Electromagnetic Waves And Antenna Basics :Polarization And Antenna Resistance

Electromagnetic waves and antenna basics

 

Electromagnetic waves are the same type of radiation as light, ultra-violet and infra red rays, differing from them in their wavelength and frequency. Electromagnetic waves have both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave.

An electromagnetic wave

The electric field results from the voltage changes occurring in the RF antenna which is radiating the signal, and the magnetic changes result from the current flow. It is also found that the lines of force in the electric field run along the same axis as the RF antenna, but spreading out as they move away from it. This electric field is measured in terms of the change of potential over a given distance, e.g. volts per metre, and this is known as the field strength. Similarly when an RF antenna receives a signal the magnetic changes cause a current flow, and the electric field changes cause the voltage changes on the antenna.

Antenna polarisation :

Polarisation is an important factor for RF antennas and radio communications in general. Both RF antennas and electromagnetic waves are said to have a polarization.

For the electromagnetic wave the polarization is effectively the plane in which the electric wave vibrates. This is important when looking at antennas because they are sensitive to polarisation, and generally only receive or transmit a signal with a particular polarization.

For most antennas it is very easy to determine the polarization. It is simply in the same plane as the elements of the antenna. So a vertical antenna (i.e. one with vertical elements) will receive vertically polarized signals best and similarly a horizontal antenna will receive horizontally polarized signals.

It is important to match the polarization of the RF antenna to that of the incoming signal. In this way the maximum signal is obtained. If the RF antenna polarization does not match that of the signal there is a corresponding decrease in the level of the signal. It is reduced by a factor of cosine of the angle between the polarization of the RF antenna and the signal.

Accordingly the polarization of the antennas located in free space is very important, and obviously they should be in exactly the same plane to provide the optimum signal. If they were at right angles to one another (i.e. cross-polarized) then in theory no signal would be received.

For terrestrial radio communications applications it is found that once a signal has been transmitted then its polarization will remain broadly the same. However reflections from objects in the path can change the polarization. As the received signal is the sum of the direct signal plus a number of reflected signals the overall polarization of the signal can change slightly although it remains broadly the same.

Polarisation catagories

Vertical and horizontal are the simplest forms of antenna polarization and they both fall into a category known as linear polarisation. However it is also possible to use circular polarisation. This has a number of benefits for areas such as satellite applications where it helps overcome the effects of propagation anomalies, ground reflections and the effects of the spin that occur on many satellites. Circular polarisation is a little more difficult to visualise than linear polarisation. However it can be imagined by visualising a signal propagating from an RF antenna that is rotating. The tip of the electric field vector will then be seen to trace out a helix or corkscrew as it travels away from the antenna. Circular polarisation can be seen to be either right or left handed dependent upon the direction of rotation as seen from the transmitter.

Another form of polarisation is known as elliptical polarisation. It occurs when there is a mix of linear and circular polarisation. This can be visualised as before by the tip of the electric field vector tracing out an elliptically shaped corkscrew.

However it is possible for linearly polarised antennas to receive circularly polarised signals and vice versa. The strength will be equal whether the linearly polarised antenna is mounted vertically, horizontally or in any other plane but directed towards the arriving signal. There will be some degradation because the signal level will be 3 dB less than if a circularly polarised antenna of the same sense was used. The same situation exists when a circularly polarised antenna receives a linearly polarised signal.

Applications of antenna polarization

Different types of polarisation are used in different applications to enable their advantages to be used. Linear polarization is by far the most widely used for most radio communications applications. Vertical polarisation is often used for mobile radio communications. This is because many vertically polarized antenna designs have an omni-directional radiation pattern and it means that the antennas do not have to be re-orientated as positions as always happens for mobile radio communications as the vehicle moves. For other radio communications applications the polarization is often determined by the RF antenna considerations. Some large multi-element antenna arrays can be mounted in a horizontal plane more easily than in the vertical plane. This is because the RF antenna elements are at right angles to the vertical tower of pole on which they are mounted and therefore by using an antenna with horizontal elements there is less physical and electrical interference between the two. This determines the standard polarization in many cases.

In some applications there are performance differences between horizontal and vertical polarization. For example medium wave broadcast stations generally use vertical polarization because ground wave propagation over the earth is considerably better using vertical polarization, whereas horizontal polarization shows a marginal improvement for long distance communications using the ionosphere. Circular polarization is sometimes used for satellite radio communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation.

Antenna feed impedance

When a signal source is applied to an RF antenna at its feed point, it is found that it presents a load impedance to the source. This is known as the antenna "feed impedance" and it is a complex impedance made up from resistance, capacitance and inductance. In order to ensure the optimum efficiency for any RF antenna design it is necessary to maximize the transfer of energy by matching the feed impedance of the RF antenna design to the load. This requires some understanding of the operation of antenna design in this respect.

The feed impedance of the antenna results from a number of factors including the size and shape of the RF antenna, the frequency of operation and its environment. The impedance seen is normally complex, i.e. consisting of resistive elements as well as reactive ones.

Antenna feed impedance resistive elements

The resistive elements are made up from two constituents. These add together to form the sum of the total resistive elements.

Loss resistance:   The loss resistance arises from the actual resistance of the elements in the RF antenna, and power dissipated in this manner is lost as heat. Although it may appear that the "DC" resistance is low, at higher frequencies the skin effect is in evidence and only the surface areas of the conductor are used. As a result the effective resistance is higher than would be measured at DC. It is proportional to the circumference of the conductor and to the square root of the frequency.

The resistance can become particularly significant in high current sections of an RF antenna where the effective resistance is low. Accordingly to reduce the effect of the loss resistance it is necessary to ensure the use of very low resistance conductors.

Radiation resistance:   The other resistive element of the impedance is the "radiation resistance". This can be thought of as virtual resistor. It arises from the fact that power is "dissipated" when it is radiated from the RF antenna. The aim is to "dissipate" as much power in this way as possible. The actual value for the radiation resistance varies from one type of antenna to another, and from one design to another. It is dependent upon a variety of factors. However a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.

Antenna reactive elements

There are also reactive elements to the feed impedance. These arise from the fact that the antenna elements act as tuned circuits that possess inductance and capacitance. At resonance where most antennas are operated the inductance and capacitance cancel one another out to leave only the resistance of the combined radiation resistance and loss resistance. However either side of resonance the feed impedance quickly becomes either inductive (if operated above the resonant frequency) or capacitive (if operated below the resonant frequency).

Efficiency

It is naturally important to ensure that the proportion of the power dissipated in the loss resistance is as low as possible, leaving the highest proportion to be dissipated in the radiation resistance as a radiated signal. The proportion of the power dissipated in the radiation resistance divided by the power applied to the antenna is the efficiency.

A variety of means can be employed to ensure that the efficiency remains as high as possible. These include the use of optimum materials for the conductors to ensure low values of resistance, large circumference conductors to ensure large surface area to overcome the skin effect, and not using designs where very high currents and low feed impedance values are present. Other constraints may require that not all these requirements can be met, but by using engineering judgement it is normally possible to obtain a suitable compromise.

It can be seen that the antenna feed impedance is particularly important when considering any RF antenna design. However by maximising the energy transfer by matching the feeder to the antenna feed impedance the antenna design can be optimized and the best performance obtained.

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