What is the formula for calculating dipole antenna bandwidth?

Dipole antennas are one of the simplest and most widely used types of antennas in radio frequency (RF) communication systems. They consist of two identical conductors, such as wires or rods, that are fed by a balanced transmission line at their center. The length of each conductor is usually a quarter of the wavelength of the operating frequency, which determines the resonant frequency of the antenna. However, dipole antennas can also operate at other frequencies within a certain range, known as the bandwidth. The bandwidth of a dipole antenna is a measure of how well it can transmit or receive signals of different frequencies without significant loss of efficiency or performance. In this article, you will learn what is the formula for calculating dipole antenna bandwidth and how it depends on various factors, such as the diameter and material of the conductors, the shape and orientation of the antenna, and the surrounding environment.

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1 Bandwidth definition

The bandwidth of a dipole antenna is usually defined as the frequency range within which the standing wave ratio (SWR) or the voltage standing wave ratio (VSWR) is less than or equal to a certain value. The SWR or VSWR is a ratio of the maximum and minimum voltage or current along the antenna, which indicates how well the antenna impedance matches the transmission line impedance. A high SWR or VSWR means that there is a large mismatch and a lot of power is reflected back to the source, resulting in poor efficiency and potential damage to the transmitter. A low SWR or VSWR means that there is a good match and most of the power is radiated by the antenna, resulting in high efficiency and performance. The typical value of SWR or VSWR used to define the bandwidth is 2:1, which means that the maximum voltage or current is twice the minimum voltage or current along the antenna.

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2 Bandwidth formula

The formula for calculating the bandwidth of a dipole antenna with a given SWR or VSWR limit is: BW = 2f_r * (1 - 1/SWR) / (1 + 1/SWR) where BW is the bandwidth in hertz, f_r is the resonant frequency in hertz, and SWR is the standing wave ratio. This formula assumes that the antenna is symmetrical and has a linear relationship between frequency and SWR. For example, if the resonant frequency of a dipole antenna is 100 MHz and the SWR limit is 2:1, then the bandwidth is: BW = 2 * 100 * 10^6 * (1 - 1/2) / (1 + 1/2) BW = 33.3 * 10^6 which means that the antenna can operate efficiently within the frequency range of 83.3 MHz to 116.7 MHz.

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3 Bandwidth factors

The bandwidth of a dipole antenna is determined by several factors that impact its impedance and radiation characteristics. The diameter of the conductors, the material of the conductors, the shape and orientation of the antenna, and the surrounding environment all influence the antenna's bandwidth. Generally, a thicker conductor has a lower resistance and higher capacitance, resulting in a higher bandwidth. Whereas, a thin conductor has a higher resistance and lower capacitance, causing the bandwidth to decrease. Additionally, conductors with high conductivity such as copper or silver have a lower resistance and higher bandwidth. Whereas, conductors with low conductivity like iron or steel have a higher resistance and lower bandwidth. Moreover, dipole antennas can have different shapes such as straight, bent, loop, or helical which affect its radiation pattern and impedance. The orientation of the antenna also affects its impedance and bandwidth as it determines the angle of incidence and reflection of electromagnetic waves. Lastly, the environment around the antenna can introduce parasitic capacitance, inductance, and resistance which can change its resonant frequency and SWR which in turn affect its bandwidth.

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Jay Zebryk

Wireless Product Development Expert
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How might a dipole be designed to have a minimum of 2:1 SWR from 470 to 700MHz? Perhaps with Conical elements of what diameters over what lengths?

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4 Here’s what else to consider

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What is the formula for calculating dipole antenna bandwidth?

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1 Bandwidth definition

2 Bandwidth formula

3 Bandwidth factors

4 Here’s what else to consider

Telecommunications Engineering

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What is the formula for calculating dipole antenna bandwidth?

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1 Bandwidth definition

2 Bandwidth formula

3 Bandwidth factors

4 Here’s what else to consider

Telecommunications Engineering

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