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. Depending on what kinds frequencies of waves we want to send, how far we want to send them, and when we want to do it, there are actually three different ways in which the waves can travel:. Artwork: How a wave travels from a transmitter to a receiver: 1 By line of sight; 2 By ground wave; 3 Via the ionosphere.
Photo: This telescopic FM radio antenna pulls out to a length of about 1—2m 3—6ft or so , which is roughly half the length of the radio waves it's trying to capture. The simplest antenna is a single piece of metal wire attached to a radio. The first radio I ever built, when I was 11 or 12, was a crystal set with a long loop of copper wire acting as the antenna. I ran the antenna right the way around my bedroom ceiling, so it must have been about 20—30 meters 60— ft long in all! Photo: Antennas that use line-of-sight communication need to be mounted on high towers, like this.
You can see the thin dipoles of the antenna sticking out of the top, but most of what you see here is just the tower that holds the antenna high in the air. Most modern transistor radios have at least two antennas. One of them is a long, shiny telescopic rod that pulls out from the case and swivels around for picking up FM frequency modulation signals.
The other is an antenna inside the case, usually fixed to the main circuit board, and it picks up AM amplitude modulation signals. If you're not sure about the difference between FM and AM, refer to our radio article. Why do you need two antennas in a radio? The signals on these different wave bands are carried by radio waves of different frequency and wavelength. Typical AM radio signals have a frequency of kHz kilohertz , while typical FM signals are about MHz megahertz —so they vibrate about a hundred times faster.
You need two antennas because a single antenna can't pick up such a hugely different range of wavelengths. It's the wavelength or frequency, if you prefer of the radio waves you're trying to detect that determines the length of the antenna you need to use. Broadly speaking, the length of the antenna has to be about half the wavelength of the radio waves you're trying to receive it's also possible to make antennas that are a quarter of the wavelength, though we won't go into that here.
Blogs Contact. At the mid point of their path the velocity will be at the maximum and at the ends of their paths the velocity will be zero. The charged particles undergo continuous acceleration and deceleration due to this velocity variation.
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. As the applied electric field varies, the positive and negative charges accumulate at the other ends. The varying charge accumulation means a varying electric voltage signal is produced at the center of the antenna. This voltage signal is the output when the antenna works as a receiver as shown in Fig The frequency of the output voltage signal is the same as the frequency of the receiving EM wave.
It is clear from the electric field configuration that for perfect reception, the size of the antenna should be half of the wavelength. In all these discussions we have seen that the antenna is an open circuit. In the past, dipole antennas were used for TV reception. The colored bar acts as a dipole and receives the signal as shown in figure. The dipole is the main driven element of it. A reflector and director are also needed in this kind of antenna to focus the signal on the dipole.
The reflector element is always longer and the director element are always shorter than the driven element. This complete structure is known as a Yagi-Uda antenna FigA. The yagi uda antenna was invented by two japanese scientists Hidetsugu Yagi and Shintaro Uda.
It is a directional antenna and used in point to point communication.
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