Friday, November 30, 2007

How the antenna works

So you know there's an antenna on that mast. and you know that somehow magically a radio wave comes out of it, do you have any idea how that happens?

An antenna is a little more than just a length of wire. If not cut to the right length, it will still radiate the carrier wave, just not in an efficient manner. Take for example the basic half-wave dipoles antenna: depending on frequency it's length will vary.

Antennas carry alternating current. This means that the antenna will have inductive reactance and resistance. In that dipole, the impedance is maximized at each end and minimized at the center gradually diminishing between the two ends reaching 73.2 Ohms in the center REGARDLESS OF FREQUENCY.

The feeder line connects to this point. This makes the two sides of the dipole behave as two separate quarter wave antennas. Here's where the A.C. ties in: for one half of the alternation, electrons flow to one side, for the other half of the alternation they flow to the other side of the antenna.Voltage will be greatest where impedance is highest (the ends) like any other circuit. The flow of electrons will be highest where the impedance is the least (the center.) No radiation will occurs at the ends.

This is where proper size matters. The electrons don't fly off into space. They rebound and return to the center. This occurs with the proper timing to reinforce the movement of the other electrons moving in that direction.

This produced both an electromagnetic field and an electrostatic field. Of course physicals tells us you cant have one without the other, I just like to point out the obvious. The electrostatic field is 90° out of phase with the electromagnetic field. Together these form a standing wave! Mission accomplished.