Many people confuse the terms “aerial” and “antenna” using them interchangeably. While that is widely accepted and understood, they actually aren’t the same thing. An antenna is used for transmitting radio waves, whether used for radio, television, cell phones or other more sophisticated equipment. We can even put satellite dishes used by television stations to send signals up to satellites in the category of being called antennas. However, a satellite dish on a home, which is only used for reception of television signals would more rightly be considered an aerial.
Aerials are used for reception, not for transmission. The most common use which most people would be accustomed to is the aerial that is used for a car radio. Before cable and satellite television became popular, all televisions had aerials on them. But, today, televisions and many other things which used to use aerials, no longer use them.
Aerials can be made out of any conductive material. Originally, they were made by stringing a piece of wire that had been cut to a particular length. Today, it is much more common to find them made out of aluminum tubing. The tubing allows the aerial to be free-standing, whereas a wire aerial requires something to attach it to at each end. So, while tubing is more expensive than wire, it provides a level of convenience that is lacking with wire aerials.
Aerials can be made in a wide variety of configurations. The actual shape of the aerial isn’t as important as its size. The length of the parts which receive the signal need to match the wavelength of the radio waves that are to be received. This varies depending upon the frequency of the signal.
We start by measuring the wavelength of the radio wave. One full cycle is needed. This is the distance from peak to peak or from trough to trough of the carrier wave.
Ideally, the aerial needs to be the same length as the wavelength for best possible reception. However, at many frequencies, the wavelength is extremely long, making it impractical to make aerials that are the full wavelength. FM radio for example, would need aerials of a little over three meters long. That would be difficult to mount on a car and would tend to hit things as the car drove down the highway.
The higher the radio frequency, the shorter the wavelength. So AM radio, which operates at a lower frequency than FM radio would need an even longer aerial than AM radio would. Cell phones, which work at a frequency of about 1.9 GHz (gigahertz) can get by with much shorter antennas.
There are several ways of compensating for the problem with not being able to make an aerial long enough. One is to use an aerial which is an even fraction of the wavelength. Half wavelength and quarter wavelength aerials are common. Another way is to use a coil to compensate for the difference between the actual length of the aerial and the wavelength of the radio waves to be received. In this manner, much shorter aerials can be used to accomplish the same thing as a longer one.
If you look at a lot of late model cars, the radio aerial is much shorter than that which is found on an older car. They accomplish this by using a coil, allowing them to install a shorter aerial on the car. However, there is one drawback; a shorter aerial won’t pick up as much of a signal as a longer one will. In order to be able to be used, the signal usually needs to be amplified.
Aerials for Multiple Frequencies
Although it has been done for years, the hardest aerial design problem is to create an aerial that can be used for receiving multiple frequencies. Perhaps the most common example of this is an old-fashioned television aerial; the type which is designed to be mounted on the roof of a house.
As we can see from this image, the aerial is made of a number of different length elements. These elements, which run across the aerial, are of various lengths. The black parts where these elements cross the aerial’s main boom are insulators, isolating each of the cross elements and making it have its own overall length.
An aerial of this type receives a wide variety of different frequencies. Each length of element is “tuned” to the frequency of a different group of channels. The longer elements are used for the VHF channels (2-12) and the shorter elements are used for the UHF channels (above 20). All of the elements are connected together electrically, although they are isolated from the main boom of the aerial.
Combining the elements electrically means that the signal going from that aerial to the television is a mixture of a number of different television signals. The tuner (channel selector) in the television is actually a filter, which filters out all the frequencies or channels that are not wanted; only allowing the frequency that is wanted to provide data to the television’s circuitry and produce the image. It really doesn’t matter if the signal is digital or analog, to the aerial it is all the same.
See also: Antennas - Radio Waves and Antennas vs Aerials
R.A.M. Rich Murphy