# How to : AM signal into an air coil

#### captoro

Joined Jun 21, 2009
207
hello,

I have an air coil about 70ohm and 2H. I would like to send an Amplitude modulation (AM) signal at 600v.
What type of circuit would I need ? CS ? CG ? should I use a JFET or MOSFET ?
I have some ideas on how to do it but the simulation so far have to given me anything positive to pursue.
Thanks for any of your ideas.

Ken

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#### Audioguru again

Joined Oct 21, 2019
6,776
A radio transmitter needs an antenna. The frequency determines the length of the antenna.
600V x 2A= 1200W. You need many parts to produce 1200W and a very powerful power supply for it.
You need a licence to operate that powerful radio transmitter and the circuit must be certified to not produce interference to other communications.

#### BobTPH

Joined Jun 5, 2013
9,245
I think ypu better go back to the drawing board. At a very low 100KHz, the impedance of your coil is 1.25M. It would require 2.5 MV to put 2 amps through it.

How about telling us what you are trying to do, since you clearly have no idea how to do it.

Bob

#### captoro

Joined Jun 21, 2009
207
I think ypu better go back to the drawing board. At a very low 100KHz, the impedance of your coil is 1.25M. It would require 2.5 MV to put 2 amps through it.

How about telling us what you are trying to do, since you clearly have no idea how to do it.

Bob
I deleted the 2 amp part in my description. But for calculatin the current I used an online calculator for air coil inductance. Using the length of wire and its dimensions I calculate 37ohm to 5kohm depending on the frequency. So using 600v. And the varying frequency The current changes. The 2 amp comes from and average of those frequencies.

#### Papabravo

Joined Feb 24, 2006
21,298
I deleted the 2 amp part in my description. But for calculatin the current I used an online calculator for air coil inductance. Using the length of wire and its dimensions I calculate 37ohm to 5kohm depending on the frequency. So using 600v. And the varying frequency The current changes. The 2 amp comes from and average of those frequencies.
And you still have no idea WTF you are doing. Stop pretending you do and wasting everybody's time.

#### captoro

Joined Jun 21, 2009
207
And you still have no idea WTF you are doing. Stop pretending you do and wasting everybody's time.
You don't need to be rude here. I dont pretend to know everything either. I have an air coil how do I pulse 600v into it.?

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#### Papabravo

Joined Feb 24, 2006
21,298
You don't need to be rude here. I dont pretend to know everything either. I have an air coil how do I pulse 600v into it.?
Actually I do, and excuse me but your requirements appear to be a bit fluid, as if you had not thought things through or maybe you are just pulling numbers and concepts out of thin air. First you want an AM signal and now you want a pulse -- which is it? Why does it matter? What are you trying to do?

You talk about a coil that is 70 ohms and 2H. Is the 70 ohms the DC resistance, the impedance at 100 KHz. Do you even know what impedance is all about? The only thing I know about a 2H coil is, I'm not putting my lips on it. It just isn't safe.

This isn't for a rail gun -- is it?

#### captoro

Joined Jun 21, 2009
207
Actually I do, and excuse me but your requirements appear to be a bit fluid, as if you had not thought things through or maybe you are just pulling numbers and concepts out of thin air. First you want an AM signal and now you want a pulse -- which is it? Why does it matter? What are you trying to do?

You talk about a coil that is 70 ohms and 2H. Is the 70 ohms the DC resistance, the impedance at 100 KHz. Do you even know what impedance is all about? The only thing I know about a 2H coil is, I'm not putting my lips on it. It just isn't safe.

This isn't for a rail gun -- is it?
No rail gun. But I was looking at some circuit and that may work. ..
I want to do research on what's happening inside an air coil. I have multiples sensors. Temp. Humidity .pressure. my control circuit uses a microcontroller increment the frequencies needed. A mix of sine wave and square wave. And a mix of two sine waves. The experiment will last 28 days so it can go though all frequencies. The circuit is finish expect for this last part I am asking advice about.

#### nsaspook

Joined Aug 27, 2009
13,530
What do you expect to see inside an air coil?

#### sparky 1

Joined Nov 3, 2018
783
I am going to put out some logical electrical values that will give some structure to this circuit even if it is wrong it will be a start.
It appears to be a modified Tesla like system without much resonance. It is to explore magnetic and electric fields inside the coil.
The signal could be 20Vpp at 20 mA. Then the voltage is raised by transformer.
The big hollow coil is excited with a custom signal fed directly into one end of the coil.

let's say that an equivalent capacitance between 1 - 60 pF exists and is calculated as a series LC circuit. It is a 2 Henry coil. (plus the self capacitance)
A signal generator is used to find a Voltage peak between 10 - 30 kHz. Find that frequency. Plug the frequency value into the LC equation, solve for C.

The wavelength is not very compatible anyway with this huge coil so I think the length will not be very resonant in a traditional sense.
These things tend to have quirks so actually doing a sweep and finding a peak voltage, note the frequency because that might be what allows it to work.

Most likely the high voltage 600 VAC will make ions. The enamel on the wire is a dielectric.
The coil should act more like a conductive cylinder to some extent. The ions should find pathways with respect to the period of the wave.
The areas at the ends of the cylinder will periodically be magnetic poles.

