Air Variable Capacitor From Scrap Aluminum Sheets




I was building a crystal set for my son, but it came to a halt. When i found out that i have no variable capacitor in my pile of junk.
Scavenging one from an old radio was not an option. Since most of new radios uses analog tuning. And the ones with air variable capacitors are very rare, and are collectors item.
I have read an article once about building an air variable capacitor. So i decided to build my own from scraps of aluminum sheets and from things that are easily found around the house.
If you have a drill, a scissors, a file and some sandpaper. You can easily build this one. It doesn't require much skills.:-)
I made mine from aluminum sheets 1.5 and 2mm thick. The housing is from an aluminum heatsink, i also manage to get 3 pieces of bolts with a few nuts from my junk box. The plastic bushing which also acts as an insulator is from a plastic pen and some plastic jar caps.
I also made my own washers/spacers from the 2mm thick aluminum sheet.
The contacts of the rotor which also acts as a tensioner. Were from a broken washingmachine timer.
You can experiment with the number of plates and with the size op gaps by adding more plates and washer, in the rotor and stator.

Step 1: Marking the Center for Pilot Holes

On this step we will do the measurements.
Lay the aluminum sheet flat on your work bench. From on end draw a square measuring 5cm x 5cm. Using a sharp object like a nail or file.
Get the center and punch a hole in it using a small nail. Punch a hole for every 5 centimeters along the straight line as shown on the photo.

Step 2: Draw Some Circles

Here we will draw circles on the sheet. We can draw circles perfectly by using a compass.
If you dont have a compass.You can improvise, by driving two small nails on a small piece of wood.
Insert one nail on the hole that you made on the sheet and turn it. It is important that the circles doesn't overlap. So that we will have enough space when we cut them out later.
You can make as many circles as you like, if you have enough aluminum sheet. It is better to have a spare you if you made a wrong cut. Than to repeat the same process if we you ran out of pieces at the middle of your project.

Step 3: Marking Lines for the Stator Plates

Using a tri-square, draw a line from the edge of the sheet up to the center of the arc. (The one that is pointed by a pen on the photo)
It is important to do this, because we need each part to be uniform in size. And this also serves as a guide when we cut them out.

Step 4: Cutting

With a big scissors and a strong grip. You can easily cut out all of the pieces. And then flatten each piece with a rubber mallet.
You can also use a hammer. But be very gentle.

Step 5: Marking the Holes for the Stator

Get a piece and draw a line from the center. Going to the left corner.
Get the centerline (from corner to arc) and punch a hole in it.
Mark this piece as pattern.
Put it on top of another piece. And puch a hole on the second piece under it. Using the hole on the first piece as a template. Flip the pattern and punch the second hole.
We do this because we will drill all the rotor and stator plates one at a time. I dont recommend stacking all the pieces and drill them all at one time. The drill bit always tend to bend. Or you may if you have a drill press

Step 6: Marking the Inner Circle

Again improvise a compass to draw the inner circle.
Using a scissors cut it from both end, up to the arc of the inner circle. Be very carefull not to cut it all the way to the center. :-)
when all of the pieces are cut. Flatten them again with a rubber mallet.

Step 7: Cutting the Arc

Here we will cut the arc that still connects the rotor and the stator plates. You can always cut it with a scissors.
But it's easier and faster if you use a curved chisel. (the one's that wood carvers use)
I have one but i don't want to ruin it.:-)
So i made one from a small pipe. Sharpen it at one end with a file. And removed half of it. So that it will be just like a curved chisel.
Dont get confused when cutting. The rotor must have the part with a hole in it. (This is where the spare pieces come in handy:-)

Step 8: Drilling

Now that we have our rotor and stator. It is time to make the holes bigger. With the use of a drill. Find bolts (3pcs.) with nuts ( i used 12 pcs. Of nuts because i removed the head of each bolt so i can open my varicap on both ends.) i used a 4mm. drill bit. Find bolts that will fit the holes perfectly, specially for the rotor

Step 9: Turning With a Drill

At this stage we will make our rotor plates perfect arcs. And to remove burrs and sharp edges. With the help of a drill and a coarse file and some sand paper.
Stack all of the rotor plates facing each other forming a circle. (we do this to avoid vibrations when we turn them with a drill) Insert the bolt and tighten the nut.
Insert the end of the bolt on the chuck of the drill. Just like a normal drill bit. Turn on the drill and use the file to smoothen the sides of the rotor plates. Be very carefull set the drill at low speed. Do not press the drill swith for a long time. (the nut may loosen) if this happens reverse the direction of the drill.
Finish with a fine sand paper

Step 10: Shaping the Stator Plates

We will shape the stator plates at this part of my instructable.
Like what we did with our rotor plates. We will also stack all of the stator plates. Insert the bolts on each hole, and tighten the nuts. If you have a bench vise so much the better.
If you can see the photo clearly. I colored the parts to be remove with a black pen. Or just simply follow the arc and avoid the head of the bolt and the nut. Be patient you can do this with a coarse file. Remember aluminum is not that hard to work with. :-)

Step 11: From This to That

At this stage, your pieces must look like the 3rd piece on this photo. Well done.
At this point you can also remove the paint. If they have paint on. And check again for curves or if ever they are crooked.

