Antennas and transmitters are the key to virtually all forms of modern telecommunication. Let's take a closer look at what they are and how they work! Suppose you're the boss of a radio station and you want to transmit your programs to the wider world.
How do you go about it? You use microphones to capture the sounds of people's voices and turn them into electrical energy. You take that electricity and, loosely speaking, make it flow along a tall metal antenna boosting it in power many times so it will travel just as far as you need into the world. As the electrons tiny particles inside atoms in the electric current wiggle back and forth along the antenna, they create invisible electromagnetic radiation in the form of radio waves.
These waves travel out at the speed of light , taking your radio program with them. What happens when I turn on my radio in my home a few miles away?
The radio waves you sent flow through the metal antenna and cause electrons to wiggle back and forth. That generates an electric current—a signal that the electronic components inside my radio turn back into sound I can hear. How a transmitter sends radio waves to a receiver. This produces an electric current that recreates the original signal. Transmitter and receiver antennas are often very similar in design. For example, if you're using something like a satellite phone that can send and receive a video-telephone call to any other place on Earth using space satellites , the signals you transmit and receive all pass through a single satellite dish—a special kind of antenna shaped like a bowl and technically known as a parabolic reflector , because the dish curves in the shape of a graph called a parabola.
Often, though, transmitters and receivers look very different. But you don't need anything that big on your TV or radio at home: a much smaller antenna will do the job fine.
Waves don't always zap through the air from transmitter to receiver. The signal is modulated to contain mono audio information left and right speakers , a pilot tone, AM suppressed carrier information, and text information. That modulated signal is sent through the air, traveling at very specific wavelengths and received by the antenna you are using. The signal with the information is called a carrier signal. When a signal has been modulated and relayed out into the airwaves, we refer to it as a carrier signal because it carries information along with it.
Just as a signal must be modulated to become a carrier signal, it has to be demodulated when it reaches the receiver. In other words, the signal is reduced down to the pertinent information. When it comes to other kinds of media such as video, the signal must be digitized. Some waves shoot straight from the transmitter to the receiver. The line of sight transmission was effectively eliminated in the 60s when fiber-optic cables were the primary method of transmitting phone calls.
Signals sent on low frequencies, often times AM stations, rely on the use of ground wave propagation to operate. These ground waves can travel a great distance when used in the lower frequencies of the spectrum. The challenge now is to find out how the electric field varies due to this movement.
The wavefront formed at time zero expands and is deformed as shown after one eighth of a time period Fig:4A. This is surprising; you might have expected a simple electric field as shown at this location. Why has the electric field stretched and formed a field like this? The old electric field does not easily adjust to the new condition. We need to spend some time to understand this memory effect of the electric field, or kink generation, of accelerating or decelerating charges.
If we continue our analysis in the same manner, we can see that at one quarter of a time period, the wavefront ends meets at a single point Fig After this, the separation and propagation of the wavefront happens. If you draw electric field intensity variation with the distance, you can see that the wave propagation is sinusoidal in nature Fig It is interesting to note that the wavelength of the propagation so produced is exactly double that of the length of the dipole.
We will come back to this point later. Please note that this varying electric field will automatically generate a varying magnetic field perpendicular to it. This is exactly what we need in an antenna. In short, we can make an antenna, if we can make an arrangement for oscillating the positive and negative charges. In practice, the production of such an oscillating charge is very easy. Take a conducting rod with a bend in its center, and apply a voltage signal at the center 7A. Assume this is the signal you have applied, a time varying voltage signal.
Consider the case at time zero. Due to the effect of the voltage, the electrons will be displaced from the right of the dipole and will be accumulated on the left. This means the other end, which has lost electrons, automatically becomes positively charged 7B.
This arrangement has created the same effect as the previous dipole charge case, i. With the variation of voltage with time, the positive and negative charges will shuttle to and fro. The simple dipole antenna also produces the same phenomenon we saw in the previous section and wave propagation occurs. We have now seen how the antenna works as a transmitter. The frequency of the transmitted signal will be the same as the frequency of the applied voltage signal.
Since the propagation travels at the speed of light, we can easily calculate the wavelength of the propagation Fig For perfect transmission, the length of the antenna should be half of the wavelength. The operation of the antenna is reversible and it can work as a receiver if a propagating electromagnetic field hits it. Take the same antenna again and apply an electric field.
At this instant the electrons will accumulate at one end of the rod. This is the same as an electric dipole.
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