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build a variable capacitor
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? |
for each facing pair, (ie with two fixed and one moving in
between, you'll get two faces) C=eA/d e=permittivity of free space, A=area, d= separation. But you must use real scientific/engineering units, not those Yank Hillbilly ones. "Jim" wrote in message ... i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? |
In article ,
Jim wrote: i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? For a parallel-plate capacitor, capacitance equals 0.0885 e[r] (N - 1) A / t where A is the area of one plate (in square inches), N is the number of plates, t is the thickness of the dielectric (separation between plates, in an air-variable cap) measured in inches, and e[r] is the dielectric constant of the dielectric (1.0 in the case of air). A cap with two stator plates and one rotor plate would have twice the capacitance of a cap with one stator and one rotor, all else being equal and neglecting parasitic effects. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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"Jim" wrote in message ... i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? ========================= In the May 2005 issue of 'Electron' ( mag of VERON ,the IARU affiliated society in the Netherlands) there is an article , by PE1LKT ,on a homebrew 'sliding 'capacitor made of pieces of double sided printed circuit board. 5 plates are fixed with 4 plates sliding in between. The fixed and sliding plates fit inside grooves ,made in 2 pieces of timber or polystyrene (alternatively inside glued strips of such material). The sliding plates are tapered at one end and straight at the other end where they are also bolted together with double sided PCB material spacers or bolts and washers. At that end is also a soldered nut which suits a threaded bar which is supported by a simple 'hose type bearing' with the end coupled via a piece of rubber or plastic hose to a fixed in-line electric screw driver. The electric motor is operated remotely via a three position switch for ' right hand rotation' - 'off ' -'left hand rotation' The fixed plates are obviously also galvanically connected at the far end away from the moving plates . This variable capacitor is used for a transmitting magloop and hence is subjected to high RF voltages ,hence the plates are appropriately spaced . Spacing could be minimised for a RX loop The above variable cap has a capacitance of approx 400 pF ,requiring fixed plates each with an area of 210 x 170 mm ( 357 cmsq equals approx 55 inch sq). The sliding plates are slightly shorter. For a RX loop requiring less plate spacing the plate area can be reduced. Frank GM0CSZ / KN6WH |
On Sat, 16 Jul 2005, Jim wrote: Date: Sat, 16 Jul 2005 13:08:18 -0500 From: Jim Newsgroups: rec.radio.amateur.homebrew Subject: build a variable capacitor i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? Do you really need to build this? I know they (365 mfd variables) are hard to find (I'd love to find a cheap source, expensive sources on the internet want $10-15 each [google on "365 mfd" to find them]), but if you go to a thrift store (Salvation Army, etc.), you may find old transistor clock radios for $1-2 each that have miniature 365 mfd variables that will do just fine if you take a few minutes to de-solder them from the circuit board. Art, W4PON |
i need a variable for a receiving loop antenna....
In the May 2005 issue of 'Electron' ( mag of VERON ,the IARU affiliated society in the Netherlands) there is an article , by PE1LKT ,on a homebrew 'sliding 'capacitor made of pieces of double sided printed circuit board. 5 plates are fixed with 4 plates sliding in between..... QST has published several homemade-capacitors articles in the last few years: * A Home-Brew Loop Tuning Capacitor (November, 1994) * A Homemade High-Power Tuning Capacitor (June, 1983) * One I recall (but can't find; my QST's are paper, someone with the QST CDs could do a search) which used concentric tubes with thin plastic around the inner/inside the outer Ok, so "few" is relative at my age!-) -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
Of course, you CAN build the capacitor, but unless you want to do it for
fun and/or education, why re-invent the wheel? Dan's Small Parts and Kits sells several nice capacitors. http://www.danssmallpartsandkits.net/ The capacitors aren't necessarily "dirt cheap", but a manufactured capacitor will be physically stable, insuring smooth change in capacitance as the shaft is rotated. Scott N0EDV Jim wrote: i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? |
"Manufacture" = "Make By Hand"
"Scott" wrote in message ... Of course, you CAN build the capacitor, but unless you want to do it for fun and/or education, why re-invent the wheel? Dan's Small Parts and Kits sells several nice capacitors. http://www.danssmallpartsandkits.net/ The capacitors aren't necessarily "dirt cheap", but a manufactured capacitor will be physically stable, insuring smooth change in capacitance as the shaft is rotated. Scott N0EDV Jim wrote: i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? |
I have recently built a very large pi-network transmatch with two very big
homemade variable capacitors. They can be seen at: http://www.metaphoria.us/hamradio/transmatch.html Jozef WB2MIC "Polymath" wrote in message ... "Manufacture" = "Make By Hand" "Scott" wrote in message ... Of course, you CAN build the capacitor, but unless you want to do it for fun and/or education, why re-invent the wheel? Dan's Small Parts and Kits sells several nice capacitors. http://www.danssmallpartsandkits.net/ The capacitors aren't necessarily "dirt cheap", but a manufactured capacitor will be physically stable, insuring smooth change in capacitance as the shaft is rotated. Scott N0EDV Jim wrote: i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? |
"Jim" wrote in message
... i need a variable for a receiving loop antenna. does anyone know how to determine the area of plates for a given value of cap? for example i need 365 pf. two half circle plates would be handy, separated by a piece of plastic or card stock. what surface area for each plate would approximate that value? does two stator plates with one rotor between them increase the value if compared to one plate each? Here is the "How to Build an Air Variable Capacitor" article you are looking for: http://www.eham.net/articles/5217 w9gb |
Without following your link.....well done that man!
