Wireless Technology
Reliable antennas for smartphones
Key to the ability of modern smartphones is still the antenna. Here, Jiangsheng Zhou, principal engineer for the wireless terminal design team, Jiang Nan Electronic and Communication Research Institute, explains the current challenges faced with antenna design
A
t its introduction in 1983, the first commercially available mobile phone was seen as more of an expensive novelty than a necessary tool. Costing the equivalent of more than $9,000 today, it weighed almost two pounds and was notoriously bulky with its rubber whip antenna extending more than five inches above its ten inch body. Despite such hefty dimensions, its battery provided only 30 minutes of talk time before requiring a recharge. Over the past three decades, mobile phone technology has evolved through many generations, with a rapidly expanding set of features contained within a smaller, lighter and far less expensive package. Modern mobile devices are more like highly portable personal computers than phones. However, a large factor in a device’s ability to provide the freedom to communicate from almost anywhere is still its antenna. Designing small profile, multi- band and wideband internal antennas with a simple structure has become a necessary challenge for the mobile phone industry. Phone manufacturers need to keep production costs low while continuing to
produce devices with more options. All of these varied capabilities affect one another, increasing the inherent challenges in antenna design.
R&D
At the Jiang Nan Electronic and Communication Research Institute in China, a team of engineers used ANSYS HFSS to design a small internal multi-band antenna that can operate across eight different frequency bands. For a wireless device to function properly as both phone and computer, it needs to send and receive wideband (824MHz to 2,690MHz) and multi-band (GSM 850MHz, 900MHz, 1,800MHz and 1,900MHz; UMTS 1,920MHz to 2,170MHz; WLAN 2,400MHz; and WiMAX 2,300MHz, 2,500MHz) signals. WLAN, WWAN and WiMAX are the most prevalent types of WiFi networks, while WLAN 2,400MHz is the frequency required for Bluetooth connectivity. GSM and UMTS bands are used for global mobile phone communications, including 2G, 3G, 3.5G and 4G LTE.
The team investigated various antenna designs using simulation. They varied the length and width of the radiating, coupling and inductive shorting strips, as well as the shorting and feeding pin positions. Changing these dimensions for the HFSS simulation led to significant variation in the scattering parameters (S-parameters), specifically the return loss (S11). Return loss can be used to judge antenna performance at different
frequencies. The team optimised the dimensions for return loss values at
frequencies between 824MHz and 2,500MHz.
Additional methods for
Modern mobile devices are more like highly portable personal computers than phones
30 July/August 2015
determining antenna performance include current distributions, far-field patterns, gain and antenna efficiency. The team simulated the current distributions and far-field patterns at 900MHz,
Components in Electronics Mobile phone technology has evolved rapidly since their introduction in the 1980s
1,900MHz and 2,600MHz. Strong surface current distributions on the radiating and shorting strips indicated that these strips are the main contributors for the lower frequency band operation. The long coupling strip controls the resonant mode at 1,900MHz, while the short coupling strip controls the resonant mode at 2,600MHz. Smooth variations in vertical polarisation in all directions indicate good antenna coverage. The radiation efficiency varied between 50 and 64% at lower frequencies to a high of 62 to 77% at higher frequencies. Antenna efficiencies greater than 45% are sufficient for practical mobile phone applications. The final optimised design required a modest 15mm x 45mm area on a small PCB that did not include a ground space at either the top or the bottom. The antenna design consists of a radiating strip, an inductive shorting strip and a coupling strip. The radiating strip is the longest, located on the top edge of the PCB’s no-ground space, while the inductive strip is only 0.5mm wide and is short circuited to the system ground via a hole in the PCB. The coupling strip is located between radiating and inductive strips. The coupling strip provides an excitation voltage to both the radiating strip and the inductive shorting strip. Together, the radiating and inductive shorting strips produce low band GSM resonance. The coupling strip has two branches that can generate a wide upper band range of 1,710MHz to 2,690MHz.
Testing and verification To confirm the HFSS results, the team tested the antenna using a vector network analyser (VNA) and an anechoic chamber.
With the antenna printed on the PCB, the team excited the antenna using a 50ohm coaxial feed at the antenna feed point. To simulate realistic conditions, there was also a 1mm thick plastic housing with a 1mm gap between the housing and the PCB, representing the phone’s case. The simulated return loss compared well with the actual return loss determined through physical experiments. Before engineers started using simulation tools, they had to build prototypes and then test each design variation individually using the same set-up required for confirming the simulation results. This was a highly inefficient method that required weeks for each evaluated design, allowing little time for optimisation and testing of innovative changes. By incorporating ANSYS HFSS into the design process, the team explored several antenna configurations and optimised the design before building any prototypes, which saved 20% in material costs. Engineers were able to run 20 different simulations to analyse various antenna configurations and optimise the final design in just three days. The antenna performance results
predicted by ANSYS HFSS for impedance matching showed close agreement with the experimental data obtained through the engineering team’s experiments using a vector network analyser. Based on both simulated and experimental results, the design is suitable to be directly printed on the system PCB of the device. Combined with its low fabrication costs, this makes the antenna a very attractive option for mobile phone applications.
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