I live in an apartment, on the second floor. I am finally finishing up a tesla coil, but I don't have an rf ground. Would placing a ground plane of wire mesh work? How big should it be? Would say,
4x4ft suffice? The coil is appx 3ft tall, 7.5kV NST, 17.5in tall secondary, really not too big.
Also, I want to build a faraday cage for the coil. It isn't necessarily a large coil, but I don't want to cause any interference for anyone. Is aluminum screen acceptable for a cage?
Also, I have a standard box power filter, mfg: corcom, model: 20VK6. To keep rfi from entering back into the power line, do I hook the filter up normally, with line going to line and load going to load, or backward, with line to load and load to line?
When you do it right, your Tesla coil resonates like a quarter wave resonator. That one needs a ground (the same way as a mono pole antenna needs a ground or counterpoise).
The second reason for a ground is to reduce RFI. Unless you make a faraday cage, you cannot avoid RFI completely. I don't know whether you are making a pulsed or CW Tesla Coil, but the power involved is that high that some interference may occur.
You can enhance the effect of the ground (and reducing probability of RFI) by adding many wires to your ground surface and spread them out (radially) in your apartment (in fact you extend the ground with the wires). You can even run them upwards (recommended). When you setup the coil in a corner, you can use the walls to hold the vertical part of the wires. In fact you make a faraday cage without a roof.
Regarding the filter, the enemy will be so called common mode noise. The best thing is to place any filtering for signal and power lines at that point where you have maximum metal or mesh surface (so the mesh and radial wires are in between the coil and cabling). So all decoupling will be done with respect to that point.
I would install the Telsa setup (coil with ground plane) of the ground and decouple all cabling (power supply, control, etc) with respect to the ground mesh. All cables (inclusive the safety ground) run beneath the ground plane (with wire extension).
When it is not possible to build the Tesla coil off the ground, I would extend the mesh to at least one side and let it go upwards (for about 6 ft). The lower corner I would use as a reference for filtering (AC decoupling capacitors), entrance of cables and safety ground connection (PE wire).
To further reduce RFI I would provide large ferrite cores to all cables (control, power, safety ground) that leave the Tesla coil setup (do not put ferrites around the radial ground extension wires.
I hope this will help you a bit. When you feel something is not clear (sorry, I have no image), you may contact me.
Sorry for not specifying the type of coil. It is a standard spark gap coil, with the basic components: hv transformer, capacitor, spark gap, primary, secondary, topload.
I think I understand what you mean. What size capacitors should I use? Based on my calculations, it should oscillate somewhere in the
500kHz region. Just to make sure, decoupling capacitors go in series with the circuit, correct)? I had planned on my control wire to just be a switch between mains and the neon sign transformer.
Also, my transformer states the center tap is connected to transformer case and ground stud. I have read to connect RF ground to the center of the spark gap as well as the center tap of the transformer. Should I do this, and just leave AC ground off the transformer, and will this cause any shock danger, being that the RF ground isn't connected to earth and could be floating?
You really should have a good earth ground. Is that out of the question? House wiring is more like an antenna then RF ground - it snakes around forever before going to ground at the entry point power panel (meter). I sunk a 10 foot copper pipe outside the wall where my TC sat with a 5 foot heavy gauge wire to it.
Most TC's of the size you are talking about would work at a lower frequency like 100 KHZ, but that is all a matter of how many turns of wire etc.. - the faraday shield/cage would also increase the capacity and lower the frequency.
Put in a safety gap at the input to the transformer. And a couple of ferrite chokes with Teflon or HV insulation on the wire for the chokes in each leg of the HV - assuming the NST is center tap grounded.
A safety gap prevents reflected RF from getting into the transformer and back into the power lines. There will be SWR problems until the coil is in tune . . . AND of course a good power line filter.
Consider liability - you aren't just talking about TVI, you could set fire to the apartment, computers on the same power line or in close proximity to the TC may die - my TC's could eat DRAM chips and reset the CMOS settings on my computer 6 feet away from a 1 KVA TC. (unplugging the computer AND modem AND all peripherals from the power line would keep it safe) Other sensitive equipment in the vicinity may be ruined (or not) my speakers took some direct hits to the wood cases and the mosfet amp didn't suffer. You probably don't want neighbor's with pacemakers around.
The pupman forum is an excellent source for all kinds of TC/HV questions. It is operated as a mailing list - you can use your email account and regular reader or use a web-based email account (recommended). You have to join the list to post but can read anything in the archives without joining.
