etc.

Low loss tangent too (they use it in pulse power capacitors, among other things).

The problem, in general, with oil is that it burns.

There are also some PCBs that make good dielectrics.  The challenge is in finding ones without dioxin contamination.  At least it's not flammable.

All liquid dielectrics are a pain to work with.. they're a mess, they leak, etc. etc.  Unless you're building a hermetically sealed enclosure.  If it's a variable capacitor, oil will leak out around the shaft bearing, or you have to mount the shaft vertically and watch the levels.


Watch out on the pulse power literature.. for 3 ns pulses distilled water makes an excellent dielectric.  For 100% duty cycle, not so wonderful.
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KE7BZI
Member

Posts: 14




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« Reply #21 on: June 09, 2011, 12:28:04 AM »

Thanks,  so much to learn.
For now I decided to build a simple loop with trombone cap to gain more experience.

One thing playing with the loop calculators points out is that when you attempt to extend the tunable bandwidth of a loop with a larger cap, loop efficiency starts to drop off quite fast.  I need to build some examples to get a better understanding of what the means for real performance.

As KE3WD pointed out earlier in this thread,  Design is never a one-input problem.

Marc
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W8JI
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Posts: 9749


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« Reply #22 on: June 09, 2011, 08:03:08 AM »

The ideal capacitor for a loop antenna, or any other high Q capacitor requirement,  is small and boxy in shape.

As the shape gets longer the length of conductors adds series inductance, and that increases losses. The worse possible shape is a long cylinder.

Adding dielectrics is always a problem, unless the dielectric is carefully selected. Things like Teflon, mica, and some other plastics and ceramics can maintain fairly high Q.

I've yet to see a high Q liquid capacitor at radio frequencies, nor a physically long capacitor with high Q. The two best capacitors are vacuum caps which can have a Q >50,000 and butterfly air dielectric caps which can have Q>5000.  Adding a dielectric generally takes them down to Q<2500.

Most trombone caps have Q<1000 and I've seen them as low as 50 or 100. The few liquid dielectric caps I've tested had Q~100 or less.

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