Bandwidth is another fundamental antenna parameter. Bandwidth describes the range of frequencies that the antenna can correctly radiate or receive energy. Typically, the required bandwidth is one of the parameters used to select the antenna type. For example, there are many types of antennas with very small bandwidths. These antennas cannot be used in broadband applications.
Bandwidth is usually quoted in terms of voltage standing wave ratio (VSWR). For example, an antenna may be described as having a VSWR <1.5 over 100-400 MHz. The statement states that the reflection coefficient is less than 0.2 across the quoted frequency range. Therefore, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. In addition, return loss S11 =20* LOG10 (0.2) = 13.98 decibels.
Please note that the above does not mean that 96% of the power is delivered to the antenna in the form of propagated electromagnetic radiation. Power loss must be considered.
Additionally, the radiation pattern will vary with frequency. In general, the shape of the radiation pattern does not radically change the frequency.
There may also be other standards used to describe bandwidth. This can be polarizing within a certain range. For example, a circularly polarized antenna can be described as having an axial ratio of <3 dB from 1.4-1.6 GHz (less than 3 dB). This polarization bandwidth setting range is approximately for circularly polarized antennas.
Bandwidth is often specified in its Fractional Bandwidth (FBW). FBW is the ratio of the frequency range divided by the center frequency (highest frequency minus lowest frequency). The "Q" of an antenna also relates to bandwidth (higher Q means lower bandwidth and vice versa).
To give some concrete examples of bandwidth, here is a table of bandwidths for common antenna types. This will answer the questions, "What is the bandwidth of a dipole antenna?" and "Which antenna has a higher bandwidth - a patch or a helix antenna?". For comparison, we have antennas with a center frequency of 1 GHz (gigahertz) each.
Bandwidths of several common antennas.
As you can see from the table, the bandwidth of the antenna can vary greatly. Patch (microstrip) antennas are very low bandwidth, while helical antennas have a very large bandwidth.
Post time: Nov-24-2023
