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The Physics of Resonance

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How Antennas Work

We can't see them but radio and television waves are just another form of light. They have a much longer wavelength than visible light but both are electromagnetic radiation.

To generate radio and TV waves we typically make electrons oscillate up and down on an antenna. This is done by applying a variable voltage or alternating current to the antenna. Antennas are generally made of metals and metals act like containers filled with a liquid made of electrons. Metal atoms have one or more weakly held electrons in their outer shells which can "float" from atom to atom.

         
Figure 1. Electric Field Around a Positive Charge
  When a negatively charged electron moves it leaves behind what is generally referred to as a positively charged hole. The hole is simply an atom with more positive protons than negative electrons.

The electrical fields for the two types of charges are shown in Figures 1and 2. These are ray diagrams. The arrows show the direction of the force that would be exerted on a unit of positive charge.

Unlike a vector diagram, the length of a ray does not indicate the magnitude of the force. Instead, the space between rays indicates magnitude. Both diagrams in Figures 1 and 2 show that the magnitude of the field decreases with increasing distance from the charge because the space between the rays increases.

 
Figure 2. Electric Field Around a Negative Charge
         

We can use a simple analogy to help understand how electromagnetic waves are produced by moving charges. Imagine for a minute that the rays or electric field lines shown in Figures 1 and 2 are like very long springs attached to a circular frame with the charge at the center, almost like a trampoline. If the charge is bounced up and down waves will propagate outward along the springs. Yes, the world of electromagnetic radiation is far more complex than our simple analogy but hopefully it gives you some idea of how a moving charge could create a wave.

The waves and variation in the electric field account for the "electro" part of the term electromagnetic waves.

A moving charge is essentially a current and currents create circular magnetic fields. In Figure 3, a positive charge moving straight out of the page would produce a magnetic field represented by the blue dashed line. The direction of the field can be determined using the right hand thumb rule. The thumb is pointed in the direction of the current and the fingers of the right hand wrapped into a loose fist. The fingers point in the direction of the magnetic field.

Note that the magnetic field lines are perpendicular to the electric field lines. This is one of the famous characteristics of electromagnetic waves.

 

Figure 3. Magnetic Field (shown in blue) Created by a positive Charge Moving Straight out of the Plane of the Page

     

Okay, you're probably wondering why we use an example of a positive charge when we just got finished saying that it's the electrons which move. It turns out that all the conventions in electricity and magnetism are set up for positive charges. Much of this can be traced back to the work of Benjamin Franklin. Unfortunately, the electron had not even been discovered in Franklin's time.

When we talk about current we pretend the positive holes are actually moving in the opposite direction as the electrons. It may seem pretty silly but it does work as a concept and so we're sticking with the tradition.

If a variable voltage is applied, it will send an electrical wave up an antenna. Free electrons in the antenna act as the media for propagating the wave. The situation is similar to longitudinal sound waves propagated in a metal rod. The sound wave is carried by alternating regions of tension and compression. In the compressed areas the rod's molecules are pushed a little closer together. In the tension areas they are pulled a little further apart. Although the molecules barely move, the sound wave can be transmitted great distances.

The very slight motion of electrons up and down an antenna is enough to cause electromagnetic waves to radiate out the sides of the antenna at the same frequency as the variable voltage applied to it. These are used for transmitting radio and television signals as well as other forms of wireless communication.

Like sound, when electrical waves at a defined frequency hit the end of an antenna they are reflected backwards and form a standing wave in the antenna. Antenna waves move at the speed of light (3 x 10 8 m/s) and so the travel time from one end of the antenna to the other is pretty quick.

The electrical waves created on antennas typically have a fixed wavelength. If the length of the antenna is wisely chosen it's possible to make it resonate. The free end of an antenna acts like an open circuit. Voltage drop is maximum across an open circuit and zero across a short circuit. Hence the end of an antenna forms an anti-node or area of maximum voltage or e-field strength. A node is a point which has zero e-field. The distance between an anti-node and node is a quarter of a wavelength.

The wavelength of an electromagnetic wave is calculated as follows:

  l =
 f
       
  Where
    l = wavelength
    C = speed of light (3 x 108 m/s)
    f = frequency
     
Figure 4 shows a dipole antenna which is generally considered the simplest form of antenna. In this case each half of the antenna is roughly 1/4 wavelength long with the antenna fed from its center. Hence, the total antenna is 1/2 wavelength long. The ends of the antenna correspond to anti-nodes and the center to nodes. This configuration causes the antenna to resonate.

An antenna will still transmit even if the length is not ideal for resonance. However, less of the power input to the transmitter will actually show up as useful output signal. In other words, the efficiency of the system will be significantly lower.

 
Figure 4. Dipole antenna
     

Dipole antennas are considered balance devices because they are symmetrical and work best when they are fed with a balanced current. In other words, the current has to be of equal size on both halves. This is usually accomplished with a balun when the antenna is fed with a coaxial cable. Coaxial cable is considered unbalanced, hence the word balun is formed from parts of the words BALanced and UNbalanced. A balun is basically a small transformer.

The optimum size of a dipole antenna is slightly different than would be expected based on wavelength alone. This is due to the interaction of the balun and antenna. However, the predicted resonance length is usually very close to the  length for optimum broadcast efficiency.

Electromagnetic waves emitted from an antenna are generally modeled as transverse waves. Since the waves have both electric and magnetic field components and are emitted in three dimensional space, the transverse wave model drawn in text books is a bit over simplified but the full picture is almost impossible to draw.

Waves emitted from simple monopole and dipole antennas tend to be polarized. In other words, if the emitting antenna is vertical the receiving antenna also has to be vertical for best reception. If the receiving antenna is horizontal the signal it picks up will be greatly attenuated.

Antenna design is very complex and requires a lot of time and study to master. However, any antenna will have to oscillate charged particles in order to transmit radio signals and will tend to do this best if the antenna is resonating.

 

For more information about wireless communication and the electromagnetic spectrum visit The Hidden World of the Electromagnetic Spectrum.

 

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