Currently, dual-band Wi-Fi modules support communication over both the 2.4GHz and 5GHz frequency bands. In the circuit design of these modules, there is an issue of antenna sharing between the two Wi-Fi frequency bands. Specifically, the RF circuits for both the 2.4GHz and 5GHz bands share the same antenna, with a switch used to alternate between the two bands. The following issues arise:
1. Sharing Logic Issue
This issue is similar to the antenna sharing problem between Bluetooth and 2.4GHz Wi-Fi. When both the 2.4GHz and 5GHz frequency bands are transmitting data simultaneously, the data is temporarily stored in a buffer and sent out sequentially through the switch. However, for receiving data, if both frequency bands have incoming data at the same time, the antenna switches to the 2.4GHz band, causing any 5GHz data to be lost. This is a critical problem—how is this data loss handled?
2. Performance Issue of Shared Antennas
The performance of the antenna for both the 2.4GHz and 5GHz bands is a concern. The antenna length is related to the wavelength, and since the 5GHz frequency is more than double that of 2.4GHz, the optimal RF performance for the two frequencies would require significantly different antenna lengths. How can a single shared antenna ensure optimal performance across both frequencies?
Sharing Logic Issue:
For dual-band Wi-Fi antenna sharing, the logic is essentially the same as the antenna sharing between Wi-Fi and Bluetooth: regardless of how the antenna is shared, time-division multiplexing (TDM) is used to achieve this. At any given moment, either the 2.4GHz band is using the antenna for Wi-Fi or the 5GHz band is. Both the 2.4GHz and 5GHz bands have their own RF circuitry, and the switch (typically an SPDT switch) can only connect one frequency at a time. Since the antenna is time-multiplexed, the throughput of each frequency is inevitably affected.
For instance, with an equal time-sharing scheme, each frequency (2.4GHz and 5GHz) only has access to the antenna for half the time, which significantly impacts throughput for each band. In particular, for RF reception, if the signal arrives at the antenna but the SPDT switch hasn’t yet switched to the matching frequency, the packet will be lost. The data can only be recovered when the switch switches to the correct frequency and a retransmission occurs. Therefore, such frequent switching will inevitably affect the RF transmission and reception throughput.
Given that time-division multiplexing impacts RF throughput, if the application requires high communication speeds, it is important to consider the potential drawbacks of antenna sharing for the 2.4GHz and 5GHz bands. This is why most routers in antenna design use separate antennas for the 2.4GHz and 5GHz bands. With dedicated antennas for each frequency, the corresponding RF circuits can provide stable service for each band, avoiding the need for frequent switching.
Additionally, for IoT applications using dual-band Wi-Fi in STA (Station) mode, the device usually connects to either the 2.4GHz or 5GHz band at a time. In these cases, there is no antenna sharing issue. For example, in IP camera products, during network configuration, the camera selects either 2.4GHz or 5GHz to connect to the router. After the initial selection, the camera will consistently use the chosen frequency band for subsequent connections. In such cases, the SPDT switch simply switches to the configured band without needing to frequently alternate between frequencies, so the antenna sharing logic issue is avoided.
Performance Issue of Shared Antennas:
The performance issue of dual-band antenna sharing is essentially a multi-frequency design challenge. A well-designed multi-frequency antenna can support communication on several bands with a single antenna.
The issue is more pronounced in mobile LTE applications, where the antenna must support multiple bands across a variety of frequencies. Modern smartphones are designed to support all LTE bands, requiring internal antennas that can handle diverse frequencies. This is more complex than supporting just 2.4GHz and 5GHz bands for dual-band Wi-Fi.
Designing an antenna that can support multiple bands and maintain good performance across each band is the core challenge of multi-frequency antenna design.
Common multi-frequency antenna design techniques include resonance branching, frequency multiplication, and parasitic branching. The following will focus on the Resonance Branching Method.
Resonance Branching Method:
In resonance branching, each frequency band has its own separate radiation branch. Since the branches are independent, tuning the resonance frequency of one branch does not affect the others, simplifying the design and adjustment process.
For example, the diagram below shows a standard dual-band Wi-Fi antenna design. The longer branch (L1+H) is for the 2.4GHz band, while the shorter branch (L2) is for the 5GHz band. These two antennas are independent and can be adjusted according to the respective frequencies.
The simulation of this antenna’s performance at 2.45GHz and 5.5GHz shows good impedance and bandwidth characteristics.
For antennas designed using the resonance branching method, the performance is optimal when supporting two frequency bands. However, when the number of branches exceeds three, interference between branches can increase, leading to a decline in performance for each band. Therefore, this method is ideal for dual-band Wi-Fi applications, where only two frequency bands need to be supported.