Antenna Resonance & Bandwidth

Radio antennas have a certain bandwidth over which they can operate satisfactorily: their bandwidth may be limited by the impedance match, directivity, etc

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

Radio antennas have a bandwidth over which they can operate effectively; even wideband antennas. Many antennas operate in a resonant mode and this gives them a relatively narrow bandwidth over which they are able to provide excellent performance.

Antenna resonance and bandwidth are two properties for antennas that are closely linked and they are very important, especially when selecting an antenna for a given application or purpose.

Whether the radio antenna is used for broadcasting, TV and radio reception, WLAN, cellular telecommunications, PMR, amateur radio, or any other application, the performance of the antenna is paramount. In this the antenna resonant frequency and the antenna bandwidth are of great importance.

Antenna resonance

A radio antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency.

Variation of impedance of antenna showing capacitive and inductive regions — Variation of impedance of antenna

The resonant frequency occurs at the point where the capacitive and inductive reactances cancel each other out. At this point the antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

The capacitance and inductance of an RF antenna are determined by its physical properties and the environment where the antenna is located.

The major feature of the antenna design is its dimensions. It is found that the larger the antenna or more strictly the antenna elements, the lower the resonant frequency.

For example antennas for UHF terrestrial television have relatively small elements, while those for VHF broadcast sound FM have larger elements indicating a lower frequency. Antennas for short wave applications are larger still.

Comparison of the sizes antennas used for different frequencies - VHF (lower) and UHF (higher) showing the different element lengths needed. — Antennas for VHF (lower) and UHF (higher) showing the different element lengths needed

Antenna bandwidth

An antenna bandwidth is governed by whether it is able to operate within the parameters required for that particular application.

The characteristics of any antenna will change with frequency and therefore it is often necessary to define the bandwidth over which the antenna will operate satisfactorily for the particular application: broadcast transmission, broadcast reception, two way radio communications, etc . .

In fact most antennas will have their operating region as part of the basic specification of the antenna: UHF TV antenna, 2 metre ham band antenna, VHF FM antenna and the like. These all indicate the part of the spectrum for which the antennas are intended to be used - it gives their bandwidth.

In some scenarios impedance may be an issue, in others it may be gain, or beamwidth. In other situations it may be the impedance match, especially where transmitting stations are using the antenna.

Accordingly, there are several ways in which the performance of an antenna bandwidth can be judged.

In most cases, antennas are operated around the resonant point. This means that there is only a limited bandwidth over which an RF antenna design can operate efficiently. Outside this the levels of reactance rise to levels that may be too high for satisfactory operation. Other characteristics of the antenna may also be impaired away from the centre operating frequency.

Impedance bandwidth

The antenna impedance characteristic and bandwidth are particularly important where radio transmitters are concerned.

If the impedance varies so that a poor impedance match is obtained between the feeder and the antenna itself occurs, then this will result in a high level of power being reflected back from the antenna towards the transmitter.

Power is reflected and sets up standing waves when feeder and load impedances do not match — Power is reflected when feeder and load impedances do not match

If the level of the reflected power is high, then this can cause damage to the output stage of the transmitter if no protection is present. It can totally destroy the output devices if the level of VSWR is so high that the peak voltage rises above the capability of the devices.

In view of this significant risk, most transmitters employ protection circuitry. This reduces the transmitter output power in the presence of a high level of VSWR so that any reflected power is not so high that it can cause damage. The result is that operating a transmitter with a high level of reflected power can reduced the transmitter power and hence its effectiveness. There can also be other issues.

For receiving purposes the performance of the antenna is less critical in some respects. It can be operated outside its normal bandwidth without any fear of damage to the set. Even a random length of wire will pick up signals, and it may be possible to receive several distant stations. However for the best reception it is necessary to ensure that the performance of the RF antenna design is optimum.

In terms of the operation of the antenna and its specification for its impedance bandwidth, one way of specifying this is to plot the standing wave ratio. The bandwidth for which an acceptable SWR or VSWR is obtained is then taken to be the operating bandwidth.

Typically the operating bandwidth may be bandwidth between which the maximum VSWR is less than 2:1, or whatever other limit is chosen.

In order to increase the bandwidth of an antenna there are a number of measures that can be taken. One is the use of thicker conductors. Another is the actual type of antenna used. For example a folded dipole has a wider bandwidth than a non-folded one. In fact looking at a standard television antenna it is possible to see both of these features included.

It is also possible to use wide band antenna designs such as a discone or a log periodic antenna. Both of these antenna types are wideband types that operate over a wide bandwidth.

However, wide band antenna types may not meet the requirements for whatever radio system is in use, and therefore it may be necessary to look at the bandwidth in terms of other factors.

Radiation pattern and gain bandwidth

Another feature of an antenna that changes with frequency is its radiation pattern. In the case of a directional or beam antenna it is particularly noticeable.

Sometimes key parameters associated with the antenna directional performance may be affected by the frequency.

In particular, the forward gain may be adversely affected over a given bandwidth.

For directional or beam antennas such as the Yagi the radiation pattern bandwidth is defined as the frequency range over which the gain of the main lobe is within 1 dB of its maximum.

Another parameter that is affected is the front to back ratio. This is the ratio of the signal in the forward direction to that in the reverse direction. This can be important in some situations where interference is an issue and it is necessary to minimise the reception of signals in the reverse direction.

Polar diagram of a Yagi directional antenna showing front to back ratio — Polar diagram of a Yagi antenna showing front to pack ratio

The front to back ratio will fall off rapidly outside a given bandwidth, and so will the gain. In an antenna such as a Yagi this is caused by a reduction in the currents in the parasitic elements as the frequency of operation is moved away from resonance.

For many beam antennas, especially high gain ones it will be found that the impedance bandwidth is wider than the radiation pattern bandwidth, although the two parameters are inter-related in many respects.

Antenna bandwidth is a key issue for any radio antenna. Whilst most antennas are operated in a resonant mode, many others are not. Whatever the radio antenna, it has a limited band over which it can operate effectively and within the parameters set out for it.

Understanding the antenna bandwidth, and also the reasons for the bandwidth specifications for a particular antenna, whether it is the VSWR, gain, or other factor, then it enables the antenna to be chosen to met its actual operational requirements, and the best performance obtained for a given situation.