Antenna Fundamentals & basic aerial theory

Antenna Basics Includes:
Basic antenna theory     Polarisation     Antenna near & far fields     Resonance & bandwidth     Gain & directivity     Feed impedance     Antenna matching techniques    

Radio antennas are a key element of any radio communications broadcast or wireless system. An antenna is required to radiate and receive the signals and therefore their performance is key to the operation of the overall radio system.

If the radio antenna performance is poor, then it will limit the performance of the overall radio communications system, or whatever wireless system is using it. Accordingly, maximising the performance of the antenna is very important.

To achieve this, an understanding of the radio antenna basics and some theory will help the maximum be gained from any aerial system. It does not have to be full of difficult mathematics - just a straightforward understanding of the principles.

In depth antenna theory can become quite complicated, but a qualitative and simplified theoretical explanation help help in understanding what is actually happening, how the radio antennas work, and how they can be optimised. This can be key when setting up a radio communications system or link.

How does an antenna work

The purpose of a radio antenna is to convert the power applied to it in the form of a radio frequency alternating current signal into an electromagnetic wave.

This electromagnetic wave is able to travel through the space between the transmitting radio antenna and a receiving antenna. At the receiving end the electromagnetic wave is converted from the electromagnetic wave back into a radio frequency signal that can be applied to the input of a radio receiver.

In this way a radio antenna is able have power applied to it from which a signal int he form of an electromagnetic wave is launched. Similarly when an electromagnetic wave is incident upon an antenna it is converted from the electromagnetic wave into a radio frequency signal that can be carried to the input of the receiving equipment.

The basic theory about the way in which antennas work can be explained using Maxwell's equations. They detail the way that as the current or charges move along the antenna they produce electromagnetic waves.

Looking at how an antenna works from a more qualitative approach, it is possible to visualise a point charge that is oscillating in line with the radio frequency signal.

As a result of the oscillation of the charge the resultant electric field will also change and this changing electric field will generate a displacement current.

In turn, as a result of Ampere's Law, this current will generate a magnetic field.

In view of the fact that the oscillation of the charge, creates the varying electric field and then a magnetic field, they all vary together.

Applying Faraday's law, a changing magnetic field will created an electric field. In turn, this electric field will again create a magnetic field and the process repeats. These waves of electric and magnetic fields constitute electromagnetic waves that propagate outwards from the original point charge.

The energy of the original oscillating point charge is converted into the energy for the electromagnetic wave - in other words the power entering the antenna is converted into the energy of the electromagnetic waves.

It can also be seen that it is the current component of the signal on the antenna that gives rise to the radiated electromagnetic waves.

Transmitter & receive reciprocity

One of the key aspects about any radio antenna is whether it will receive and transmit in the same way. A passive antenna, i.e. one that does not use an embedded electronic circuit such as an active antenna will normally work in the same way transmitting and receiving.

It will have the same gain, the same directional pattern, polarisation, the same impedance and other aspects for both transmitting and receiving.

Often it is easier to visualise factors like the gain, and directional pattern using the image of a transmitted signal, but the antenna will have the same gain and directional pattern, etc when receiving as well.

Key antenna theory topics

There are several basic topics that are common to all radio antenna types and which form part of the basic antenna theory.

• Polarisation:   Radio antennas are sensitive to polarisation. In just the same way that electromagnetic waves can be polarised, so too are antennas. It will be seen that some antennas have their elements in a vertical fashion and others are horizontal. This is to accommodate vertical and horizontally polarised electromagnetic waves.
  
  Vertical and horizontally polarised antennas receive electromagnetic waves having the same polarisation - the polarisation of an electromagnetic eave is defined by the plane in which the electric field is contained. If the polarisation of a wave is not aligned, then the signal level will be reduced - cross polarised antennas will not receive any signals transmitted by the other. It is therefore important to ensure that the polarisation of antennas in a radio communications system are the same.
  
  Apart from linear polarisation, electromagnetic waves can also be polarised in a circular fashion - there are obviously two directions, i.e. clockwise and anticlockwise. Similar to linear polarisation, the circularly polarised antennas must have the same direction of polarisation to recieve signals transmitted by the other.
  
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• Resonance & bandwidth:   Resonance and bandwidth are key issues for antenna theory. Essentially the bandwidth of an antenna is the band of frequencies over which the antenna will operate within its specification. Although this may seem like a vague definition, it is actually the one that is the most useful as there are often different criteria for different antennas in different scenarios.
  
  Two aspects of antenna performance may limit the bandwidth. One is the reflected power, and the other is the gain.
  
  As many antennas are operated as resonant antennas, there is only a limited band over which they can operate. Outside these limits the level of reflected power increases and they may not be able to operate as effectively.
  
  The other common limitation is the gain. Many antennas like the Yagi, commonly used as a TV antenna. These antennas operate well within their given bandwidth. Outside this the directional patter will change and they will not be as effective.
  
  The bandwidth of antennas can be important. For some applications a very wide bandwidth is required. For example, television antennas often need to have a wide bandwidth - not only do the transmissions occupy a reasonably wide bandwidth, but more importantly the different television signals can be be spaced over a wide band, and the antenna will need to be able to receive them. For other applications, for example various wireless systems, the system may operate on a single frequency using a narrow band transmission, and for these applications antenna bandwidth can be narrow.
  
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• Gain & directivity:   Antennas do not radiate equally in all directions - only an isotropic source radiates equally in all directions and this is a theoretical entity only . In some directions practical antennas exhibit gain where the available power is focussed in a particular direction, and they have a directional pattern. Antenna theory for directivity and gain is important in many areas whether for various wireless systems, radio communications or broadcasting.
  
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• Feed impedance & matching:   The input connection to an antenna presents an impedance to the feeder to which it is connected. For optimum power transfer source and load should be matched. Accordingly antenna theory associated with the feed impedance is important for the optimum operation of the antenna.
  
  There are many factors associated with feed impedance and there are various methods of ensuring that a good feed and matcha re obtained for any particular antenna to ensure its performance is optimised.
  
  Read more about . . . . feed impedance & matching.
  
  

Although radio antenna theory may appear to be daunting, a working understanding of how antennas work, and some of the key concepts is very useful. It can be invaluable when setting up a radio communications system or link or even for installing broadcast receiving antennas, or radio antennas for any one of a number of applications.

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