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Build Your Own Air Variable Capacitors!

David F Hammack (N4DFP) on April 29, 2003
Website: http://www.qsl.net/n4dfp/buildcaps.html
View comments about this article!

Build Your Own Transmitting Air Variable Capacitors!

By: David Hammack, N4DFP

As I was building my transmatch, I became increasingly dissatisfied with my choice of capacitors. They were too small for more than about a hundred watts, and the ganged capacitor was in pretty sad shape. The frame was rusted and I could not remove all of the oxidation from the plates. I had a very nice Hammerlund 450pf that would do well for the Input Tuning, but I didn't have an equivalent ganged capacitor to use for Output Tuning. I am temporarily disabled and have no income at the moment, so it looked as if my transmatch would either be constructed of inferior parts or put on hold indefinitely. As I was looking over the Web, I came across an article by DL5DBM, Anwar von Sroka on building your own capacitors. It looked pretty simple, but I was concerned that the materials would strain my very limited budget. I did some thinking and looked around my local hardware store to check the price of materials. I found that the heavier gauge sheet metal was beyond my price range, but a 10' roll of aluminum roof flashing was only about $4.00. I figured I could manage that, so I looked for the other materials. A 3' stick of 1/4" all-thread was only $0.99. I couldn't find the spacers, and did not have a tubing cutter to make them with. I knew from experimentation that the spacing on the Hammerlund Capacitor's plates was the same as a 1/4" nut, so I decided to use nuts as spacers. I didn't feel the flashing would be suitable to make the contact spring for the rotor, so I found a small compression spring to use for tensioning the rotor to the contact plate. I also decided to use nylon filled stop nuts for securing and adjusting the rotor. The total bill of materials came to less than $15.00, and all materials were found at my local hardware store. I had some Lexan on hand to use for endplates, but any good non-conductive material such as PlexiGlass, Teflon, Nylon, Lucite, or phenolic would work. Usable scraps are usually available at reasonable prices at most glass shops or plastics suppliers. I would recommend not less than 1/4" thickness.

Construction

Rather than use Anwar's measurements, I modeled my capacitor on the Hammerlund. Since it was 25 plates on the rotor and 24 plates on the stator, rated at 450pf, I figured that 12 rotor and 12 stator plates would give me something on the order of 220pf. I measured the Hamerlund as illustrated:

This layout was drawn with Cadvance 6.5b, a free program available at www.cadvance.com

One major advantage to using flashing is the ease with which you can work it. It can be easlily cut with a good pair of utilty scissors. Do NOT use your XYL's favorite dress shears for this!

When making your plates, lay out one of each, cut them out, and then use them as templates to layout the rest. Here are the plates, all cut out. Since no one is perfect, make sure you mark an index on each plate as you cut it out. This will insure proper alignment when you assemble the device.

Drill the endplates. It is a good idea to drill both together so the holes will match front and back. To make the contact plate for the rotor, cut a strip of flashing long enough to cover the rotor shaft hole on both sides. After bending the strip over the endplate, drill a small hole to accommodate a 1" 6-32 brass screw.

Next, drill the shaft holes in the rotor plates. Again, in order to insure the uniformity of the parts, you should drill all the holes at once. I used scrap Lexan to hold the plates in the jaws of a Vice-Grip. Dimple the top plate of the stack at the center of the hole. It is a good idea to drill a small pilot hole before drilling the 1/4" hole.

Now, take a rotor plate and a stator plate and tape them together in full mesh. Make sure the shaft tab on the rotor plate is properly aligned with the cut out in the stator plate.

Align the shaft hole in the endplate with the shaft hole in the rotor plate and square the endplate over the parts to be marked. I used a carbide tip scribe for marking, but any sharp pointed object could be used. In a pinch, you could use the point on your pencil compass for marking.

Separate the rotor and stator plates you just marked. Stack all of the stator plates together insuring that the rotor tab cutout matches and tape them. This will keep them from slipping when you clamp them for drilling.

Cut the all-thread for the stator rails. For a 220pf capacitor of 12 plates, 4-1/2" is adequate. Mount these in the back end plate, leaving 3/8" to 1/2" beyond the outside nut. Secure them to the inside using 2 nuts per rail to leave adequate space for the rotor.

Begin mounting the stator plates. First flatten them as much as possible, removing the bows, twists, and curls. Make sure they are properly aligned. This is where the index marks you put on the plates come in handy.

As you mount the plates, tighten the nuts finger tight - do NOT use a wrench. Excess torque will deform the plates, making alignment extremely difficult. Look for a bowing in the plate. Either the plate was not flat, or the nuts are too tight. Correct this now before proceeding to the next plate.

Once the stator is assembled, assemble the rotor, again flattening the plates as much as possible. The shaft should be cut an 1-1/2" to 2" longer than the stator rails. Use a stop nut as the last nut in the rotor assembly,and then a regular nut.

