Electromagnetic Waves And Antenna Basics: Antenna Resonance & Bandwidth

Antenna Resonance & Bandwidth

Two major factors associated with radio antenna design are the antenna resonant point or center operating frequency and the antenna bandwidth or the frequency range over which the antenna design can operate. These two factors are naturally very important features of any antenna design and as such they are mentioned in specifications for particular RF antennas. 

Antenna resonance

An RF antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency. This is the frequency where the capacitive and inductive reactances cancel each other out. At this point the RF antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

 

Impedance of an RF antenna with frequency

The capacitance and inductance of an RF antenna are determined by its physical properties and the environment where it is located. The major feature of the RF antenna design is its dimensions. It is found that the larger the antenna or more strictly the antenna elements, the lower the resonant frequency.

Antenna bandwidth

Most RF antenna designs 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 center operating frequency.

The antenna bandwidth is particularly important where radio transmitters are concerned as damage may occur to the transmitter if the antenna is operated outside its operating range and the radio transmitter is not adequately protected. In addition to this the signal radiated by the RF 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 RF antenna design is optimum.

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.

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