The lack of separation between antennas causes interference issues.
Antenna isolation is the technique of separating antennas that coexist so that there is only acceptable levels of interference between systems.
S-parameters S21 and S12 describe the effect on port 2 due to port 1 and vice versa and determine the insertion loss and isolation between the antennas.
Antenna separation leads to fewer interference issues
As a result of the expansion of wireless technology, there is a proliferation of antennas in smart devices. For example, there are several transmitter and receiver antennas in a smartphone. In certain devices, different antennas might operate under the same bandwidth. In such systems, there is a high probability for transmitted power to interfere with the receiver signal. This interference can cause the loss of data, signal degradation, or circuit component failures.
The consequences of tight coupling between antennas are aggravated by the trend of compact sizing in smart devices. For this reason, it is essential to include antenna isolation in such devices. Antenna isolation measurements are carried out using specialized RF equipment. Large values of antenna isolation measurements indicate good antenna separation, meaning fewer interference issues.
Why Is Antenna Isolation Important?
Multiple antennas are often embedded in a single device. For example, you can find GPS and WiFi antennas in a smartphone functioning simultaneously. The crowding of antennas in a single unit can overdrive the receiver signal strength or degrade the receiver signal quality. The lack of separation between antennas causes different interferences in the form of direct radiation, enclosure resonances, waveguide mode excitation, and noise coupling. Providing sufficient antenna separation can help mitigate the following issues:
Transmitter noise - High-density communication generates broadband RF noises in the transmitter, called sidebands. The sidebands generated in the exciter stage get amplified in further stages until they reach the output stage. Noise levels in the transmitter can be worsened by the type of modulation as well.
Receiver desensitization - Receiver desensitization occurs when the receiver gets overloaded by off-frequency signals. The strength of the signals is so high that the receiver becomes dysfunctional. The receiver stops responding to any signals under these conditions.
Intermodulation issues - Receiver designs sometimes overload the front-end of the system, leading to intermodulation distortion. Receiver signals are mixed with unwanted interference signals, generating intermodulation distortion products in the receiver.
How to Achieve Antenna Isolation
Antennas can be isolated in several ways. Some antenna isolation methods are:
Physical separation - Physical distance between the antennas is increased to prevent interference. This kind of antenna isolation is achieved through the vertical or horizontal separation of antennas.
Antenna pattern optimization - The correlation coefficient between the antenna patterns can be reduced by directing the peak radiation from the antenna in different directions.
Polarization - The polarization of the antenna generally allows the antenna to either receive or transmit signals with a particular polarization. Using different polarization minimizes the interference between antennas.
Filtering - When physical separation, polarization, and orientation are not sufficient, more antenna isolation can be ensured by incorporating RF filters. RF filters minimize the received power of unwanted frequencies.
Antenna Isolation Measurements
The close proximity of the antennas and the co-site interference can be identified using antenna isolation measurements. In general, antenna isolation measurements can be taken as the power transfer from one antenna (transmitter) to another (receiver). The effectiveness of the antenna isolation can be measured in terms of loss in dB. The ideal value of an antenna isolation measurement is a high value. Engineers use antenna isolation measurements to plan co-site interference mitigation methods and assess filtering requirements.
How to Measure Antenna Isolation
S-parameters are useful when it comes to antenna isolation measurements. To measure the isolation between two antennas, two-port network S-parameters can be used. The S-parameters S21 and S12 describe the effect on port 2 due to port 1 and vice versa. These parameters determine the insertion loss and isolation between antennas. It is also possible to measure the isolation between co-located antennas using a multiport vector network analyzer.
Antenna isolation measurements are significant in reducing the engineering time required for introducing antennas into a system design. You can design antenna-based systems using Cadence’s PCB design and analysis tools. These layout and simulation tools help you to isolate antennas to minimize interference in devices.
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