EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: cdev on April 20, 2021, 02:19:51 pm
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I am building a loop antenna that I hope to transmit into in the future, so I am wondering about good fixed capacitors for RF at medium power. Best caps for power?
If I used fixed non-variable caps I can parallel it with a single variable cap to support a wider range. Ceramic, probably?
Preferably high Q. Mica? Vaccum? Ceramic? What are the best/cheapest caps for filters, loops and other high Q applications if you're on s budget?
(Unfortunately seems most vacuum caps are too high in price for me now, although its clear they are the best ones to use if you have them. ) :(
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How much RF current do you intend to put through the capacitor? In a "tank" circuit, the circulating current is higher than the input or output current of the resonant network.
There are ceramic and porcelain capacitors with wide ribbon leads, and sometimes a traditional NP0 non-monolithic disc ceramic capacitor, such as the (former Sprague) Vishay Ceramite will do, because of their relatively large area.
The power dissipated in the capacitor will depend on the current and the ESR, which depends on the Q of the capacitor at the resonant frequency. At not very high frequency, the old (huge) mica capacitors (for example, https://www.rfparts.com/capacitors/capacitors-transmitting/cm55-01-600.html (https://www.rfparts.com/capacitors/capacitors-transmitting/cm55-01-600.html)) could handle large currents due to their large volume for dissipating the heat.
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You must also consider that the voltage across the tuning capacitor(s) on a transmitting loop will be very high. It can be over 1kV with as little as 10W of transmit power!
Also the capacitor ESR and dielectric losses must be kept to an absolute minimum to maintain a high Q and thus achieve high efficiency.
The issue with using fixed capacitance with a smaller variable capacitor is that you will have limited tuning range. Might be able to get away with it if designing a monoband loop, but otherwise you'll need a much larger capacitance range.
On a budget, the most practical and functional is an air-variable capacitor. There are many ways of constructing air-variable capacitors yourself for relatively low cost. Air variable capacitors have low loss, high Q, can handle high voltages depending on the spacing between the plates, and can offer a very wide tuning range from a single capacitor (covering 3-4 bands on a single loop is achievable). Power levels of 100W or a bit more are achievable on a suitably constructed air variable capacitor, but the required plate spacing makes these quickly get unreasonably large if you're going for 500W, 1000W... that's where vacuum variable capacitors really shine.
Vacuum variable is the best of the best, especially for high power, but of course more expensive and you can't build one yourself.
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I wonder why vacuum variable capacitors are so expensive? It must be difficult to make them in terms of time or ... something..
The effect of a high -Q magnetic loop is hard to beat. Its my understanding that they beat out almost any other HF antenna and often their size is the most manageable especially for the lower frequency bands. If somebody has a high location they can really perform spectacularly well. They even work surprisingly well indoors.
I know that they often develop surprisingly high voltages across the variable capacitors. I have a huge collection of variable capacitors of many kinds and except for two that are not already in use none of them would be appopriate for use with a transmitting loop. I'd have to make one, which I agree would be the best way to go considering the cost. My problem as far as making the usual style is I dont have any CNC or similar metal fabrication gear. If I did I would try my hand at making a butterfly cap.
You must also consider that the voltage across the tuning capacitor(s) on a transmitting loop will be very high. It can be over 1kV with as little as 10W of transmit power!
Also the capacitor ESR and dielectric losses must be kept to an absolute minimum to maintain a high Q and thus achieve high efficiency.
The issue with using fixed capacitance with a smaller variable capacitor is that you will have limited tuning range. Might be able to get away with it if designing a monoband loop, but otherwise you'll need a much larger capacitance range.
On a budget, the most practical and functional is an air-variable capacitor. There are many ways of constructing air-variable capacitors yourself for relatively low cost. Air variable capacitors have low loss, high Q, can handle high voltages depending on the spacing between the plates, and can offer a very wide tuning range from a single capacitor (covering 3-4 bands on a single loop is achievable). Power levels of 100W or a bit more are achievable on a suitably constructed air variable capacitor, but the required plate spacing makes these quickly get unreasonably large if you're going for 500W, 1000W... that's where vacuum variable capacitors really shine.
Vacuum variable is the best of the best, especially for high power, but of course more expensive and you can't build one yourself.
Some of them have quite complicated internal structures.. But still its not rocket science.
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for magnetic loop usually vacuum capacitors are used. As alternative you can use air capacitor with extended gaps between plates. Also air capacitor should not use moving mechanical contacts like it is used in air capacitors for a usual tube-receivers. For example you can use two sections connected in series through a rotor to avoid moving mechanical contact.