The various waveforms can be mixed and the composite of those signals can be buffered.
A transformer having a ratio of 1:30 could be made to raise voltage from 20V to 600V

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#### Ian0

Joined Aug 7, 2020
10,226
What frequency is it going to work at?
If it’s <50kHz then the circuit looks like an Audio amplifier.
if it’s >500kHz then it looks like a medium wave radio transmitter.
Why such a high voltage? The same magnetic field could be generated with lower voltage and higher current?
Does the magnetic field have to be sinusoidal? If triangular will do, then driving the coil with a squarewave will achieve that much more easily and much more efficiently; and amplitude modulation can be achieved by varying the pulse width.

#### captoro

Joined Jun 21, 2009
207
I am going to put out some logical electrical values that will give some structure to this circuit even if it is wrong it will be a start.
It appears to be a modified Tesla like system without much resonance. It is to explore magnetic and electric fields inside the coil.
The signal could be 20Vpp at 20 mA. Then the voltage is raised by transformer.
The big hollow coil is excited with a custom signal fed directly into one end of the coil.

let's say that an equivalent capacitance between 1 - 60 pF exists and is calculated as a series LC circuit. It is a 2 Henry coil. (plus the self capacitance)
A signal generator is used to find a Voltage peak between 10 - 30 kHz. Find that frequency. Plug the frequency value into the LC equation, solve for C.

The wavelength is not very compatible anyway with this huge coil so I think the length will not be very resonant in a traditional sense.
These things tend to have quirks so actually doing a sweep and finding a peak voltage, note the frequency because that might be what allows it to work.

Most likely the high voltage 600 VAC will make ions. The enamel on the wire is a dielectric.
The coil should act more like a conductive cylinder to some extent. The ions should find pathways with respect to the period of the wave.
The areas at the ends of the cylinder will periodically be magnetic poles.

The various waveforms can be mixed and the composite of those signals can be buffered.
A transformer having a ratio of 1:30 could be made to raise voltage from 20V to 600V
Thought about using a transformer before it goes into the Coil, but that will modify the waveform. In my square wave I have 15ns rise time, I wont be able to anything near that in a transformer. I am aware I wont be in resonance since I am sweaping frequencies, I am not going to make a coil for every frequencies. I am also aware that at some frequencies there will be losses and the efficiency will be very low. .

#### DickCappels

Joined Aug 21, 2008
10,245
Do you want to use a linear amplifier so your waveforms are reproduced nearly faithfully or would you prefer an amplifier that switches on/off at a fixed duty cycle (50% is popular)? Do you really want amplitude modulated or continuous wave (fixed amplitude)?

If you tell us what you want to do with it you will obtain advice customized for your project.

#### captoro

Joined Jun 21, 2009
207
Thought about using a transformer before it goes into the Coil, but that will modify the waveform. In my square wave I have 15ns rise time, I wont be able to anything near that in a transformer. I am aware I wont be in resonance since I am sweaping frequencies, I am not going to make a coil for every frequencies. I am also aware that at some frequencies there will be losses and the efficiency will be very low. .
I also thought of using a vacuum tube in a common source amplifier

#### captoro

Joined Jun 21, 2009
207
Do you want to use a linear amplifier so your waveforms are reproduced nearly faithfully or would you prefer an amplifier that switches on/off at a fixed duty cycle (50% is popular)? Do you really want amplitude modulated or continuous wave (fixed amplitude)?

If you tell us what you want to do with it you will obtain advice customized for your project.
I have many experience to do. I will focused on the first one. Carrier wave at 3mhz and the modulated wave between 100hz to 500khz. Square wave.

#### Danko

Joined Nov 22, 2017
1,855
What interesting you want to see inside coil at different frequencies?

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#### Papabravo

Joined Feb 24, 2006
21,298
I also thought of using a vacuum tube in a common source amplifier
No such animal. A vacuum tube does not have a "source" terminal. That only applies to a Field Effect Transistor.

#### captoro

Joined Jun 21, 2009
207
No such animal. A vacuum tube does not have a "source" terminal. That only applies to a Field Effect Transistor.
Would it act like a mechanical switch? And put a high power resistor in series to get the voltage.. or put it in common terminal. I'm just throwing ideas.

#### Alec_t

Joined Sep 17, 2013
14,397
In my square wave I have 15ns rise time, I wont be able to anything near that in a transformer.
Nor will you in a 2H coil. If you apply 600V to a 2H coil the coil current will rise at a rate of only about 0.3 microamps per nanosecond.
I'm just throwing ideas.
We can see that.
Are you sure of your coil inductance value? An air coil would normally have an inductance in the microHenry or milliHenry range.

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#### captoro

Joined Jun 21, 2009
207
Nor will you in a 2H coil. If you apply 600V to a 2H coil the coil current will rise at a rate of only about 0.3 microamps per nanosecond.

We can see that.
Are you sure of your coil inductance value? An air coil would normally have an inductance in the microHenry or milliHenry range.
Not finish winding. But its 9inch in diameter and 22 inch high using 23 awg enamel wire. Using 900feet of that 23 awg wire. Single layer construction.