You download the list mailing via a regular news reader or web based email. Read some of the archives before posting so you have a feel for how things are done.
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If these folks don't know it, no one does.
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Thanks to everyone for the replies. I will have to look at the forum.
I can't put in a good earth gound because I'm on the third floor, and the apt won't let me run any wires outside etc... so all I have is the mains ground, which I'm sure is probably shared with a few units.
So, that's why I was trying to find out if just the Faraday cage and ground plane would be sufficient. If it's iffy, and I could hurt someone elses equipment, I'll just wait until I get a house. I'm not out to make anyone elses life difficult.
I would think about finding somewhere other than an apartment to run the coil, as it is quite likely to upset the neighbours, and damage their appliances etc. Do you know anyone who lives on a farm by any chance? If not, the Faraday cage is a good start. If the coil is inside the cage, and the ground of the coil is connected to the bottom of the cage, then the thing you need to take care of is any wires that go in or out of the Faraday cage. At the point where any wire goes through the cage, it needs to be connected RF-wise to the cage. It doesn't matter whether the cage is connected to the real ground two stories below, but the cage should be connected to the ground of your power cable, and the live and neutral wires should go into the cage by way of a filter, the ground casing of the filter being connected directly to the cage. You will need a very good filter. The most important thing about the Faraday cage is that any joins in the mesh need to be really well connected, and all the way along, not just in one place. It is unlikely that you can get a really good connection between pieces of aluminium mesh if it is anodised or painted - you would have to get that coating off and it would be difficult. If you can get hold of some bare metal mesh then that would be ideal, maybe chicken wire (galvanised iron wire with about half inch holes) might be OK if you could solder the joins.
If you go for the faraday cage, try to find wire screen that is hot-dipped after it is woven - like chicken fence or "hardware cloth." Hot-dipping in zinc insures a connection at the joints between wires.
A clean wire brush in an electric drill will buff the zinc so it will take solder - then you just need something like a large soldering iron and some sheet metal to make mounting points for the cage filters.
A few of the pupman list subscribers are in the business of building and displaying large TC's they build cages for them to use inside shopping malls and museums with temporary facilities and probably no earth grounds Richard Hull (one list subscriber) had some of his coils shown in National Geographic years ago (it may be July 93) the article was about lightening and they sensationalized it but the pictures were worth the price. Richard has moved on to "Farnsworth Fusers" but may still put in a word from time to time. He was working in the class of devices that used "pole pigs" (pole mounted distribution transformers) reverse connected to produce the excitation sparks for his coils.
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I think you're right. I may just wait until I have a house. I don't want to risk damaging someone elses equipment, that wouldn't be very responsible of me.
So, if the cage itself is connected to power ground, which in my case is AC ground, as long as all wires going in and out are filtered properly, there should not be any RF present on the ground wire?
Just to clarify, how do I properly decouple the wires going through the cage? I'm used to seeing feedthrough capacitors in RF equipment, something along this principle? I'm having some trouble figuring out which ground is acceptable to connect where, but I will do my research, and likely put this project on hold until I have a more acceptable location.
Even a small unit like a Violet Ray may toast all of your electronics - and your neighbors' as well. You need a rock solid filter for the AC line alone - and good insurance.
Yes feed through filters are good for this - cap and inductors in one device. The NEC power cord filters one sees on computers and such may work well. Switching power supplies are up in the TC coil range these days, so those types of filters may work.
A device used in auto radios of old (50's) was something called a "spark plate" (had nothing to do with sparks). It was a piece of mica (high dielectric constant, low loss) on the outside of the chassis often with a rivet or bolt carrying the battery connection to the outside world, and a square piece of copper. Only a few pico farads but very effective for high frequency noise. The chassis formed one side of the capacitor and the copper the other (somewhat harder to make safe with mains voltage).
I assume it got the name "spark plate" from auto mechanics that accidentally grounded the hot copper piece exposed on the outside of the chassis.
I've used that idea to decouple gunn diodes used in radar jammers. Solder a nut to the thin pcb material and it decouples the power and provides an isolated mount for the diode. (gunn diodes won't oscillate if the voltage lead has inductance).
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So, if I decide to change gears completely, lose the Faraday cage and ground plane altogether and put in a proper RF ground, I assume the same filtering still applies to the mains wiring to keep RF out of the
115V supply, it just won't be connected to a cage? I will probably put this project on hold until I have the proper facilities, at which point I'll do the proper research and figure out how to do it right.
Thanks to everyone for their help, I think this will wait until a later day.
