Representing EM Radiation

Learning Objectives

• Understand how a radio transmits and receives EM signals.
• Ability to interpret EM waves propagation using four types of displays: wave display, phase display, ray traces and signal strength displays.
• Be able to describe the advantages and disadvantages to each of these display methods.
  

Radio Animation

Click on this radio. This will open a new window with a FLASH animation that demonstrates the principles we will discuss below. Keep the new window open while you read the rest of this page. You may need to rearrange the windows to see everything.

Radio

Transmitter
Figure 1 is a simplified representation of how radios work. Sound waves from the women's voice on the left are converted to electric signals by the microphone. A modulator converts the relatively slow sound wave frequency of the electrons into much higher frequency EM waves that carry the sound signal by small changes in frequency (FM) or changes in amplitude (AM). The amplifier boosts the voltage to a transmission level.

Antenna
The electronic pulses travel to the transmitting antenna where electrons move back in forth in response the varying voltage. These accelerating electrons generate EM radiation. This simple rod antenna is an omni-directional. This type of antenna causes radiation to be transmitted in all horizontal directions, but above and below the horizontal plane bisected by the antenna the strength of the radiation signal decreases. Other types of antennas will create different radiation patterns. For example most radars produces relatively narrow beams. If there is nothing that interacts with the waves, for example in empty outer space, the waves will decrease in strength away from the transmitting antenna. The strength of the EM signal is 1/r² where r is the distance from the transmitter. This normal decrease in power away from the antenna is called r-squared loss.

Receiver
Electrons in the receiving antenna move up and down in response to the EM radiation and generate a small voltage which drives an electric current. This voltage is amplified and demodulated into a voltage that varies at the original sound frequencies. The current produced then drives a speaker that produces sound waves that reproduce the women's original words and can be heard by the man.

Representing Radio Waves

We can't see most EM waves, but we want to be able to represent them graphically in order to demonstrate how they propagate in the atmosphere. In this class we will use four display methods, all of which are shown in Figure 1.

1. Wave Display
   We've already used this type of display to show how the electric and magnetic fields are related to each other. This display shows wiggly lines that represent the EM waves. The distance from the peak of the waves to the valley of the waves is called the amplitude. Sometimes the amplitude is used to display the EM signal strength, i.e. the maximum strength of the varying electric and magnetic fields. In Figure 1, notice how the amplitude is less for the waves traveling above and below the horizontal plane representing the weaker signal strength in these areas. The r-squared loss is not shown. Sometimes the wavelength (or frequency) is represented by the distance between wave peaks (or wave valleys). Usually it is not practical to represent the actual wavelength or a correct wavelength scaled to diagram because it is too large or too small. Although actual EM radiation has both electric and magnetic fields, wave displays typically show only the electric field.
   
   
2. Phase Display
   The phase display in Figure 1 shows lines that connect all the wave peaks. These are called phase lines. This type of display is useful for showing how radiation spreads out from a source and how reflected and direct path waves interact. The distance between the phase lines can indicate wavelength or frequency. A disadvantage to this display method is that it does not usually give an indication of signal strength. In some cases, we indicate a decrease in signal strength by making the phase lines duller.
   
   
3. Ray Trace Display
   Lines drawn at right angles to to phase lines are called ray traces (Figure 1). Ray traces can be thought of as photon paths and indicate direction of EM energy transmission. Ray traces are a simple way to show how radiation reflects or curves as it propagates. Often we show multiple rays representing radiation propagating at different angles from the same antenna. Sometimes the distance between rays can be used as qualitative measure of signal strength (e.g. when rays converge due to some focusing mechanism). However if converging rays are out-of-phase, the result can be a decrease in signal strength. This points out a disadvantage to ray traces, they do not show phase relationships and cannot show a quantitative signal strength. Representations of high frequency (HF) and optical radiation typically use ray traces. The black line with an arrow in Figure 4 on the previous page is a ray trace. Note that the electric and magnetic force lines in EM radiation are always at right angles to the ray traces (and each other).
   
   
4. Signal Strength Display
   This method shows the EM signal strength by using contours or colored ranges. This is the most quantitative display method and is used by the Advanced Refractive Effects Prediction System (AREPS) and most other UHF and VHF prediction tools. This is the only method that can accurately show subtle details of how signal strength varies in space. Although this type of display cannot directly show phase relationships and reflections, it is especially good for displaying the resulting signal strength interference patterns from such effects. In Figure 1, the greatest signal strength is indicated by red with yellow, green, blue and purple, respectively, indicating. Note how the signal strength shows both r-squared loss and a weakened signal above and below the transmitter level.

Summary

Accelerating charges, such as the electrons in an antenna, generate EM waves. These waves propagate through the atmosphere and accelerate other electrons in a receiving antenna, allowing the transmission of voice and data. There are four common ways that the EM waves can be visualized: wave display, phase display, ray traces and signal strength displays.

Study Questions

1. What nuclear particle in transmitting antennas accelerates to produce EM waves?
2. What nuclear particle in receiving antennas accelerates to produce a current?
3. What is the relation between electric or magnetic force and ray traces?
4. Do phase lines ever line up with lines of electric or magnetic force?
5. Can ray traces alone show phase relationships?
6. What is the advantage of using signal strength displays?
   

Last Update: 1/12/2004