Air Variable Capacitors

Building my first Air Variable Capacitor, my goals and what steps I am taking.

Air Variable Capacitor

by

Johnny McCown

22 October 2013

 

Air Variable Capacitor 

As of 22 Oct 2013 I have made 6 Stator and 6 Rotor plates that are complete.  These plates have field down to remove the sharp edges.  In addition to the 6 sets of plates I have also made 8 other sets, these sets have not been field down or drilled out.

Spacing between Plates

With the 6 set of plates, I did a preliminary capacitance test using my MFJ-269 antenna Analyzer seeing good results.  During this test I noticed that the spacing between the plates was not even.  There are two brass nuts between each plate.  It turns out that nuts are not made to the same thickness.  These nuts were ranging from 3.47 to 3.87mm in thickness.  Talking with Dennis, we think for now I should try using star washers to make sure the spacing between the plates is the same.  The ideal situation would be to use brass spacers, though these can be expensive.

Rule of Thumb

The larger the plates the more voltage they can handle.  When you move plates closer together the capacitance goes up, and if you mover them further apart the capacitance goes down.  If the plates are two close you can experience sparking between the plates, so to counter this we increasing the spacing though this decreases the capacitance.  To insure we get the capacitance we need, we have to add more plates.

Tune the Capacitor

This first capacitor that I am building will be outside at my screwdriver antenna, I do not want to walk outside and tune the capacitor to tune the antenna.  So what are my options?  Could teach my dog to do it, but I don’t think he can do a good job without a thumb.  Instead I will use a motor.  I could use a regular DC motor geared down, so it would turn slowly.  The drawback to this is that it would be slow.  Without an optical encoder we would not be able to easily return to a previous state, we would have to watch an antenna tuner for the optimum dip.  An alternative to the optical encoder would be to use a reed switch, and that would close a sensor circuit each time the motor made one revolution, though this would be less precise.  Instead of using a regular DC motor with an optical encode, I would like to use a Stepper Motor.  With the stepper motor I can control how many steps it makes one direction or the other.  Each step correlates to a degree of a turn; for example one step can equal 1.8 degrees.  If we think this is too much of a course adjustment we can gear down the motor, with a small gear on the motor and a larger gear on the capacitor’s rotor shaft.  Keep in mind we will need isolation between the motor and the shaft.  This can be accomplished with a ceramic coupler if driving directly or use a rubber timing belt if using gears.

Control the Motor

For initial control of the motor I am going to use a Netduino, I could use a regular Arduino, but I only have the Netduino Plus (is a version of the Arduino microcontroller that is made for networking and to be programmed in C# rather than the Arduino’s C language) on hand.  Latter on I could switch out between the microcontrollers, it is just a matter of writing new code.  At first I will use a Motor Shield from www.adafruit.com, though I will most likely build my own circuit down the road to reduce the cost of the project.  I will be building two of these loop antennas so I want to keep the cost down.  Also, I will be using this same setup when I go to build a remote tuner.

 

 

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