Vacuum capacitor provide you with better Q. But on the other hand very high Q leads to very narrow bandwidth, very sharp resonance and extreme high voltage and current in antenna. So, it will be more hard to tune and requires more expensive capacitor.
By the way, using magnetic loop for transmission is not safe, because it has extreme high field strength near the loop.
Just look at simulation for a magnetic loop at 14 MHz with a 100W power. The black line on the color grade is official safety limit.
As you can see, the magnetic loop has:
- E field strength more than 15000 V/m, while safety limit is just 130 V/m
- H field strength more than 60 A/m, while safety limit is just 0.34 A/m
For comparison, half wave dipole has 8 times smaller E field strength and 15 times smaller H field strength.
I don't recommend to use more than 10-15 W power with magentic loop. And when TX, keep away from antenna as much as you can.
Some years ago I was played with magnetic loops and it was a funny when you keep the gas-discharge lamp in your hands and it glows like it is plugged to the mains. It was just 10 W power and I had a many very painful burns on my fingers when tried to tune air capacitor. But later I read more about electromagnetic flux, antenna design theory and underlying physics and I came to the conclusion that it is not safe for the health. Especially if you use more than 10-15 W power with a shortened antennas.
Using a full size (half wave dipole) antenna is much more safe and give you better results. Full size antenna has much better efficiency , much less noises, it's more easy to match it and it has much more safe field strength near it.
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I am building a loop antenna that I hope to transmit into in the future
A transmit loop antenna may be a nice experiment, but beware, the voltage at the ends of the top capacitor will be VERY HIGH, and if using the loop indoors you'll have a LOT of RF around you
so I am wondering about good fixed capacitors for RF at medium power. Best caps for power?
If I used fixed non-variable caps I can parallel it with a single variable cap to support a wider range. Ceramic, probably?
for TX loops, usually vacuum variable capacitors (http://www.ur4ll.net/#caps3_1) are preferred for a number of reasons, including the fact that, if the loop is placed outside, the capacitor won't arc due to a bit higher humidity in the air; never tried using fixed capacitors but given the voltages I believe that some "door knob" capacitors (see here (http://www.ur4ll.net/#caps2) for some examples) would fit; keep in mind that a tuned loop has a very HIGH "Q" so you'll also need to find a way to remotely tune it (and avoid getting burned when touching the variable cap shaft), again, it may be a nice experiment, but sincerely I'd rather use a multiband antenna like an OCFD and a separate, dedicated RX antenna which then may be a loop (RX ones are fine in my book)
Then ok, if you absolutely want to go for a TX loop, check this one (http://www.radioamatoripeligni.it/i6ibe/loop/loop_mega.jpg)
(http://www.radioamatoripeligni.it/i6ibe/loop/loop_mega.jpg) (http://www.radioamatoripeligni.it/i6ibe/loop/loop_mega.jpg)
;D
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+1 for a vacuum cap; the smaller the loop and the lower the operating frequency, the higher the voltage over the cap; on the higher SW band, it might work with silver mica caps, but 160m on a 1m diameter loop no chance; but it's a rather unusable setup as well, efficency will be well below 1%
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I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.
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I wonder why vacuum variable capacitors are so expensive? It must be difficult to make them in terms of time or ... something..
Low demand, since they are used commercially in just a limited number of niche applications. This means they aren't mass produced, and thus cost a lot more.
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There is an interesting and informative video on Youtube done by a gent who used 22mm copper tube, plastic tube and 15mm copper tube to create a trombone, driven by a motor to slide in and out. A bit agricultural possibly, but works fine.
https://www.youtube.com/watch?v=j3Q1VgDrWYA (https://www.youtube.com/watch?v=j3Q1VgDrWYA)
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Don't worry folks, I am not even licensed yet and when I am I fully intend to put any loops far away from my body, like in the back yard, hanging from a tree, or between two trees most likely. A safe distance even from my friends the squirrels.
The video is indeed of the guyI mentioned and his dual magnetic loop. And its a great idea, in terms of cost. I think building a variable capacitor is the way to go.
There is an interesting and informative video on Youtube done by a gent who used 22mm copper tube, plastic tube and 15mm copper tube to create a trombone, driven by a motor to slide in and out. A bit agricultural possibly, but works fine.
https://www.youtube.com/watch?v=j3Q1VgDrWYA (https://www.youtube.com/watch?v=j3Q1VgDrWYA)
What did you mean by use of the term "agricultural" here? Reminding one of farm machinery perhaps? Inventive ?
