Radio transmission

When a coil is energised using an AC voltage, its magnetic field is detectable through a small distance only. Its intensity drops rapidly. What causes an aerial to transmit waves through large distances. Is it the capacitor in parallel with the coil?

Please clarify.

Actually there are two components of electromagnetic transmissions.

One : Induction or Near field Component.

This is exactly the field you are talking about. Its strength decreases very rapidly. The maximum distance it could travel is 0.16 times the wavelength of the signal.
Now you ask how could it still progress ,the answer is the other component.

Two : The radiation or far field component.
It is found that a far field component exists which falls comparitively very slow with the distance. In transmission thhrough a aerial this field is utilized.
As far as Capaciator is concerned, its not used for transmission but only for supporting circuits like genreation of signal,resonance. Actual transmission occurs via Inductor or a simple antenna (which is nothing but a wire in a defined shape ).

Please refer any book on Antenna and Wave propagation for further details.
One good book is Antenna and Wave prop. By : Krauss.

I once energised a coil using an AC signal. Then I brought an AM radio near to it.
The radio gave the signal at its output as the signal lied in the audible frequency range. When I tuned the radio, nothing happened to the loudness of the sound.

Does it mean that the NEAR effect has a band enough broad to affect the radio at all tuning levels.

Can you please provide me with a link so that I may download that book.

Regards.

Sorry , but i am not clear with your problem.
Please eloborate more.
Like which signal you gave and which band AM reciever you used and how you proceeded.

All i can say u this time is tuning and loudness of sound is not actually related.
If you know what is tuning basically,
Its frequency selection phenomenon, a capacitor or resistor is varied to match with varous incoming signals.
Loudness would relate to the power (amplitude ).

I can explain more if you can explain more

And about that book, actually i have the hard copy of that and i need to scan that for uploading.
Also it voilates the rules of our forum( no free distribution copyright )
Hence i can't upload the book here.
But i can scan and send you fundamental chapters personally.
But unfortunately i'll require time for that as i am currently busy with my projects.

Bye

I was using normal AM radio (500 to 1500 kHz). The coil was energised using a small circuit which produced a small musical sequence. I want to say that the radio could not select any frequency. It seemed that all of the bandwidth was effected by the music (550 to 1500).
How?

I think because of the strength of the field generated by the coil and distance betwee the coil and radio, this signal is inducted inside the radio on every track, wire etc and is just amplified by the audio amplifier .. so tunning the input stages doesn't really do anything ..