Step 12: Making Your Own Washers

Make your own washers/spacers. From a heavier guage or from the same sheet. But you will need 2 pcs. Of washers/spacers for each gap. I recommend using the same guage as the stator and rotor plates.Lighter guage is much easier to cut.
Cut a strip of aluminum about 1cm. wide. Drill holes in it using the same drill bit you used for the stator and rotor plates. Cut the strip into squares. Making sure that the hole you drilled is on the center. Insert the bolt with its head on top. Then cut the corners using the head of the bolt as a guide, as seen on the photo.

Step 13: Finishing Your Washers

As what you did with the rotor plates. Hammer them gently to make them flat. Stack them up, insert the bolt, tighten the nut. And again with the use of a drill turn them and smothen with a file and finish with a fine sand paper.

Step 14: Rotor and Stator Assembly

Remove the head of each bolt. Put one nut on one end of eah bolt. Turn the nut until 5cm. of the bolt protrudes from the nut. Get one of the bolts, insert on one stator plate followed by two washers. And again a stator plate and two washers. Repeat this step until you finish. Leave enough space for 2 bolts and two washers and dont forget the shaft on where to put a knob

Step 15: The Housing

I made the housing from an aluminum heatsink salvaged from a television chassis. It is more or less 2mm. thick.
I placed the rotor and stator on top of it. leaving a distance of 1mm. Between the rotor and stator. Marked the 3 holes to be drilled using the rotor and stator holes as template. Then drilled all the holes using the same drill bit used for the rotor and stator plate. Then i just cut it in a triangular shape and rounded all of the corners.

Step 16: The Insulator

Look for something that will insulate the rotor's axel from the end plate/chassis. Like for example a rubber hose from a fuel line of an automobile. Since i don't own a car. I just used a plastic pen for a bushing. Note: look for the right pen before you make the top hole bigger. Don not drill until you have your bushing:-)

Step 17: Plastic Jar Caps

Cut two triangular shaped plastic from jar caps. Or any plastic of your choice. This plastic will insulate the rotor contact from the aluminum housing. I just glued my contact temporarily. I will later change it with a copper connector. The ones that look like washers like the ones used for ground connection.
Now you just put the two triangular plastic on both sides. Followed by the two triangular shaped aluminum and you are done.
Remember if you have two stator plates, you must have three rotor plates. 4/5, 5/6 and so on.
I hope that this instructable will be of much help. Thanks :-)

Step 18: Finish

Here is the finished variable capacitor. Installed on a crystal shortwave receiver.



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    70 Discussions


    11 months ago

    This is a great "from scratch" piece. To those that say that commercial ones are hard to find, they are not, a web search will turn up lots of them, but you won't have the satisfaction of doing your own. With too much store bought stuff, making is reduced to assembling.


    3 years ago

    Would this work with copper? I'm very ignorant about crystal radios but would like to give it a try.

    1 reply

    Reply 1 year ago

    It will work with any metal. Aluminum is just easy to work with, is soft, light, cheap and doesn't rust.

    spark master

    2 years ago

    Nice job and you could make it bigger and with a longer shaft you could make several in series of different capacities that all tune together


    3 years ago

    Where are you making your wire connections to your variable capacitor?


    3 years ago

    Wow not only did you give a very good write up, but the detail to your craftsmanship / work is great for the tools you used. I can use this to make variable air caps for my magnetic loop antenna project(s) when I get around to doing them. This will even help someone If you have one variable air cap and need more of the same or with less or more rotors and stators, all you need to do is disassemble it and copy (trace ) the parts and follow this to help make it correctly. Thank you very much for making this with such detail.

    2 replies

    3 years ago

    What are the dimensions for the parts for the capacitor?


