How does a dipole antenna work

ASidd

How does it interact with the magnetic and electric fields of the radio waves while receiving them?

mfb

Radio waves are electromagnetic waves. Imagine a plane wave, polarized in some way. If you put a conductor parallel to the direction of the electric field, you get an electric field which changes direction with the frequency of the radio waves (or 2 times this frequency, if you just count direction switches). This electric field leads to an alternating current in the antenna, and therefore the potential at the ends of the antenna change - this can be detected by electronics.

Most radio waves are not plane waves and not polarized nicely, but usually you get some component of radiation which has the correct orientation for the antenna to pick up.

ASidd

This may sound stupid but then what happens to the magnetic component of the waves? Does it also interact with the antenna somehow to create electrical current.

sophiecentaur

A changing magnetic field will induce an emf in a wire (it needn't be a loop).

webberfolds

There are many ways of interpreting that line, please specify which interpretation is the correct one.

mfb

This thread is more than 1 month old.
I cannot see multiple ways to interpret that line, can you show me some?

webberfolds

Well the direction of the electric field is at a right angle to the direction the EM wave would be going. So parallel to that would be under or over the direction of the electric field? I'm pretty sure there's more ways of interpreting that but is that the correct one? I'm not very good at reading in general, but I want to make sure, thanks.

davenn

what light ??

we are talking about radio wave E-M field


Dave

webberfolds

When I wrote light I meant an EM wave, some animals may be able to see radio waves so I included them as light. I changed my previous post, it's more clear now that way.

davenn

try the ARRL Radio Handbook
it gives a very good background to radio transmission and reception including antenna theory


cheers
Dave

mfb

What do you mean by "over" or "under" the electric field? In planar waves (or waves similar to them), the electric field is the same everywhere in the plane perpendicular to the propagation direction of the wave.

webberfolds

So where exactly is parallel? Nevermind about the over and under thing.

webberfolds

Thanks!

mfb

Parallel

As examples:
If the polarization of light is vertical, the electric field is vertical, and your antenna has to be vertical to pick up the signal (at least with some vertical component).
With a circular polarization, the electric field is vertical->horizontal->vertical->..., and your antenna direction does not matter unless it is orthogonal to the direction of propagation of the wave (or has some orthogonal component).

webberfolds

So if the antenna is vertical and the electric field is vertical and the photon moves by the antenna but is a bit too far left to hit it directly, could the magnetic field induce current in the antenna?

davenn

Even with the antenna and the electric field being at 90 degrees to each other there will still be some signal induced into the antenna .... just not very efficiently

In the field, in pratical situations, we see a 25 to 30 dB difference in signal strength when the polarisation is out by 90 deg

Dave

webberfolds

If the polarization of the signal is vertical and the photon meets the vertical antenna straight on, will that generate the most current? I hope this has to do with dipole antennas.