This will hold the regular nut allowing you to torque the first plate into position, and will later be used to adjust the turn tension. Assembly of the rotor will require a wrench. As illustrated, use some kind of spacer to tighten the plates against. This will insure that the plates remain level and allow you to torque the nuts fairly tightly.

Now you are ready for final assembly. Hold the completed rotor in a full unmesh position and insert the shaft into the back endplate using 2 Teflon or Nylon washers on either side of the plate at the rotor shaft. You may need to add a washer to each side of the stator to allow space. Secure the rotor with a stop nut . Rotate the rotor to full mesh and install the front plate. Secure the plate with regular nuts on the stator. Place a flat washer, the compression spring, and another flat washer on the rotor shaft. Secure these with a stop nut. Tighten the stop nut till the spring compresses, tensioning the rotor shaft. Now tighten the stop nut on the other end of the rotor to center the rotor plates in the stator. Judge proper centering at the shaft.

It is possible that you find the rotor turns too freely to suit you. You can adjust turns tension by tightening the inside stop nut on the rotor shaft against the backplate to provide a more pleasing tension. Likely, you will need to adjust the rotor plates. This is accomplished using a small screwdriver to slightly bend the plates at the shaft. Be gentle, a little at the shaft translates to a large movement at the outer radius. If you have developed a slight bow in a stator plate, use the screwdriver in the same manner at the nuts on the stator rail. Only as a last resort should you attempt to adjust the capacitor with needle nosed pliers.

Now your capacitor is complete and ready for your project!

Conclusion

As I assmbled this unit, I came to the conclusion that Anwar's use of spacers, rather than nuts is probably a good idea. If I were using spacers, I would use a stop nut on either end of the assembly to prevent loosening. Flashing is not the best material to use. If you can find it easily, I would recommend a heavier gauge material. Anwar made a very valid point of which I was not aware when he critiqued my design, which I will pass on here. Sharp corners should be radiused to prevent arc over. It would be a simple matter to round of the corners with a mill file as you drill the holes. I suspect if I use significant power, I will probably have to rebuild the capacitor with radiused corners after it arcs over.

Transmitting air variables are becoming scarce and expensive, even at Hamfests, and the kit capacitor from TenTec is over $50.00. This is a viable alternative for medium power projects. Though I have not tested it, I suspect a carefully aligned capacitor of this type would easily carry a kilowatt. This would be perfect for a Transmatch or the coupling capacitor in a Loop Antenna without breaking the bank. Ham Radio is a fun hobby, but it doesn't necessarilly have to be expensive, as long as you carry on the HomeBrew tradition!

I would like to thank Anwar von Sroka, whose excellant article inspired this project.

A small little program to calculate capacitor parameters (along with many other GREAT design programs) can be found at: http://www.qsl.net/ve3sqb/

Make sure you look at my Transmatch Project in which I used a homebrewed double ganged capacitor.

Member Comments:
This article has expired. No more comments may be added.
 
Build Your Own Air Variable Capacitors  
by N4DFP on April 8, 2003 Mail this to a friend!
One point I forgot to make in the article: When making your plates, lay out one of each and cut them out, then use them as templates to make the rest.
 
Build Your Own Air Variable Capacitors!  
by K1VV on April 29, 2003 Mail this to a friend!
This is REAL HAM RADIO !!!
Whitey K1VV
 
Build Your Own Air Variable Capacitors!  
by VK4JAM on April 29, 2003 Mail this to a friend!
This articale is brilliant !

Not sure that I agee with "Do NOT use your XYL's favorite dress shears for this!" .... it suspect the successful use of this type of shears will depend on several key factors: (1)how fast you can run, and (2) how well you have planned your getaway ! :-)

Thank you for a great article.

Andrew
VK4JAM
 
RE: Build Your Own Air Variable Capacitors!  
by N0UY on April 29, 2003 Mail this to a friend!
Nice Article David.
Like every project, when it's done you can always find something you'd like to do differently. I find that to be true at my job also. As far as using the XYL's shears for the project, I would consider it as fair payback for all the times my tools were used to stir paint, chip ice, or serve as temporary markers in the vegetable garden. But seriously, good materials are the core of any successful project. Hope you enjoyed the effort, it's the most important part.

73, Ray N0UY
 
RE: Build Your Own Air Variable Capacitors!  
by KE4MOB on April 29, 2003 Mail this to a friend!
This is neat! Good Job!

Steve, KE4MOB
 
Build Your Own Air Variable Capacitors!  
by K8AG on April 29, 2003 Mail this to a friend!
Fantastic article. I will bookmark this and reference it.