The design is good because the "pea" of a magnetic look lke this one is likely to be very sharp, so the tuning is best when its got a reduction gear.
If ou are using a standard "bread slicer" capacitor that means that the peak is likely to be so sharp you will just blast through your desired frequency in a second or less and will be unlkely to be able to hear the noise peak as it blasts by.
But with the long linear motion, that seems likely to be almost ideal. .
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I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.
I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.
Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
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check this
http://sm0vpo.altervista.org/antennas/frameant.htm (http://sm0vpo.altervista.org/antennas/frameant.htm)
not perfect, but will get you on the air on 160/80 and since it's easy to put together, it will allow you to evaluate a loop and decide if you want to go on
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A typical "magnetic loop" antenna gain figure is -10dBd to -20dBd, for a correctly installed dipole (which achieves 8+dBi on the main lobe).
They are attractive when space is extremely restricted, especially on the 160m/80m/40m bands. However, on lower bands the resonance bandwidth becomes extremely narrow, to the point that it barely fits the SSB spectrum (2.5kHz or so) and tuning is absolutely critical.
On higher bands (20m and higher), better performance, simpler and much cheaper solutions (including shortened verticals or even random wires & tuner) are more viable.
They are mostly a novelty rather than a real solution, but they could work. The complexity makes it less than ideal for beginners.
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That kind of coil is easy to make and works well. Multiple strands of wire definitely will mean higher Q.
check this
http://sm0vpo.altervista.org/antennas/frameant.htm (http://sm0vpo.altervista.org/antennas/frameant.htm)
not perfect, but will get you on the air on 160/80 and since it's easy to put together, it will allow you to evaluate a loop and decide if you want to go on
I find the online calculators are not so accurate.
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I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.
I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.
Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
The voltage is not only dependent of the TX power but also on the size of the loop. A larger loop will give a lower voltage. By making the loop as large as possible while still being a STL the voltage can be kept under the dielectric strength of the coax.
A fixed capacitor can also be made of a sheet of 1,6mm PCB material or two sheets of thin copper separated by a sheet of picture frame glass.
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The voltage is not only dependent of the TX power but also on the size of the loop.
The voltage depends on Q factor and the coil impedance.
Since half wave dipole Q factor is about 10, the voltage is multiplied by 10 times.
Since magnetic loop Q factor is about 2000, the voltage is multiplied by 2000 times.
Magnetic loop impedance is about Z=0.25 Ω. If you put 100 W into load with Z=0.25 Ω, you will get U=sqrt(100*0.25*2)=7 Vpk. And since LC has Q=2000, the voltage after 2000 cycles at resonant frequency will be about U=7*2000=14000 Vpk.
This is how magnetic loop works. It uses high Q at resonant frequency to accumulate oscillations amplitude to a very high voltage in order to compensate low efficiency of a small size radiator. As a drawback it needs 2000 oscillations in order to radiate the same energy as antenna Q=1. This is why bandwidth of magnetic loop is reduced 2000 times.
And this is why full size antenna with lower Q is more safe and has less energy loss on heating.
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The voltage is not only dependent of the TX power but also on the size of the loop.
The voltage depends on Q factor and the coil impedance.
Since half wave dipole Q factor is about 10, the voltage is multiplied by 10 times.
Since magnetic loop Q factor is about 2000, the voltage is multiplied by 2000 times.
Magnetic loop impedance is about Z=0.25 Ω. If you put 100 W into load with Z=0.25 Ω, you will get U=sqrt(100*0.25*2)=7 Vpk. And since LC has Q=2000, the voltage after 2000 cycles at resonant frequency will be about U=7*2000=14000 Vpk.
That's a very high voltage that I am sure would fry most home-made capacitors Ive built. How far a gap would that jump, I wonder. A lot.
Sometimes my loops when receiving a strong signal develop enough power to make the reciever completely lose its ability to differentiate signals from one another.
This is how magnetic loop works. It uses high Q at resonant frequency to accumulate oscillations amplitude to a very high voltage in order to compensate low efficiency of a small size radiator. As a drawback it needs 2000 oscillations in order to radiate the same energy as antenna Q=1. This is why bandwidth of magnetic loop is reduced 2000 times.
And this is why full size antenna with lower Q is more safe and has less energy loss on heating.
What do you mean by "full size" ?
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What do you mean by "full size" ?
The half wavelength
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I have seen pices of coaxial cable used as cabacitor on fixed frequency magnetic loops.
There is a certain capacitance between center-conductor and screen of the cable.
I think coax cable as capacitor is a bad choice. Its voltage limit is just 1000-2000 V, this is too small for magnetic loops.
