Electromagnetic wave?

Thread Starter


Joined Jul 21, 2008
LC circuit ?

if I hook a 6 or 9 volt battery up to an LC circuit

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Where do I hook it to?
Either way I can figure out the reasonating frequency by
1/(2 * pi * sqrt( L * C) ).
So if i want to generate a electromagnetic wave with a given frequency.
I just use the formula. And get the required capacitors/inductor pairs.

But when I hook up the battery. Won't the battery fill the wireing with charge. So the only way to have these electormagnetic waves is to repeatively connect and disconnect the battery?

I want to beable to generate the same electromagnetic waves (frequency)
without having to manual disconnect and reconnect the battery?

Also if I want to make a reciever for it do I just use the same LC circuit but keep the battery constantly on it.

Anyway I am just trying to understand how to build simple reciever / transmitter.



Joined Apr 20, 2004
Well, the original radio transmitters used a spark gap transmitter with an LC circuit to grab energy for the antenna. But the spark radiates all over the place, so only a tiny fraction of the spark energy actually gets to the antenna.

The receiver for a spark radio used a device called a "coherer" that might be hard to find. The operator had to look for a tiny spark through a microscope. And that was with a transmitted signal of hundreds of watts. The only way to see a signal on a tuned LC ircuit would be an oscilloscope - or go further and make a crystal radio receiver.

You just can't get enough out of a 6 or 9 volt source to make a spark, much less radiate it out of an antenna. But you can put together a little transmitter like this one - http://english.cxem.net/radiomic/bug3.php.

Thread Starter


Joined Jul 21, 2008
Yes I will try this.

I just have a few questions why cann't you just do it using one LC circuit and a mic.

mic --------------|----------- antenna

Say I want a frequency of just 103 MHZ
If my calculations are correct all I would need is a C = 470 pf
and a L = 5.13 nH

Does the battey matter where I place it in the circuit.

Maybe the reason why they use many resistor's, capacitors , inductors ,..ect instead of just one is that you cann't just get L = 5.13 nH by looping coil's.

I never can follow why you need so many circuit elements? I wish I could follow why you need this?


Joined Apr 20, 2004
Radio is a transfer of energy from the transmitter to the receiver. Circuits on both ends are tuned to resonate for maximal energy transfer. The inverse square law says you need a fair amount of power at the transmitter - although dedicated types have communicated by bouncing the signal off the moon using only 1 watt out.

Question - what part of your circuit is oscillating to produce the carrier? The closest setup to yours was a voice transmitter someone made back in the 20's. He had a 50 KHz alternator feed an antenna with a water cooled carbon microphone in between for direct AM modulation.

You might find some references on the subject interesting - http://en.wikipedia.org/wiki/Radio.

Thread Starter


Joined Jul 21, 2008
My problem is I understand the theory (i.e the physic's )

But building these things is a different ball game.

I just need to know.

Why do you have to go and use all these different circuit elements.
Why cann't you just do the math get a capacitor and coil a wire enough times to get the correct frequency. Then just add a mic and an antenna to the loop.

Really all electromagnetic waves are is just an acceleration of charged particles. So is the antenna even essential. If so I don't understand what the length depends on for optimal antenna. Would it be the size of the wave you are trying to propogate. What about the different shapes ?

Anyway I would real want to understand how somebody came up with

Do you need the battery for the reciever end.
Because the (electrical )wave is going to be send down the antenna

I am sorry for sounding stupid I just don't understand why we need some of the things in this circuit and why we need a carrier wave. I thought the carrier wave would be the wave your trying to transmit.

Thanks again


Joined Dec 27, 2007
My problem is I understand the theory (i.e the physic's )
One of the boldest statements I have ever heard on this forum. EM Physics is a very difficult subject!

The problem with your understanding, I believe, is that you do not seem to understand that there are losses in the inductor and capacitor in the LC tank. So, when energy is put into the tank, the L and C will create an oscillation which will exponentially decay to zero. This is why oscillators typically create a negative resistance, which is equal to the losses, thus sustaining an oscillation.


Thread Starter


Joined Jul 21, 2008
Ok is that

R4 100 without any units ohms ? (I for get the standard omitted)

C5 1-30p trimmer is this p (pico farada's)

The bottom pictures are a little confusing should it work if I duplicate the top


Joined Oct 22, 2008
You're asking good questions but even a short explanation of that circuit would take several pages. A full explanation would amount to an entire course in electronics.

In short, for every part in that circuit, there is an explanation for why it is needed and why that particular value was selected, and what the tradeoffs would be if the part was changed in value.

Why don't you get the parts and make it? Use an alligator clip for an antenna. See if you can receive it on an existing FM radio.

R4 is 100 ohms. C5 is in picofarads - you can make a capacitor this size by twisting a few inches of insulated wire together. (Old-time radio guys used to call these gimmick capacitors.)

Thread Starter


Joined Jul 21, 2008
Ok I went to get the parts but I can't get some of them.
They don't have them and I don't want to buy online.

I have all the resistor's.

