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Flat Sliding Air Variable Capacitors




 
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Author Topic: Flat Sliding Air Variable Capacitors  (Read 4071 times)
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aa5wg
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« on: February 05, 2013, 09:04:43 AM »

Hi to all,

The shape of the variable plates for Straight-Line Capacitance, Straight-Line Wavelength and Straight-Line Frequency air variable capacitors are shown in Radio Engineering book by Terman, 1937, 2nd ed., page 30.

(1)  Instead of the above traditional am shape design, what would the shape of the straight-line frequency air variable plates look like it if they were laid flat plates in the horizontal plane sliding over each other?   i.e. A brass plate sliding over another brass plate separated by glass, cardboard or air on top of my work bench.

For practice the stator plate could be taped horizontally flat to the work bench and the rotor plate could be taped to a piece of cardboard and laid flat and laid on top of the work bench.  Then, this flat rotor could be pushed over the fixed stator to change capacitance.

What shape shoulc these two flat plates look like to act as an straight-line frequency air variable capacitor?

Chuck
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VE3LYX
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« Reply #1 on: February 05, 2013, 09:24:20 AM »

Having played around with crystal sets (for those who previously said they havent and never will, you have no idea what you are mssing!) I made one with cardboard and aluminum foil seperated with wax paper. Cant say it works wonderful but it does work. Pics later. (still have it) Also have made variable caps from soup can lids (POW regen)
Want to try small and big tin cans. I like though your idea. That would work very well. I would cut both plates on a 60 degree angle and counter position them to have a very smooth gradual increase per centimeter adjustment rise in  capacitance. I suspect it would work very very well.
Don VE3LYx
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Don VE3LYX<br />Eng, DE & petite Francais
aa5wg
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« Reply #2 on: February 05, 2013, 10:49:48 AM »

Hi Don:

When you say 60 degrees do you mean a 60 degree side of a triangle?

Yes, please send us some pictures please.

Thank you.

Chuck
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VE3LYX
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« Reply #3 on: February 05, 2013, 04:41:28 PM »

I whipped this up out of paper just to give an idea. The long angle makes the rise in capacitance very gentle giving you very good control. here we have wide open  which is minimum pfs, half colsed giving mid range and fully closed giving max puffs. The extra wing standing up in the fully closed postion could be trimmed square just for looks since it doesnt do anything.  
Cutting aluminum. If you to cut it very well with out twisting and bending it sandwich the sheet of aluminum you are using between two pieces of plywood fastened together and cut you pattern out on  a normal table saw with a normal wood blade. (Aluminum cuts very well on a wood blade) Then after it is cut it will still be flat and straight.  Wear safety glasses!
Make it mechanically strong and you will have a very very good variable capacitor
Don VE3LYX


* open min pfs.jpg (10.42 KB, 423x125 - viewed 312 times.)

* partly engaged.jpg (11.44 KB, 439x129 - viewed 288 times.)

* fully engaged max pfs.jpg (10.84 KB, 390x133 - viewed 300 times.)
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Don VE3LYX<br />Eng, DE & petite Francais
aa5wg
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« Reply #4 on: February 06, 2013, 10:12:57 AM »

Don,

Your pictures were very helpful!  Thank you.  I get the idea. 

Any body else want to jump in here?

Chuck
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W3RSW
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Rick & "Roosevelt"


« Reply #5 on: February 06, 2013, 11:46:58 AM »

The slopes of the 'rotors' or top sheet shown so far are linear, straight edged, and will yield a non-constant frequency change per slide unit regardless of the angle of engagement unless your rotated the 'rotor' or top sheet as you slide it. 

For a straight motion, one axis only slide, one of the plates has to have a natural (base "e") curve.  Then it will yield a linear change in frequency.

When you slide one straight plate over another with a constant skew angle between, you get an exponential change in frequency, i.e., faster rise in frequency at the lesser plate engagement per unit of travel, and slower change in frequency at the greater engagement, just like an ordinary var. cap. with plates having the same radius.   

So cut that top sheet with an exponential curve, big (tall) on one end tapering down to small on the opposite end.  You can try half of a parabola as a first approximation. 

Don't forget the dielectric in-between.  Grin
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RICK  *W3RSW*
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« Reply #6 on: February 06, 2013, 02:46:18 PM »

Linear?  "It Works" would be good . When you made yours how did you do it?
Don Ve3LYX
 
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Don VE3LYX<br />Eng, DE & petite Francais
aa5wg
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« Reply #7 on: February 06, 2013, 06:56:38 PM »

Rick, how do I determine the correct exponential shape of this "rotor" sliding capacitor plate?

Chuck
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W3RSW
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Rick & "Roosevelt"


« Reply #8 on: February 06, 2013, 07:22:43 PM »

I don't think you need a math book or have to work up drafting from any formula for a neat first trial.  Just look up the time constant curve for an RC circuit, you know the one that shows the discharge voltage over time of a capacitor and resistor.  

 You can use either the charge diagram or the discharge diagram.  Just scale up and trace that curve and cut your top sheet to match.  Try both "halves" of the cut sheet for your top sheet, one concave, one convex or the mirror image.
Try them both.  -should be fun.  O

Of course when you slide the sheet try to achieve a constant speed, heh, heh.

Oh, I meant to add; because of this very non-linearity of commercial variable capacitors, especially on the high frequency or smallest radius portion even if cut for somewhat linear dial read-out, some manufacturers decided to make variable permeability coils with a constant value capacitor to achieve a linear frequency scale.  -Drake, Collins 32V, etc.

Here's a bunch of exponential curves to play with.
http://www.google.com/search?hl=en&safe=active&biw=1243&bih=904&gbv=2&tbm=isch&sa=1&q=exponential+curve&aq=f&aqi=g10&aql=&oq=
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RICK  *W3RSW*
aa5wg
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« Reply #9 on: February 18, 2013, 09:02:24 AM »

Rick,

Thank you.

Chuck
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