Build Instructions

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• 1

  Introduction and a very simple STEP 1

  Introduction

  This design uses rectangular brass plates cut from one long sheet. The amount of overlap between plates and the spacing between plates determine the capacitance. In this design, we use 3.06 mm thick brass nuts on threaded rod to space the plates apart. Thus, there are only 3 things to decide:
  

  1. The length of the cut sheet
  2. The number of brass nuts between sheets
  3.  The number of sheets that will be interdigitated with each other.

  A fixed capacitor to explain how it works

  Let me explain the design using a fixed capacitor.  My variable capacitor is actually simpler, but going over this design is helpful for explaining. This diagram shows the arrangement, dimensions and equation for a fixed capacitor. Some of the parts are in imperial units (e.g., inches), but I convert everything to millimeters to make the calculations easier. 

  The brass plates are held together by the threaded brass rods and insulators on the ends. (Plexiglas makes a great insulator.) Each brass plate has 2 holes. When the nuts are tightened, the structure is rigid. I will not discuss the relationship between breakdown voltage and plate spacing right now, but the spacing must be wide enough to tolerate the voltage you will be using. The capacitance is calculated as C (in the figure) times the number of sections of the capacitor.  You can build a capacitor like this in an hour or two. It takes a hacksaw, drill, file and some wrenches.

  Example

  The figure has plates that overlap each other by 40 mm. Each plate is 50.8 mm wide.  Thus, the area of each overlapping plate is 40 x 50.8 = 2032 mm2.  The spacing is 3 nuts. Subtracting out the thickness of a plate, the nuts produce a gap between any two plates of 3.06 mm.  The capacitor has 4 sections (4 gaps between plates).  

  Simplified equation

  We can ignore the k value, since it is 1 for air.  The value for ɛ is in Farads/meter. That is annoying. Fortunately, we can just throw away the exponent to get picoFarads (pF). ɛ = 8.85 pF/m.  For mm, we divide by 1000: 

  ɛ = 0.00885 pF/mm. I will round it to 0.009 pF/mm.  Now the equation is much simpler.
  

  C =  0.009 * (Area / Distance) * sections   (all in mm)

  For this example with 4 sections

  C = 0.009 * 2032/3.06 * 4

    = 24 pF

  A simple variable capacitor

  To make a variable capacitor, you need a structure that lets you move one set of plates. In my design, I move one plate in between two fixed plates, in a simple linear fashion. Most traditional capacitors rotate. Mine is linear for two reasons:

  1.  A 3D printed frame makes assembly very easy.
  2. The moving plate can have a soldered wire for a very low impedance connection. This is especially important for loop antennas.

  Here is the basic design of the linear capacitor. Note how simple it is.  The fixed plates are spaced by 3 brass nuts. The sliding plate goes in between. 

  This design has 2 sections: section 1 is the capacitance between the sliding plate and the top plate, section 2 is the capacitance between the sliding plate and the bottom plate. The spacing is 3.06 mm, based on the thickness of these brass nuts and the thickness of the sliding plate. The plates are 50.8 mm wide (made from 2" wide brass sheet). Together, two sections gives you just under 0.3 pF for each mm of overlap. For my purposes, 40 mm overlap should provide 12 pF. In practice, the capacitance is a little higher. 

  Here is Step 1 

  • Cut 3 plates from the brass stock. The plates should be long enough for the 40 mm of overlap and to drill holes for the threaded brass support rods. 
  • Cut two lengths of threaded rod 
  • We will need 12 nuts for the fixed plates and two for the sliding plate. 

  The drawing shows two nuts for the sliding plate, but we will revisit how the sliding plate is attached. 

• 2

  3D printed frame

  The 3D printed plastic holder is used to support the two fixed plates and provide a path for the sliding plate.

  The STL file is in the Files section of this project. You can use it to make a 15 pF capacitor, or use it as a guide to designing your own. 

  After cutting and drilling the brass plates, the edges of the plates should be smoothed with a file. The fixed plates should mount easily on two threaded rods using the correct number of nuts to keep them spaced. The moving plate should ride easily along the guide. The mounting holes are for 8-32 screws.  Mounting can be done with 2 side screws (there are two on either side) or the two front screws. 

• 3

  Mechanical and electrical connections

  The Files section has a STL file for printing a simple coupling. The coupler proved handy for attaching to a motor. However, almost any type of insulated coupling can be used. The coupler can be seen in the picture.
  
  

  Electrical connections can be done in several ways. The fixed plates can be screwed to a solderless lug or soldered to a wire. It is best to solder the threaded rod when not assembled to the printed plastic piece. I opted to use wire braid for both the moving plate and the fixed plates.  You can see the braid coming from the sliding plate in the picture above. You can see the braid on the fixed plate, below. The braid goes only a short distance to a heavy solid copper wire. This is inside of a tuning unit for a 100 W loop antenna, so low impedance connections are important. 

  The underside of the moving plate. 

  The braid is positioned so that it moves freely without touching other components when the moving plate is moved. 

  Construction is done!

  The Easy Peasy variable capacitor is done!  However, while you are here, we are going to dig deeper into testing and remote control. 

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