# Electromagnetic radiation production and detection

Discussion in 'The Projects Forum' started by xeroshady, Oct 7, 2012.

Aug 1, 2012
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Hey guys, I'm working on a physics project about the study of electromagnetic waves including their production and detection. I need help with the electronics part.

My questions regarding the production:

1) Will just a tank circuit be enough to produce electromagnetic waves, propagated through an antenna?

2)How do I supply power to charge the capacitor? (where do I connect a voltage source?)
And do I need a constant power supply or provide feedback to the circuit?

3) Where do I connect the antenna?

Regarding detection:

P.S. The circuit diagram probably looks very stupid. Please excuse me.

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2. ### donpetru Active Member

Nov 14, 2008
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1. What is the frequency range of electromagnetic waves emitted by the antenna?
2. Are you fed circuit required to 5V or you can also use another voltage?
3. More specifically, what energy want to charge capacitor? Want to load the energy emitted from the antenna? If so, schedule changes slightly.

3. ### DickCappels Moderator

Aug 21, 2008
2,773
669
Your circuit will work. It will produce a damped sine wave at the resonant frequency of the tank. You don't need an antenna to transmit short distances. This can easily be heard on an AM broadcast receiver (if your tank resonates on that band) as clicks every time you open the button/switch. I suggest a resistor of 330 ohms in series with the 5 volts to limit the current.

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Reminds me of something I came up with a very long time ago. Just leave off the op amp...

• ###### Tank Circuit.GIF
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Aug 1, 2012
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donpetru:
The easiest frequency to transmit and receive, I guess. So, anywhere in the infrared - microwave - radiowave region. The 5V DC source is just arbitrary, can be modified. I read that all you need to do is charge one plate of a capacitor, and then connect it to an inductor, to get sustained oscillations. so I guess I need to supply energy to the capacitor in the form of current through a source.

DickCappels:
What do I need to do to sustain the oscillations? Take off the switch and just make it a wire? or provide feedback to the tank circuit?

Bill_Marsden:
Do I need to include the resistor parallel to the other components (for some reason?), or take that off too?
And should the switch be switched at regular intervals? Is there a way to make the transmission self-sustained?

6. ### wayneh Expert

Sep 9, 2010
12,388
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You might take a look at the AM radio I built for this project. It's very simple and it works, to a point.

For the receiver, you could make the old standard crystal radio set. Or you could use a cheap AM radio and rig a tone detector to light up when your little transmitter sends the right tone. Just depends on the requirements of your project.

7. ### vk6zgo Active Member

Jul 21, 2012
677
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I'm sorry,but Mr Hertz,Mr Marconi,Mr Tesla et al,have done this already!

As it stands,this thing will produce a "damped wave",& not a very good one,at that!

If you were to look at the voltage across C1 with an Oscilloscope,you would see,every time you make the switch,a fairly sharp increase in DC voltage,with at the top edge a burst of RF energy,which rapidly tails off to nothing.
When you "break" the switch contact,you will get something similar.

You cannot produce Continuous Waves in the manner you suggested,you need an amplifying device of some sort,in an Oscillator circuit.

Prior to the introduction of vacuum tubes,there was no way to produce a Continuous Wave,apart from very high speed alternators,which were large & cumbersome,so most Radio Communications used Damped Waves.

These were not a simple "operate a switch & hope for the best" system though,as "Spark Transmitters" were developed to a high degree.

By the way,if your circuit makes any RF,you should hear it at the high frequency end of the AM Broadcast band,around 1.6MHz.

Last edited: Oct 8, 2012

Jul 21, 2012
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9. ### vk6zgo Active Member

Jul 21, 2012
677
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Just correcting my earlier comment:-

"If you were to look at the voltage across C1 with an Oscilloscope,you would see,every time you make the switch,a fairly sharp increase in DC voltage"**,with at the top edge a burst of RF energy,which rapidly tails off to nothing."

**
As the inductor is in series to earth,you won't see the DC increase,but the rest is correct!

I just had a play with an Oscilloscope calibration output,& a similar circuit to yours,with the resulting display being a set of damped waves following each transition,with nothing in between.

I had a go at switching a 9V battery ,but couldn't get fast enough rise times (or repetition rates).

Another problem is,of course,that you are putting a fairly low resistance across a DC supply,so your inductor will get quite warm,so you need a series resistor to prevent excessive current.

The downside of this is that the resistor will increase your risetime.

Bill's change over switch is a much better idea,& will work better,but you will still battle to get the repetition rate fast enough.

Aug 1, 2012
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thanks for all the suggestions guys!

one more question. Is it practical to connect like, 50 capacitors, each of 1 pF or so, and do the same with 50 inductors, maybe 1 nH each, so as to get a higher frequency?
Any ideas as to what the lowest practical values of a single capacitor and inductor are, and the highest achievable frequency, by this method?

11. ### DickCappels Moderator

Aug 21, 2008
2,773
669
No, its not practical to put 1 pf capacitors in parallel -better to just buy the value you need, or if you don't know what value to use but know the range, use a variable capacitor.

Similarly, putting inductors in series will bring with it a lot of stray capacitance and stray inductance, and so is not recommended.

Putting capacitors in parallel increases the total capacitance just as putting inductors in series increases the inductance. Increasing one or both will LOWER the resonant frequency.

To answer your earlier question, the reason the oscillation dies out is because the circuit is continuously using up the energy stored in the tank circuit. To sustain the oscillation, you need to have something that will automatically replace the power lost on each cycle. A transistor is the popular choice. Take a look at circuits for oscillators; they are basically transmitters.

Such an application is the topic of a current discussion on this forum:

By the way, before going off and building a spark transmitter, please take note that except in one special case, the kind of signal that spark transmitters generate (Class B emissions) is prohibited in the United States. If you only connect a small tank circuit to a low power power supply, such as in Bill's circuit, you won't have to worry about creating interference and getting "caught".

12. ### takao21203 Distinguished Member

Apr 28, 2012
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1nH correlates to quite a very short piece of wire. Even 1uH is only a small piece of wire.

Practical circuits would use something like 100uH for a starting point, and at least a few nF. As soon as you go higher than let's say 10 Mhz, you get various stray effects.

I made one particular transistor oscillator (with a LED inlined) which visibly reacted when I touched the antenna coil (open wire end) with a large screwdriver. Just into open air, no capacitance towards anything. Except maybe through the isolated handle but that would be a few pF at best.