"Jozef" wrote in message ... I have recently built a very large pi-network transmatch with two very big homemade variable capacitors. They can be seen at: http://www.metaphoria.us/hamradio/transmatch.html "Polymath" wrote in message ... "Manufacture" = "Make By Hand" |
The discussions in this thread make me wonder... it seems like the
tough part of making variable caps in the "classical" shape is making the plates. In my distant past, I was involved in making modest quantities of punched aluminum parts, and found a shop that was willing to make a punch/die set for the parts, even though my initial order was for only a couple hundred of them. (Eventually I ordered enough to wear out the punch/die set and they made another one...probably did 10,000 or so total.) Once you pay for the punch/die, the parts become pretty cheap, and the uniformity is vastly superior to what you could reasonably do by hand. So, the question becomes, if the plates (rotors and stators) were available in maybe two or three different basic sizes, how many frustrated hams would be interested in buying them? Might it be enough to make the punch economical? Would you be willing to pay, say, two dollars per plate for 0.0625" thick plates with 3" rotor diameter? A set of 25 such plates, using 0.1" gap (good for maybe 7kV peak?--provided the edges are properly rounded), would give you about 190pF. Do the people who would be trying to build such things already have the ability to do the rest of the parts, or would they need to be included too? Does someone already sell kits of capacitor parts? That's obviously way beyond what the OP needs for a receiving loop, of course! For that, I'd probably use a varactor diode...or find a radio receiver to scrap one out of if I wanted to stay mechanical. Cheers, Tom |
K7ITM wrote:
The discussions in this thread make me wonder... it seems like the tough part of making variable caps in the "classical" shape is making the plates. In my distant past, I was involved in making modest quantities of punched aluminum parts, and found a shop that was willing to make a punch/die set for the parts, even though my initial order was for only a couple hundred of them. (Eventually I ordered enough to wear out the punch/die set and they made another one...probably did 10,000 or so total.) Once you pay for the punch/die, the parts become pretty cheap, and the uniformity is vastly superior to what you could reasonably do by hand. So, the question becomes, if the plates (rotors and stators) were available in maybe two or three different basic sizes, how many frustrated hams would be interested in buying them? Might it be enough to make the punch economical? Would you be willing to pay, say, two dollars per plate for 0.0625" thick plates with 3" rotor diameter? A set of 25 such plates, using 0.1" gap (good for maybe 7kV peak?--provided the edges are properly rounded), would give you about 190pF. Do the people who would be trying to build such things already have the ability to do the rest of the parts, or would they need to be included too? Does someone already sell kits of capacitor parts? For short runs today laser cutting would be better -- the programming and setup costs are much lower than tooling. The quality will approach that of a good die set and exceed that of a poor one. There will be a point where things'll be cheaper to punch out with a die set, but it'd probably be in the thousands if not tens of thousands of plates. That's obviously way beyond what the OP needs for a receiving loop, of course! For that, I'd probably use a varactor diode...or find a radio receiver to scrap one out of if I wanted to stay mechanical. Cheers, Tom I wouldn't use a varactor to tune a recieving loop because of intermod. Antique Radio Supply and others have various variable caps for sale. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
Good point about other tooling methods (laser cutting not the only one
available), but the question remains, how many people would actually be interested in buying plates, at what cost levels? (At the time I was making the other parts, soft tooling was cheaper for very short runs, but the shop that did the punch/die was set up to make such tools at very low cost and easily beat the soft tooling for modest runs. You just need to decide how many of your parts you want, and go shopping.) As for using varactors on receivers loops, if you use back-to-back diodes and don't have to deal with seriously big signals, distortion should be at worst a very minor problem. For a fairly high Q tuned antenna, the signal voltage across the diodes for signals you're not tuned to is pretty small. Of course, the reason it's so hard to find small (receiving size) mechanical caps now is that their job has been taken over...by varactors. Cheers, Tom |
On 2005-07-20, K7ITM wrote:
So, the question becomes, if the plates (rotors and stators) were available in maybe two or three different basic sizes, how many frustrated hams would be interested in buying them? This would be a good emachineshop.com order. You can choose any of the processes that people are discussing on this thread, depending on the expected volume. When I played with their estimating software, my experience was that pretty much everything cost roughly: $100/ea in qty 1 = $100 $11/ea qty 10 = $110 $1.20/ea qty 100 = $120 $0.13/ea qty 1000 = $130 etc. I figured the biggest risk in using them would be the temptation to get 10x more units for 10% more and then have a zillion whatevers (eg rotor plates :) around the garage. I've been trying to think of a project that needs 100 identical widgets, and a homemade air variable might be it. -- Ben Jackson http://www.ben.com/ |
In article .com,
K7ITM wrote: Good point about other tooling methods (laser cutting not the only one available), but the question remains, how many people would actually be interested in buying plates, at what cost levels? (At the time I was making the other parts, soft tooling was cheaper for very short runs, but the shop that did the punch/die was set up to make such tools at very low cost and easily beat the soft tooling for modest runs. You just need to decide how many of your parts you want, and go shopping.) At the price you suggested (a couple of dollars per plate), the cost of putting together a good-sized air variable is going to be fairly fierce. It may be somewhat less than the cost of a new, commercially manufactured AVC, but it's probably not competitive with the cost of used-but-serviceable air variables at hamfest/fleamarket prices. You might find that it was appealing only to those who needed a fairly specific size/capacitance/standoff-voltage combination, not available used. You might find it attractive to specialize a bit. One not-uncommon application for large air variable caps these days, is people who want to make a magnetic-loop transmitting antenna. In this application, keeping resistive losses in the cap to a minimum is very desirable to help keep the Q and efficiency as not-terribly-low as possible. One way to do this is to use a welded rotor/shaft assembly, and welded stator plates, to reduce resistive losses. This is beyond the capabilities of most homebrewers, I think. An alternative is to use a soldered construction, which can't be done easily with aluminum but which is relatively easy if your shaft and spacers are brass, and the plates are either brass, or PC-board material (double-sided would work best). Hence, you might increase the demand for such stamped or laser-cut plates if you were able to offer them in materials other than aluminum. The materials cost for brass would probably be higher than for aluminum, but you might be able to offer plates in double-sided FR4 for prices no more than aluminum. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Cool, Ben. Thanks. I'm wondering what part you put in to get 13 cents
each at 1000 parts. That would be very attractive, but I keep getting over a dollar a plate for 3" diameter semicircles with a tab/hole for the shaft, for rotor plates. Admittedly, the 0.063" aluminum thickness is more than needed for many applications, but dropping it to 0.024" doesn't help a lot. I don't figure that making your own cap for low power applications is going to be very attractive, so I'm figuring the larger sizes. Also, what machining technique did you pick? Cheers, Tom |
Dave Platt wrote:
In article .com, K7ITM wrote: Good point about other tooling methods (laser cutting not the only one available), but the question remains, how many people would actually be interested in buying plates, at what cost levels? (At the time I was making the other parts, soft tooling was cheaper for very short runs, but the shop that did the punch/die was set up to make such tools at very low cost and easily beat the soft tooling for modest runs. You just need to decide how many of your parts you want, and go shopping.) At the price you suggested (a couple of dollars per plate), the cost of putting together a good-sized air variable is going to be fairly fierce. It may be somewhat less than the cost of a new, commercially manufactured AVC, but it's probably not competitive with the cost of used-but-serviceable air variables at hamfest/fleamarket prices. You might find that it was appealing only to those who needed a fairly specific size/capacitance/standoff-voltage combination, not available used. You might find it attractive to specialize a bit. One not-uncommon application for large air variable caps these days, is people who want to make a magnetic-loop transmitting antenna. In this application, keeping resistive losses in the cap to a minimum is very desirable to help keep the Q and efficiency as not-terribly-low as possible. One way to do this is to use a welded rotor/shaft assembly, and welded stator plates, to reduce resistive losses. This is beyond the capabilities of most homebrewers, I think. An alternative is to use a soldered construction, which can't be done easily with aluminum but which is relatively easy if your shaft and spacers are brass, and the plates are either brass, or PC-board material (double-sided would work best). Hence, you might increase the demand for such stamped or laser-cut plates if you were able to offer them in materials other than aluminum. The materials cost for brass would probably be higher than for aluminum, but you might be able to offer plates in double-sided FR4 for prices no more than aluminum. At two bucks a pop for plates you _ought_ to be burning up most of your money in setup, tooling and handling, with very little going to the actual material. Hence brass shouldn't be too much more expensive. I might be more interested in a kit that would let me saw plates out of FR4 and assemble a precision cap -- so if you supplied frame & shaft pieces that would accept .031 or .062 FR4 that would be a cool thing. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