Also coax cable has high loss in the insulator.
Good magnetic loop has Q up to 2000. With 100 W power and magnetic loop impedance 0.25 Ω, the voltage will be up to 14000 Volts.
The voltage is not only dependent of the TX power but also on the size of the loop. A larger loop will give a lower voltage. By making the loop as large as possible while still being a STL the voltage can be kept under the dielectric strength of the coax.
A fixed capacitor can also be made of a sheet of 1,6mm PCB material or two sheets of thin copper separated by a sheet of picture frame glass.
A sheet of plexiglass would make a good dielectric. I've seen some easy to make homemade caps made which probably cost almost nothing made with plexiglass and copper sheeting and or even tape. Have two sheets hanging down with bungee cords on the back to defaultto wide open. The control of the capacitance can be adjusted by increasing or decreasing the amount of fulling on two pieces of parachute cord. (unlikely to stretch much and lose calibration. )
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A sheet of plexiglass would make a good dielectric.
As I know, fiberglass has smaller loss than plexiglass, but it depends on material.
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Other plastics available in sheet or plate form, such as polyethylene and polypropylene, have very low dielectric loss and high dielectric strength (but cannot be glued easily).
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why not glass?
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Glass is probably a very good dielectric, with a material as good as glass the main determnant of the voltage capacity is the distance between the two conductors. The further apart they are the more voltage the capacitor can handle without flashover.
Glass might be very good.
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Looking at eBay, there seems to be a good number of vacuum FIXED caps available. For example, 100pF 25KV for $35 from Russia. 50pF 20KV from US is $75. Doesn't seem to be excessively expensive. Otherwise, I think one of more practical options are door knob capacitors. They were often used in tank circuit on a very large transmitters.
I don't think you ever gave a specific number. What kind of power are you thinking? QRP at 10 watts? Or the usual 100 watts? That makes huge difference in what's suitable and what's not.
Home brew approach isn't bad either. Overlapping copper pipe with teflon insulation will work reasonably well. Or, if power is low enough, air insulation will be even better.
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Again, the real question is how much current will flow through the capacitor. In a tank circuit, the circulating current is higher than the load current. For very high tank-circuit current, vacuum is the best and most expensive. Old clunky mica capacitors can handle current due to their large volume.
For simple flat-plate capacitors, glass works but polyolefins such as polypropylene are easy to cut. The important thing is adhering the copper electrodes without air gaps between metal and dielectric. (Polypropylene is hard to glue.)
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loctite medical primer with loctite plastic superglue works fine for polyethylene, very strong. adhesion promoter not curing agent. nothing special to it, just clean prime on the hard to bond side, inspect with UV if you want, then glue. not sure about pp
dp8805 or dp8810 epoxy might work too but I have not tried it for polypropylene yet. you probobly should scratch it up with silicon carbide sand paper first (very sharp). the 8805 and 8810 are loaded with glass spheres also, Iirc, that should stabilize it and ensure thin proper bond lines.
Loading the glue with ceramic spheres or such may benefit capacitors too to ensure standard size spacing, but I am not sure how that will work out in theory.
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I built a ten foot dia mag loop and used a vacuum variable cap. The big problem with small loops is that they do not radiate well, The radiation resistance is very low. Therefore you must keep real ohmic resistance to very low levels, less than one ohm. This is impossible without good tuning caps. Even soldering joints is frowned upon, you need to weld or braze or real silver solder. Anything with a normal solder joint will have too much resistance and you will waste power.
There are some air caps that seem to work. Trombone variable caps work and are used by some people who use transmit power into the tens of thousands of watts in vertical antenna tuning. I think they go short is it rains on them. Almost anything will work for RX.
I assembled mine without any solder joints by putting the ends of a Vacuum Variable Cap into the ends of the circular bent 2 inch copper pipe.
Try the Mag loop group on Groups IO https://groups.io/g/MagLoop/topics
Or this group if you want to be a bit different: https://groups.io/g/HelicallyLoadedMagLoop/topics
Feeding small mag loops is another matter, They can be fed with a small loop or a gamma or even a delta match, more fun
About the supposedly huge dangerous voltage, my loop is in the next room and my cat walks by it all the time and her fur still looks OK.
I use it on 20 Meters with about 300 Watts.
Wally
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Wow that's a big loop!
THANK YOU! This looks like a great resource. I have a thee turn loop made of approximately half inch tubing and Ive been wondering how to make the rest of it and using big huge battery clips for now, not so great but OK for testing, You answered a major question hat I have had.