Thanks,

John Pawlicki, K8AG
 
Build Your Own Air Variable Capacitors!  
by WB4ONA on April 29, 2003 Mail this to a friend!
Nice Article, some comments and questions:

Stack up the plates and use a drill then hole-saw. Cut the stack in half and solder/braze the plates to the shaft. Maybe a way to offer these hard to find caps to the Ham community if someone wanted to produce them. (Those machining excellent hand keys and bugs for us may take note; a new market for you?)

A good place to get specifications on and order stock materials is McMaster-Carr, http://www.mcmaster.com/ I haven't ordered from them for years, but they are alive and well. Good site for for metal sheet, teflon/insulators, mechanical goodies like bearings etc., all kinds of stuff. Plenty of specifications too on their Web site. (I have no relation with McMaster-Carr).

Perhaps a Lost-Art? "Linearizing" air-and dielectric spaced variable caps: Does someone have the equations to cut the plates on an air-variable to make "cardiod" plates that "band-spread" the tuned filters they are a part of so tuning is linear (or otherwise) in terms of shaft rotation? Remember the old 365pf "front-end" tuner caps in old tube AM receivers? They did this and I suspect there's a legacy of information on how they were designed that may be lost.

73's, Dave
 
RE: Build Your Own Air Variable Capacitors!  
by K3AN on April 29, 2003 Mail this to a friend!
Great info. We need more how-to articles like this.

If anyone can direct me to information on home-brewing a set of tires for my car, I'd appreciate it. I'd even settle for learning how to do retreads. ;-)
 
Build Your Own Air Variable Capacitors!  
by WR8Y on April 29, 2003 Mail this to a friend!
Two great articles in a row? I hope this trend continues.


Mark
WR8Y
 
RE: Build Your Own Air Variable Capacitors!  
by N8FVJ on April 29, 2003 Mail this to a friend!
Refreshing article. Removes that 'buy it now, pay later' mentality & reinforces the Amateur Radio self-sufficiency some of us now keep in our back pocket (not the wallet side folks).
 
RE: Build Your Own Air Variable Capacitors  
by AB5Q on April 29, 2003 Mail this to a friend!
Very creative, thanks for sharing your capacitor project with us.
 
Build Your Own Air Variable Capacitors!  
by KB9ERU on April 29, 2003 Mail this to a friend!
GREAT job on the project.
Thanks for sharing with the rest of us!
This is INDEED REAL HAM RADIO!!!
...now where are those scissors....hihi
 
Build Your Own Air Variable Capacitors!  
by WN9V on April 30, 2003 Mail this to a friend!
That's a great article, using common hardware store goods to build an otherwise hard to find component.

The base design, could be expanded into multi gang capacitors, differential capacitors, and, with some modified stators, even butterfly capacitors. Who will be the first to use all home made caps in a copy of the Johnson Matchbox?

Here are a couple of suggestions for refinements:

Whether the plates are made of thick or thin stock, I would recommend smoothing the plate edges and corners using wet and dry sandpaper of approximately 200 to 300 grit then finishing with 400 or finer grit. If you do this sanding while keeping the work wet with water, the sanding will go quickly. The rounded off edges will help avoid arcing.

Some capacitor applications involve significant circulating currents. The Z match is one. For simplicity and a reliable low-loss connection I suggest using a short, solidly connected jumper instead of the sliding rotor contact. Assuming the capacitor is limited to turning only 180 degrees, an inch-long jumper made of braid or finely stranded wire would last a long time. One end of the jumper would be soldered into a hole drilled in the center of the back end of the capacitor shaft. The jumper would extend straight out the back of the shaft and be secured to an insulated tie point which would then serve as the rotor terminal. Use a heat sink when soldering the stranded jumper so that the part that needs to twist does not wick up the molten solder.

I hope the article by David N4DFP inspires others to present more hands-on projects that are easy on the budget and don't require a machine shop to build.




 
Build Your Own Air Variable Capacitors!  
by AG5P on April 30, 2003 Mail this to a friend!
Great article, David. Thanks for the ideas
and extra URLs. Very well done!

Just a note, if you are building air caps for
VFO tuning or just need a small air cap, the
brass washers cut in half make nice plates,
just use tiny brass split ring washers for
the spacing and aircraft nuts on each end
of the all-thread rods. Like David, mentioned
be sure to smooth all sharp edges on the plates.
For tension to avoid free turning you can use
several nylon washers between the first rotor
plate and the front housing to snug up the
assembly and use a small amount of teflon
spray lubricant for smooth turning.

My preference is to make split-stator air
caps and get away from the electrical contact
problems of the standard air cap.

Walter - AG5P
 
Build Your Own Air Variable Capacitors!  
by K0TRL on April 30, 2003 Mail this to a friend!
What a great article. It is refreshing to see someone build something useful from scratch today and then share how to do it with everyone! This capacitor even looks great. Maybe this the the 21st century internet Elmer. I hope so.
 