    4 years ago on Introduction

    Beautiful piece of work. I got a start in electronics when I was a kid by my Dad who bought me a crystal radio. I learned to make my own and went all the way to building amps and digital circuits.
    How well does your crystal radio design work? I noticed just a telescoping aerial, I had to use about 50 feet of wire strung across our back yard. I also gave up on variable capacitors and used smaller coils with a sliding rod of ferrite for tuning. I was listening to the AM band not shortwave so that would make a difference.


    9 years ago on Step 18

    Here is the finished radio. All the wirings are done under the plastic base. I will publish an ible of this crystal set soon.

    8 replies

    Reply 8 years ago on Step 18

    How would you measure this type of Capacitor? I'm guessing it would have a resistance range? Do you think the number of plates would add variability to those values? Perhaps adding in a few extra plates at the top end only. You could do this by using 1/4 and 1/8 of a circle for a few of the rotor plates instead of the 1/2 circle rotor plates. I'm not sure this will translate into capacitance the way I'm image it. Varying the degree of the rotor plate to achieve a larger range?


    Reply 4 years ago on Step 18

    You can get a reasonable estimate of the maximum capacity (totally meshed) by simple geometry. The capacity in Farads of two parallel conductive plates separated in air is approximately 8.85×10−12 times the Area of the plates in square Meters divided by the Distance apart in Meters. This applies to the area of plates meshed but some capacity still exists between the unmeshed plates so you can't get down to zero capacity.


    Reply 7 years ago on Introduction

    You need a meter that measures Farads. An Air Variable like this for Broadcast band should go from 0-365 pF.
    Air Variables can be had from for about $12 +/- US.
    Adding or subtracting plates will increase or decrease the caps value.

    If you can make caps like you suggest then you should have a Ham Radio License and build your own Transmitter and Receiver to operate.


    Reply 7 years ago on Step 18

    A little FYI goes a long way:
    the equation for a air plate capacitor is:

    C= {k*0.2248*A}/d

    this is in inches:
    C is the capacitance of 2 plates
    k is dielectric constant air and vacuum are 1
    A is the area of one plate in square inches
    d is the distance between the plates in inches

    The maximum capacitance of a multi plate variable capacitor would be:

    Ct= {n-1}*C

    Ct is the total maximum capacitance
    n is the number of plates
    and C is the capacitance in pico farads of just 2 plates

    If you do like the Japanese do and put a dielectric between the plates it can make the capacitor smaller no real advantage in a crystal set. but if you were shipping tens of thousands of them it would save you sum money.

    Say you take a 3 inch diameter circle the area fro the caqpacitor would be r^2*PI= A
    or about 7 square inches then you only will be using half of the circle so making A=3.5 inches for the capacitor equation.

    say you make the distance .04 inches between the plates
    {1*3.5*.2248}/.04=79.4 pf for 2 plates

    so 365/79.4=4.59 so 5 plates total will get you close

    6 plates would be

    it is perfectly ok to have even about of stator and rotor plates
    or you can tweak the diameter with a spread sheet to get an odd number of plates

    de N8ZU


    Reply 5 years ago on Introduction

    Or.. for the purpose of building a crystal set... why bother with the meter. Just hook it up and see what frequencies you get. That could be part of the fun!


    Reply 5 years ago on Introduction

    When I build an X-Tal set I tend to use what I have in the junk box.
    I have BCB air variables from the tube era that I use that are not near 365pF but they usually have trimmer caps on their sides. If that do the trick I have been known to adjust the capacitance with fixed caps and micro switches. My last X-Tal set I had set up under the transmitting antenna, at a Boy Scout Jamboree, of a 100 watt HF transmitter. My long wire was lower and parallel to the antenna on the transmitter. With the switched caps on my air variable I was able to have complete separation of frequency's. HF radio did not bleed over to the BC X-Tal set. I had 2 meters on the thing to detect null spots for each tank circuit on the X-Tal set. I had a Push Pull coil and a spider web coil all working together with a matched pair of 1N34 detector diodes. No "Taps" on any of the coils. The results I had were astonishing to me. I was able to tune from below 53kHz AM all the way into the SW Bands to around 40 meters somewhere.
    The separation was great. The 2 Meters I used as tuning aids. Best X-Tal Set I ever had. Next time I use 600/44 Litz wire for the coils.

    WH7WP de


    Reply 5 years ago on Introduction

    That capacitor doesn't look like it would go as high as 365pf. But.. he didn't say Broadcast Band, he said Shortwave. That wouldn't require as large a capacitor so it makes sense.

    If you are going to build something like this shortwave might be more fun anyway. AM broadcast radios are still so common. I'm sure we all have several even if we never use them. It's fun to build anything, just to be able to say you built it yourself but better yet when it is something you don't already have!