Bandwidth

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Bandwidth is another fundamental antenna parameter. Bandwidth describes the range of frequencies over which the antenna can properly radiate or receive energy. Often, the desired bandwidth is one of the determining parameters used to decide upon an antenna. For instance, many antenna types have very narrow bandwidths and cannot be used for wideband operation.

Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, the return loss S11=20*log10(0.2)=-13.98 dB.

Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of EM radiation; losses must still be taken into account.

Also, the radiation pattern will vary with frequency. In general, the shape of the radiation pattern does not change radically.

There are also other criteria which may be used to characterize bandwidth. This may be the polarization over a certain range, for instance, an antenna may be described as having circular polarization with an axial ratio < 3dB (less than 3 dB) from 1.4-1.6 GHz. This polarization bandwidth sets the range over which the antenna's operation is approximately circularly polarized.

The bandwidth is often specified in terms of its Fractional Bandwidth (FBW). The FBW is the ratio of the frequecny range (highest frequency minus lowest frequency) divided by the center frequency. The antenna Q also relates to bandwidth (higher Q is lower bandwidth, and vice versa).

To give some concrete examples of bandwidth, here is a table of the bandwidths for common antenna types. This will answer such questions as "what is the bandwidth of a dipole antenna?" and "which antenna has a higher bandwidth - a patch or a spiral antenna?". For a fare comparison, we set the center frequency for each antenna to 1 GHz (1000 MHz).

Table I. The Bandwidth for Several Common Antennas.

bandwidths for common antennas

As can be seen from Table I, the bandwidth for antennas can vary widely. Patch (Microstrip) Antennas are very low bandwidth, while spiral antennas have a very large bandwdith.


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