What is the Difference between a Single Band and Multi Band Antenna?

The key difference between a single band and multiband antenna lies in their ability to operate on a specific frequency band or multiple bands of frequencies. This article explores the technical details, advantages, and disadvantages of both types of antennas, providing a comprehensive guide for understanding their unique characteristics and applications.

Single Band Antenna

A single band antenna is designed to operate on a specific frequency band. It typically consists of a radiating patch fed by a 50 ohms microstrip line. In the study, a single band antenna was simulated on an FR4 dielectric substrate with a relative permittivity of 4.4 and a loss tangent of 0.02, at a height of 1.6 mm. Due to its narrowband operation, the single band antenna ensures high efficiency and precise tuning for the specific frequency it is designed to work with.

Multi Band Antenna

A multiband antenna, on the other hand, is designed to operate across multiple frequency bands. Unlike the single band antenna, one part of the multiband antenna is active for one band while another part is active for a different band. This design allows the antenna to cover a broader range of frequencies, making it more flexible for various applications. However, this flexibility comes with challenges in terms of impedance matching and radiation pattern.

Technical Differences

The primary technical differences between single band and multiband antennas arise from the number of frequencies they can work with and the resulting consequences. A single band antenna can only operate on a specific frequency, which ensures high efficiency and precise performance. In contrast, a multiband antenna must manage the interaction between different bands, leading to potential issues with impedance matching and radiation pattern.

Short Story

In a short story, a single band antenna is better than a multiband antenna. A multiband antenna can be a good choice when there is no other option, such as in situations where multiple frequencies need to be accommodated with limited space or resources. However, if high efficiency is required, a single band antenna is often the preferred choice.

Long Story

Upon closer examination, both single band and multiband antennas have their unique characteristics. An antenna operates with two oscillating standing waves: one for the current standing wave responsible for the H magnetic field and another for the voltage standing wave responsible for the E electric field. These standing waves are 90 degrees out of phase, resulting in field intensity proportional to the amplitude of the standing waves.

When the antenna is half the wavelength of the signal, it produces a peak on the current standing wave, which in turn results in peaks in the H and E fields. As the input signal wavelength is halved, the current standing wave breaks into two pieces or more, creating multiple peaks that effectively act as multiple smaller antennas. This phenomenon is the basis for the multi-band behavior of an antenna.

However, there are significant challenges with multiband antennas. First, efficient energy delivery is required without losses, as the impedance of the antenna changes with frequency. Transmitters and feed lines must match the antenna's impedance to deliver energy without losses. Second, the radiation pattern of the multiband antenna is affected by the interaction between different frequency components, leading to non-uniform patterns.

Solutions and Trade-offs

Several solutions can address the challenges posed by multiband antennas:

1. Multiple Single Band Antennas with Switching

Using multiple single band antennas with switching can allow operation on one band at a time, ensuring efficient energy delivery and avoiding the complexity of impedance matching. This approach, however, sacrifices flexibility in terms of immediate multi-band operation.

2. Harmonic Frequency Operation

Another solution is to use frequencies that are harmonics of each other, such as 144 MHz and 432 MHz. This approach minimizes changes in antenna impedance and reduces losses, making the system more efficient.

3. Antenna Tuner and Matching Devices

Antenna tuners at the feed point or discrete component matching can solve impedance matching issues but do not address the radiation pattern problem and may introduce losses, especially on UHF and higher bands.

4. Combined Multifunctional Radiators

Using multiple antenna radiators combined in different ways, each tuned to its own frequency, can solve impedance matching but may affect the radiation pattern. This approach is useful but results in a larger antenna.

5. Traps and Log Periodic Antennas

The use of traps, which are LC filters on the antenna body, can pass only the required frequencies, solving discrete band issues but introducing losses that reduce antenna efficiency.

6. Logarithmic and Fractal Structures

Log periodic and fractal structures have minimal changes in impedance when input signal frequency changes, making them suitable for high-frequency bands. Phased arrays combined with logarithmic structures offer even better matching and radiation pattern control, used in modern radar systems.

Advantages and Disadvantages of Multi Band Antennas

The advantages of multiband antennas include lower space requirements and fewer materials, making them suitable for short-range applications or high-gain antenna radiators like parabolic dishes. They are particularly useful for broadband or UWB ultra-wideband data applications at higher UHF, SHF, and higher bands. However, these antennas have several disadvantages, including increased complexity, inconsistent radiation patterns, and reduced efficiency compared to single band antennas.

In conclusion, while single band antennas offer high efficiency and specific frequency performance, multiband antennas provide more flexibility and can be more space-efficient. The choice between the two depends on the specific requirements of the application.