You need an RF-ground, because you are using a quarter wave resonator. When you don't make a construction that serves as RF-ground, all return current will flow through the power cable (so the power cable and everything connected to it serves as RF-ground). This is undesirable.
The reason for the faraday cage or large ground plane is to force the E-field lines into your artificially created ground, so the induced current can return to the base of the coil via a known safe path. In other words the cage or ground plane is the RF-ground. For safety reasons you connect your RF-ground to PE. When you did de construction correctly, there will be little RF current in the wire that is between RF-ground and PE.
Off course, when you have a large terrain available, you can use mother earth as an RF-ground (in same way as medium wave broadcast transmitters).
Yes, I received your mail. Thanks for the picture, it was very clear. I tried sending you a reply using two different email accounts, both undeliverable. Thanks for all your help, Steve
You would still need the cage with any scheme where the coil is operated in your apartment, both for radiated interference and to stop an arc from just directly hitting your light fittings or wires in you walls - that would cause some conducted interference!
Good point. Just to clarify, by proper facilities, I meant moving into a house where it's just my wife and I, but I may still do the cage so I have no worries. I am a paranoid person by nature.
Your main danger is the primary circuit - it is easy to mistake a gap that has stopped firing for one that is turned off, check, double check, and put the plug in your pocket while making adjustments with one hand only. I good bright rotating beacon wouldn't be a bad idea.
Folks that use distribution transformers will add a time delay and horn sounding before the mains voltage can be connected. Switches have be operated from a safe distance, and in sequence.
Make sure others don't try to work the equipment when you aren't there.
Make sure the tank cap has a way to discharge and know how long it takes to reach a safe voltage. (normally only a second or two by discharging through the NST) - but all it takes is a loose connection to keep it charged, so use a shorting bar when you get ready to get both hands in it.
Use ventilation - these things produce ozone like you won't believe. Situate it so that an open window can exhaust the ozone.
Put a lock on it if you have children, or children will be present.
Warning signs aren't a bad idea
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It seems if you use a counterpoise or whatever you want to call an RF ground plane on the 3rd floor, you will reduce lossy current flow through the floor, etc., and maybe reduce current flow through other wiring.
But the effect of doing that is to increase the efficiency with which the resonator radiates. That will probably decrease its Q through loading. But is radiation what you're trying to achieve? I would think that the last thing one wants to do is efficiently radiate RF energy into everything in the far-field as well as the near-field! One can have a resonant LC circuit without significant radiation.
Does that make sense?
I concur with suggestions to use filters on the AC wiring. And the Faraday cage may provide benefits as well.
Chuck NT3G
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Hi Chuck,
As all (most) coilers want to get the sparks at the top of the coil (and not at the bottom), you need an electrically quarter wave circuit (or LC series circuit where the top capacity + coil capacity forms the capacitor). Another option may be a center fed (dipole) structure. The problem with these is that air-breakthrough is not that symmetrical, so when the air on the left side breaks first, you get strong asymmetry that may cause a breakthrough from center to primary feeding coil.
The quarter wave monopole has low impedance (in particular when it is a short one [3ft at 500 kHz). All quarter wave circuits (whether shorted or not) require a ground or something to pull out the drive current for the coil.
When you make it large enough (or even extend the wire mesh on the walls), you provide a return path for the displacement current (and in the end a return path for streamers).
With respect to radiation. You are right, some energy will leave as radiation, but it will be a very small fraction of the input power. As le/lambda = (about)
0.0017, the radiation efficiency will be very low (compare it with a
0.13m monopole antenna at 80m). Over an infinite ground plane the radiation resistance will be about 1m Ohm. Assuming about 30 ohms of AC resistance of the coil, the radiation efficiency will be 0.004%. When someone adds a large top capacitor, the radiation resistance will increase, but with factor 4 maximum.
As soon as air-breakdown occurs, all excess power will be converted into heat. I think, the Q-factor is not limited by radiation losses, but by conduction losses and at higher power levels Q-factor will reduce due to air-breakdown. The same is valid for small loop antenna's, Q is limited by heat losses, to much input power causes the tuning capacitor to arc.
You can reduce the far field radiation (and near field to the adjacent apartment) by bending the ground plane upwards (on the walls of the apartment). When you extend it higher then the top of the coil, you made a coaxial quarter wave resonator (with low near field and far field radiation).
So in my opinion providing a defined "return path" for the bottom feed current of the coil will result in maximum power in the ionized air. When you also want to minimize near field and far field radiation, you should construct a faraday cage (or similar structure with wires).
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