I have capacitors
radioshack 272-802 pkg of 20
radioshack 272-801 pkg of 100 (ceramic disc but don't know how to read them. What is the 10 underline mean is that 10 henries)

For Transistors
radioshack 276-2016 (2N3904 NPN)
radioshack 276-1617 pkg of 15

radioshack 270-090 (condenser mic) all they had in store

Batteries are pretty easy to get I think I have 1 , 6 ,9 volt .

Other stuff I have is 276-1715 , and 276-1731

I have a few different size perf boards , a 30 watt general purpose solder gun, some wire striper's that can strip almost all gages of wire I came across, a few different color LED's , and alot of different gage wire I think I got 30 gage down to 15 or 16 gage.

Their should be some type of transmitter/recevier pair I can build?

Also I have a few question about some of the measurements

For a capacitor it lists the voltage why is this? Is this like some max voltage that the capacitor can operate at before not storing a charge.

For some components it list power dissipation why is this important.

For my condensor microphone it gives Electrical characteristics
supply voltage pretty obvious what this is for.
nominal supply 4.5 VDC <--direct current but is nominal just average
current drained 0.3mA don't full understand this won't the current be effected by the voltage or resistance how is it constant?
signal/noise 60db (min) I kind of get why this is good to know. And have done alot of stuff with sound and db unitless measurement.
sensitivity -64 +- 2db don't really get it other then the fact it fluctuates at most 4 db.

Output Impedance 1 k ohm don't real understand impedance's purpose. Other then it is like the equivalent of resistant's but now your using AC as opposed to DC.

Some times I even see differential input voltage and input voltage what is the difference and why is differential important?

They also have input offset voltage, output current sink/source ?

I have read alot of thing's even done Finding I , V ..etc in basic circuit
using kirsicoff's current and voltage formulas. Setup matrices to solve more complex ones as well. I read about some of the components and the differences such as NPN as opposed to PNP transistors...etc

The list goes on but I just still don't know how somebody cam up with why it works and why it is the best or only way to do it?

I am more about understanding it enough so I can make my own designs and create new circuits. I would think other then learning to solder
(which I am getting better at) it is fairly easy to make it once you have the design in front of you. That is just copying. The real hard part is being able to create the design that works.

Either way I want to first get practice in making these things then learn the why (for right now)

Thanks for any help and sorry for the miss spells

O I do have some diodes and rectifers (not the germanium diode they don't have it in stock any more)

If you make a transmiter could you just copy the transmitter and take the mic out and put in a speaker instead.
Because they would both have the same resonanting frequency. So once you make one is the other just trivial to make.
(i.e just copy it over again)???

Hope I gave enough info.
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Joined Apr 20, 2004
I think that as some time and experimenting goes on, you may come to appreciate the difference between theory and practice. RF circuits are particularly good at bring the differences home.

There are a lot of ways to get into electronics. One is to build kits or predesigned circuits and then find out how and why they work. Another is to read up on the basic theory so you can start to understand why a certain circuit got built that way.

For more understanding of what you are kind of trying to do, look up a copy of "Elements of Radio Communication", by Ellery W. Stone. My third edition is dated 1926. It covers spark gap theory and practice quite thoroughly. It will also give you an appreciation of why people got interested in vacuum tubes and oscillators.

I sometimes think of a very old Tom Swift novel ("Tom Swift and the Giant Searchlight") in which the crucial discovery is made by accident. His sort of servant/sidekick (did I mention this novel and others in the series were just a tad racist?) accidently knocks loose some connections to the eponimous searchlight somewhere in the dirigible. In somewhat of a panic, he throws some wires back on the panel, and the searchlight gets much brighter. This is very handy, as a girl gets rescued, and old Tom gets credit for the "invention".

In reality, not many people make significant advances in the state of the art without doing some homework. That basic knowledge makes a solid framework to build on. I would further suggest that our Ebook would be a great place to start reading in order to become proficient in electronic theory (significantly different from Dr. Maxwell's equations, a sort of unified theory).

This may sound as if I am trying to duck your questions, but there are better ways to get an education than by simply asking questions (sorry Montessori). That is why a number of people went to the trouble of writing the Ebook - to make it easier to learn electronics. Books are excellent teaching tools. It will certainly answer most, if not all, of your questions.

Thread Starter


Joined Jul 21, 2008
That is why a number of people went to the trouble of writing the Ebook - to make it easier to learn electronics. Books are excellent teaching tools. It will certainly answer most, if not all, of your questions.
I have read most of this book but that is the thing. I can duplicate a circuit if given a diagram, I can understand LC circuits RLC , RC...etc

But I just don't know why you cann't use one L and one C of the correct size to get a given frequency?

I am lost in when some thing is insignificant and when something is important.

Like you said in an LC circuit the osillation is dampered out exponentional fast.

I always keep reading stuff what is the best way to start understanding this stuff? (The building of circuits not the theory?)


Joined May 16, 2005
A resonant tank is not an oscillator. It is only a part of an oscillator. The tank by itself is analogous to a bell with no clapper. No energy in means no energy out. The wrong energy in means only heat out.