Can a solenoid emit/receive electromagnetic radiation?

[Lojzek]

1. Can a solenoid connected to alternating current generator be used as transmitter of electromagnetic radiation?

2. Can a solenoid be used as a receiver of electromagnetic waves?

I think point 2. is possible, since alternating magnetic field will induce voltage in the loops of solenoid.
An alternating current in a solenoid would generate an oscillation of magnetic dipole moment. I guess this would emit electromagnetic radiation based on the similarity between electric and magnetic field (since an oscillating electric dipole moment does emit electromagnetic radiation).
Are this considerations correct? Also why transmitters/receivers (for example radio antennas) never have the form of a solenoid?

 

[yungman]

I think it is possible.

As a transmitter, the magnetic field just emmit from the two end. As a reciever, then it is very directional, only the magnetic field going through the coil can cause current in the coil as per Ampere's law. Are they effiecent? I doubted.

In time verying field, E and M are together, one generate the other.

You seen the old am fm radio( maybe not so old ), they have coil with rod in the middle as antenna? As a kid, I torn up a lot of them and unwinded the coil on the antenna! Yes, that's how I started my EE career that lasted over 27 years.

Remember the famous loop antenna is a single turn coil!

 

[mohammed.omar]

Hello Lozjek,

There is a type of antennas called 'Helical Antennas' that resemble a solenoid more or less. They can be used in two main modes of radiation, the end-side and backfire mode.

You can find plenty of information on them in any Antenna Design books, check out Antenna Theory by Constantine Balanis for example.

Regards,
Mohammed Omar

 

[Bob S]

There are also magnetic loop antennas. See

http://en.wikipedia.org/wiki/Magnetic_loop

Bob S

 

[K^2]

Ideal solenoid has no external field, but you get these from the same box as massless string and frictionless pulleys. Real solenoid produces external field from the edges. If you run alternating current through it, that gives you alternating magnetic field. And whenever you have an alternating magnetic field, you are emitting RF.

 

[Lojzek]

Thanks for the replies.

I have another question:

Is it obvious that any object which causes an oscillating
electromagnetic field in it's neighbourhood will emit
electromagnetic power? Or does such object exist which has a
zero time average of emitted power?

For example, to prove the effectiveness of a transmitter with a
form of a solenoid (or a singe loop), is it necessary to prove
that the integral of energy flux over a large sphere (averaged
over time) would not converge to zero?

 

[mohammed.omar]

Hello Lozjek,

Any time varying electromagnetic field, i.e. one that has both time varying electric and magnetic field will cause electromagnetic radiation.

Thus, any oscillating electromagnetic field will ultimately cause radiation.

What differs in one source from the other is their radiation efficiency, i.e. the amount of real power radiated by the source. Let me try to explain:
If you look at the Poynting vector, any electromagnetic radiation can be considered to be composed of real and reactive power. What you make use in the form of radiation is the real power. Its ratio to the excitation used, which controls the radiation efficiency of any antenna, depends on many factors which include the shape of the antenna.

If the time average of the energy flux increases, the real power increases, and as it decreases then the real power will decrease.

Please note that this integral will depend on the distance from the antenna, there are two main regions, the near field which is mostly reactive power, and the far field which is mostlr real power.

Regards,
Mohammed Omar