On 2005-07-20, K7ITM wrote:
Cool, Ben. Thanks. I'm wondering what part you put in to get 13 cents each at 1000 parts. That was just an example. I just made a quick 3" diameter rotor with center hole, laser cut from 6061T6 .047 and 1 is $103, 10 are $123, 100 are $264. It's not quite as steep a curve as the other parts I was playing with. Since that's laser cutting you can group parts, so if I throw on a stator too it only goes up to $404 for 100. If I go to slightly thinner mild steel it's $254 for 100 pairs. Not sure how much difference Al vs steel makes for this application. I bet the turret punch would be cheaper, but it can't make the convex curves. If you make a die for the rotor it looks like the tooling is about $1k but the per-part cost can go down to 15 cents each at quantity 10k (for a net price of about 27 cents each). On the other hand, the lead time is 72 days! -- Ben Jackson http://www.ben.com/ |
Jozef wrote in message ... I have recently built a very large pi-network transmatch with two very big homemade variable capacitors. They can be seen at: http://www.metaphoria.us/hamradio/transmatch.html Jozef WB2MIC Very nicely done... I toyed with the idea of designing a couple of flat dies to punch out the stator and rotor plates and market my own series of capacitors. I agree, the high price of those capacitors are unwarranted. There was no need for two variables in that tuner... a single DIFFERENTIAL capacitor would do nicely and eliminate the extra adjustment during tune-up. For some unknown reason Hams still need a tuner with two capacitors---it was nice to see MFJ offer the Differential-T tuner Model 986. Many years ago (before the MFJ 986 tuner) as an experiment I built a field expedient tuner using a differential capacitor made from two telescoping cardboard tubes and aluminum foil (two plates outer and one plate inner tube) and slid the inner tube in and out to acheive an antenna match with a 28 microhenry tapped coil. It worked fine at 100 watts and many contacts were made with the end fed wire but dilectric heating of the cardboard tubes was observed. RG |
RadioGuy wrote:
Jozef wrote in message ... I have recently built a very large pi-network transmatch with two very big homemade variable capacitors. They can be seen at: http://www.metaphoria.us/hamradio/transmatch.html Jozef WB2MIC Very nicely done... I toyed with the idea of designing a couple of flat dies to punch out the stator and rotor plates and market my own series of capacitors. I agree, the high price of those capacitors are unwarranted. There was no need for two variables in that tuner... a single DIFFERENTIAL capacitor would do nicely and eliminate the extra adjustment during tune-up. For some unknown reason Hams still need a tuner with two capacitors---it was nice to see MFJ offer the Differential-T tuner Model 986. Many years ago (before the MFJ 986 tuner) as an experiment I built a field expedient tuner using a differential capacitor made from two telescoping cardboard tubes and aluminum foil (two plates outer and one plate inner tube) and slid the inner tube in and out to acheive an antenna match with a 28 microhenry tapped coil. It worked fine at 100 watts and many contacts were made with the end fed wire but dilectric heating of the cardboard tubes was observed. RG RG, is there an example of a differential capacitor I could look at? I'm not sure I've ever seen one? Is there a Tuner project somewhere using one I could look at (grin)... Regards, Mike |
In a differential capacitor, as the rotor disengages from one
set of fixed plates, it engages with the other set, instead of engaging with fresh air. The two sets of fixed plates are adjacent to each other and separated by a gap. (If it were not for the gap, there would be a solid square of metal with the axle dead centre) "Mike, K8LH" wrote in message ... RG, is there an example of a differential capacitor I could look at? I'm not sure I've ever seen one? Is there a Tuner project somewhere using one I could look at (grin)... |
Polymath wrote:
In a differential capacitor, as the rotor disengages from one set of fixed plates, it engages with the other set, instead of engaging with fresh air. The two sets of fixed plates are adjacent to each other and separated by a gap. (If it were not for the gap, there would be a solid square of metal with the axle dead centre) "Mike, K8LH" wrote in message ... RG, is there an example of a differential capacitor I could look at? I'm not sure I've ever seen one? Is there a Tuner project somewhere using one I could look at (grin)... Thank you... I have seen variable capacitors like this... Vy 73, Mike - K8LH |
"Jozef" wrote:
: http://www.metaphoria.us/hamradio/transmatch.html excellent demonstration for others to learn from. |
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