Antennas - Radio Waves and Antennas vs Aerials

Antennas

In 1897, a young Italian inventor, Gugliemlo Marconi made the first successful long-distance radio transmission. His invention, using a telegraph key and Morse code, sent a series of electronic sparks, which were able to be picked up at a remote location by a radio receiver. The age of radio was born.

 

Marconi wasn’t the first to have the idea of wireless communications, nor was the only one to be working on it. Various scientists and inventors had been working on the idea for 50 years. However, he was the first to succeed, sending his first public transmission 6 miles and later transmissions over 2000 miles.

 

One of the things that made Marconi successful for long distance radiotelegraphy was an almost accidental change he made to his antenna. Rather than using a horizontal antenna, like most who were experimenting with radiotelegraphy, in an effort to improve his range, he switched to a vertical antenna and then connected it to the ground. This greatly increased his range, although the reason for that increase was not known for many years.

 

How Radio Waves are Produced

Any radio wave transmission (which includes television, cell phones, WiFi, Bluetooth and any other wireless technology) is somewhat amazing from the point of view of electrical theory. One of the requirements for any electronic circuit is that there be a complete circuit, running all the way from the positive pole of the battery or power supply, all the way through the circuit and back to the negative pole of the battery or power supply. With radio, there is a break in the circuit; a break between the transmitter and the receiver.

 

Radio transmitters produce an oscillating or alternating current. When applied to an antenna, this causes the antenna to produce radio waves. To understand this, we must understand how electricity and magnetism are interrelated.

 

As electrical current passes through a wire, it produces a magnetic field. We don’t normally recognize the existence of this electronic field, because it is small enough that we can’t discern it except with very sensitive instrumentation. However, if we make a coil of wire, the successive layers of wire add to the magnetic field, increasing it. This then becomes an electromagnet, becoming magnetized every time that power is applied.

 

Automatic solenoids are essentially electromagnets. A magnetized ferrous rod is inserted into the coil. When electricity is applied, the bar moves, actuating a car door lock, opening the trunk or a number of other things.

 

Radio depends upon this magnetic field as well. When Marconi connected his vertically mounted antenna to the ground, he created a magnetic field between the antenna and earth ground, increasing the signal strength and greatly increasing the effective radiating power of his antenna. Although it is his work with radios that made him famous, it should have been his work with antennas.

 

What the Antenna Does

For radio transmissions, the antenna that you see is only half of the antenna, the other half is what is known as the ground plane. For Marconi, the Earth itself was the ground plane. For a police or citizens band radio mounted in a car, the vehicle becomes the ground plane. Antennas mounted on buildings are often built with ground planes that consist of a number of arms sticking out from the base.

 

The radio wave is produced by a combination of the vertical (or sometimes horizontal) element and this ground plane. The magnetic field produced by the alternating current passing between the converts into this radio wave, which is sent in all directions.

 

Radio waves can be steered by using either a reflector or by the way that the radiating element of the antenna is located in relation to the ground plane. If there is more ground plane to one side of the radiating element of the antenna, the radio waves will be stronger in that direction, radiating over a longer distance.

 

For radio waves to be effective, their length must match that of the frequency of radio wave that they are transmitting. A 30 meter radio wave would therefore require a 30 meter long or 30 meter tall radiating element as part of the antenna. However, most modern antennas are much shorter than this. Instead of using a “full wave” antenna, a half-wave, quarter-wave or five-eighths-wave antenna is used.

 

Not all of this length has to be in a straight line either. A compensating coil can be attached to the base of the antenna, acting as part of the overall length. This allows a shorter radiating element to be used.

 

Citizens Band (CB) radios operate in the 11 meter band. A quarter-wave antenna would therefore be 9 feet long. Yet CB radios usually use antennas that are only about 30 inches long. The difference is made up by one of these coils at the base. It is rare that you see a vehicle with a CB radio in it that uses a 9 foot long whip antenna, although there are some that do.

 

Much of our modern electronic equipment uses much shorter antennas than this, as the radio frequency is much higher than the 27 MHz (megahertz) that CB radio operates on. Cell phones, one of today’s most common radio transmitters, operate in bands from 7 MHz to 21 MHz.

 

The length of the antenna must be tuned exactly to the frequency that is transmitting it. Failure to accomplish a perfect tuning reflects part of the radio’s power back to the transmitter, instead of transmitting it. This is referred to as a “standing wave.” The ratio of radio frequency (RF) that is leaving the antenna to the RF which is reflected back to the transmitter is called the “standing wave ratio” (SWR). If the SWR is too high, it can cause damage to the transmitter.

 

Tuning the length of the antenna to the frequency is not much of a problem with a fixed frequency radio transmitter, but it is virtually impossible for a variable frequency radio transmitter, as each frequency needs a slightly different antenna length. To accommodate this, the antenna is usually tuned to the middle frequency in the range, in extreme situations, multiple coils are used to make up for the different frequency ranges.

 

Antennas vs. Aerials

Although commonly referred to as “antennas” radio receivers collect the incoming radio waves with an aerial, rather than an antenna. There are a few differences between the two, notably that aerials do not have to have a ground plane and do not have to be as accurately tuned to the frequency of transmission.

 

This is not to say that just any old length of aerial will work effectively. The most effective length of an aerial, like the antenna is the wavelength of the radio wave being received or a fraction thereof. However, if the wrong length of aerial is used, there is no risk of damaging the radio receiver, as there is with the wrong length of antenna. The only effect it will have is to reduce the strength of the signal received.

 

See also: Antenna Aerials - Basics of Aerial Design and Multiple Frequency Aerials

 

Rich Murphy