Antenna resonance and bandwidth

Antenna resonance and bandwidth

Two major factors associated with radio antennas are their resonant or centre operating frequency and the bandwidth over which they can operate. They naturally are very important feature of the operation of the antenna and as such they are mentioned in specifications for particular antennas and are a particularly important facet. Whether the antenna is used for broadcasting, WLAN, cellular telecommunications, PMR or any other application, the performance of the antenna is paramount, and the resonant frequency and bandwidth are of great importance.

Antenna resonance
An antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency at the frequency 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.

Impedance of an antenna with frequency

The capacitance and inductance of an antenna are determined by the physical properties of the antenna and its environment. The major feature of the antenna 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, whilst those for VHF broadcast sound FM have larger elements indicating a lower frequency. Antennas for short wave applications are larger still.

Bandwidth
Most antennas are operated around the resonant point. This means that there is only a limited bandwidth over which it 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.

The bandwidth is particularly important where transmitters are concerned as damage may ccur to the transmitter if the antenna is operated outside its operating range and the transmitter is not adequately protected. In addition to this the signal radiated by the antenna may be less for a number of reasons.

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 antenna is optimum.

Impedance bandwidth
One major feature of an antenna that does change with frequency is its impedance. This in turn can cause the amount of reflected power to increase. If the antenna is used for transmitting it may be that beyond a given level of reflected power damage may be caused to either the transmitter or the feeder, and this is quite likely to be a factor which limits the operating bandwidth of an antenna. Today most transmitters have some form of SWR protection circuit that prevents damage by reducing the output power to an acceptable level as the levels of reflected power increase. This in turn means that the efficiency of the station is reduced outside a given bandwidth. As far as receiving is concerned the impedance changes of the antenna are not as critical as they will mean that the signal transfer from the antenna itself to the feeder is reduced and in turn the efficiency will fall. For amateur operation the frequencies below which a maximum SWR figure of 1.5:1 is produced is often taken as the acceptable bandwidth.

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 which is described fully in Chapter 3 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.

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

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.