I do have some silver "eutctic" solder saved just for something like this. Its a bit more brittle than most of my solders when solid.
Cant transmit yet so I have been using my bench multimeter and DE-5000 to measure its equivalant series resistance awith the big clips and my nanovna to try to measure the sharpness of the peak / Q .
This makes me want a new receiver so I can see the effect over a wide bandwidth.
wonder if I could find a good vacuum cap with a wide low to high capacitance ratio commercially, thats not a one of a kind unit and not too expensive.
Thank you!
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I would try to make a omega shape with a bushing/collet/fuse holder type thing for the vacuum capacitor on the bottom with a key that inserts into it to adjust its spacing
I wonder if a flaring tool and some slits could make the pipe into a proper electrical contact, but looking at those capacitors I think it should just clip in on the bottom like a cheap two part PCB fuse holder (the two metal bits you solder on)
I think if it was possible to make a good 'fuse holder' for the capacitor it would be very interesting, if its possible with just a press in fit
https://imagesvc.meredithcorp.io/v3/mm/image?url=https%3A%2F%2Fstatic.onecms.io%2Fwp-content%2Fuploads%2Fsites%2F37%2F2017%2F10%2F16021207%2FSCW_007_03.jpg
none of the solutions I see are quite that elegant though, to have a press in fit, its maybe non viable, it all uses bolts i.e.
https://forums.qrz.com/index.php?threads/mounting-vacuum-variable-capacitors-on-mag-loop-antennas.700414/
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2 inch Copper and aluminum "pipe" is 2 inches internal dia
2 inch copper or aluminum "tube" is 2 inches outside dia.
It's hard to "flair anything without a good tool.
You can however slit the opening of the 2 inch "pipe" to make it easier to insert the VVC. Yhen you can use big hose clamps on the outside to make a tight connections. Some folks use silver paste to make a better connection.You need another hole in the pipe for the tuning rod, this does not have to be isolated since the tuning rod is connected to the end of the cap anyway, however a piece of new fangled PEX pipe can be used to connect the tuning rod to a knob or a tuning motor, to insulate it from the user.
You need to consult the charts to determine the value of your cap. The surplus Russian ones have had good reviews by some folks.
Count on $100 or so for a transmitting VVC. For Rx any good, not corroded air variable cap will do nicely.
Some VVCs come with mounting brackets for connecting. You might get lucky and find one like this. But again, the more connections the worse the performance.
Silver solder is mostly silver, many solders only contain some silver (1-5%)
Anyway. these things are hard to tune. They have sharp tuning and the better they are made, the sharper the tuning.
The more the surface area the more the radiation resistance and better performance. Hence the helically wound loops.
If you have a Vector Network Analyzer, you can look at you antenna's characteristics with just regular solder joints and then with good connections.
The more the turns of this type of antenna the lower the radiation resistance, I think someone did get an award for making a two turn 80 Meter loop.
Wally
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I think the flaring tool would need to be custom made on a lathe
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Harbor Freight sells flaring tools that work somewhat
A muffler shop may be able to flare your ends.
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it also might be an option to use thick copper pipe for the loop then make a braze to a thin copper pipe of same dimensions to make the holder
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Most vacuum variable caps are adjusted simply by turning a rotating part on one of the two ends.
Only a few companies seem to be selling them new, and they are very expensive it seems for what they are.
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Relatively cheap for NOS former Soviet ones on ebay though.
Edit, They must be pretty labour intensive to build though, you've got the large glass to metal seals, metal bellows and screw mechanism acting against a fair amount of static force etc.
Eg. https://www.ebay.co.uk/itm/164070350233 (https://www.ebay.co.uk/itm/164070350233) (1nF 20kV 100A).
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I used a thick copper pipe for my loop.
It worked.
It was very difficult to bend and did kink.
I used a hydraulic bender.
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Relatively cheap for NOS former Soviet ones on ebay though.
Edit, They must be pretty labour intensive to build though, you've got the large glass to metal seals, metal bellows and screw mechanism acting against a fair amount of static force etc.
Eg. https://www.ebay.co.uk/itm/164070350233 (https://www.ebay.co.uk/itm/164070350233) (1nF 20kV 100A).
their main use I think was to determine if we are about to kill each other, so naturally the price drops until another cold war follows ww3 which destroys the advanced technology, then the price will drop when we develop back into current circuits
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Earlier on this thread, I made an error.
My loop is NOT 10 feet dia
It is 10 feet in Circumference, one piece of 10 foot copper pipe.
SORRY SORRY SORRY
Wally