Build Your Own Air Variable Capacitors!  
by W8OB on April 30, 2003 Mail this to a friend!
Super article this is just what ham radio should be like again. The author of this can say he is really homebrewing a project
 
RE: Build Your Own Air Variable Capacitors!  
by N3NYC on April 30, 2003 Mail this to a friend!
Dave, WB4ONA,

There's a section on how to design a capacitor for straight-line-frequency operation in "Principles of Radio Communication" by J.H. Morecroft (John Wiley and Sons, 1921, 1927). The book needs a bit of interpretation, sometimes, due to the use of obsolete units, but it's a gold mine of old, half-forgotten stuff.

73,

Hale, N3NYC
 
Build Your Own Air Variable Capacitors!  
by KE4ZHN on May 1, 2003 Mail this to a friend!
Excellent article Dave! This is what ham radio is all about! Very few hams these days homebrew anything due to the ease at which one can buy about anything you can imagine. Its great to see people like yourself still keeping alive the true spirit of ham radio by homebrewing your own air variables. Keep up the good work.
 
Build Your Own Air Variable Capacitors!  
by WA8QNN on May 3, 2003 Mail this to a friend!
Great article Dave! This takes back to my amatuer radio beginnings, when ops made their own equipment, antennas and modifications.

Larry
 
Build Your Own Air Variable Capacitors!  
by W3DCG on May 7, 2003 Mail this to a friend!
Wow!
 
Build Your Own Air Variable Capacitors!  
by VK3BSF on May 11, 2003 Mail this to a friend!
What a great project!! I built a switched inductor for L1 as rotary inductors are also impossible to buy. Maybe someone can come up with a homebrew design using the same philosophy that David used. http://w1.859.telia.com/~u85920178/ under antenna projects..tuners..describes how to build a switched inductor instead of using the roller type. I can recommend this design as I built one for my tuner and works well.
 
Build Your Own Air Variable Capacitors!  
by KD5INM on September 7, 2004 Mail this to a friend!
I've found that Dollar General Stores carry a cheap cutting board made out of some tough white plastic. This material is probly a type of nylon and is hard against knife cuts, but can be sawed into squares or whatever shape and would make good end pieces, probly much more resistant to heat like that is generated in amplifiers and such than what maybe lexan or plexiglass.

This is an excellent article, I enjoyed reading it.

73' John Hamilton KD5INM
 
Build Your Own Air Variable Capacitors!  
by LOWTECHLOU on September 28, 2004 Mail this to a friend!
I have been looking for plans on this subject for a while. I want to build a bigger set and see how the dx-ing comes through? ill let you know...


lowtechlou
 
Build Your Own Air Variable Capacitors!  
by AP2WF on December 14, 2004 Mail this to a friend!
I am a Home Constructor myself and was very much facinated with the idea and available drawing tool on the web site. The availability of these mechanical electronic components like Gang Capacitors is a problem here. So I am inspired with the idea and want to thank the writer of the Article.
 
Build Your Own Air Variable Capacitors!  
by GEOCORONA on February 23, 2005 Mail this to a friend!
I've been playing with variations of this project for the past two weeks. Some things I've learned:

* Aluminum flashing has "memory"-- it wants to stay curled in the direction it was rolled. Lay out your pieces so they curl across the shortest dimension, and keep the same side of all pieces facing the same direction to reduce warp & rubbing.

* Using cylindrical spacers (such as washers) instead of nuts doesn't help the rotors grip the axle. They often fan out as the axle is turned, even when rough-sanded for texture. Since aluminum doesn't solder with normal resin-fluxed tin, creating a sturdy electrical & physical contact to the axle can be tricky.

* Drill bits tend to grab and torque sheet metal. Soft steel (like that used in cookie tins) is worse than aluminum for this, but at least it solders.

* Using 10 mil Lexan as a dielectric insulator (4.4 dielectric constant) helps to drastically reduce the size of the capacitor, but I suppose the wattage rating is going down drastically, too.

If I ever get a successful capactor, I'll let you know.
 
RE: Build Your Own Air Variable Capacitors!  
by GEOCORONA on February 25, 2005 Mail this to a friend!
Okay, I think I have it solved. When using spacers instead of nuts to hold the rotors, I drilled two tiny holes near the 1/4 inch axle hole of the rotors, and ran a piece of copper wire bent like a long staple through the holes. After the last rotor, I bent the two wire ends together and soldered them.

Now if one rotor grabs, they all grab.

Also, a droplet of superglue on the assembled axle where extra "stick" is required doesn't hurt (surface only -- not for joining metals). Nor does a little wipe of WD-40 on the rotors when using plastic film instead of air space.
 
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