The Brainstorm: Broadband Antenna Design
What current trends in broadband antenna design help boost broadband capacity for mobile devices?
Edgardo Menendez, Field Applications Engineer, AVX
Recent advances in antenna design and manufacturing have enabled manufacturers to embed antennas into printed circuit boards, providing significant size and cost reductions. PCB antennas have become an essential element in today’s RF mobile applications, in which size and cost are always on the chopping block.
A popular PCB antenna design is the planar inverted F antenna (PIFA), which is often incorporated into the housing of today’s size-constricted RF mobile devices. The PIFA is constructed by shorting one end of a rectangular patch antenna to ground to reduce its size and then adding a feedline in the middle of the antenna. The antenna sits on top of a ground plane, which acts as a barrier between the emitted electromagnetic waves and the user, effectively reducing the component’s specific absorption rate (SAR) in cell phone applications, which may make users vulnerable to electromagnetic radiation.
PIFA antennas exhibit high gain in vertical and horizontal polarizations. Variances in cell phone designs leave the potential for antenna mismatch, which results in losses in efficiency and gain. Furthermore, the construction of the PIFA antenna reduces its capacitance while increasing its inductance by adding the shorting pin connected to ground. To compensate for the loss in capacitance, an RF capacitor can be placed in parallel between the open end of the antenna and the feedline to balance the antenna for maximum gain.
Careful capacitor selection is important in antenna design to solve issues due to mismatch. Thin film capacitors are manufactured using a tightly controlled semiconductor process that allows for accurate control of electrode and dielectric thickness. Primary advantages of thin film capacitors include an extremely repeatable frequency response, tolerance as tight as ±0.01 pF, high Q, and low ESR. For example, values from 0.05 to 68 pF are easily achievable with increments of 0.01 pF. Thin film capacitors provide consistent matching from lot-to-lot, over time, and across a broad temperature range.
Paul Tornatta, Vice President of Product and Customer Engineering, Cavendish Kinetics
First, let’s understand what antenna issues exist in a broadband world. Broadband means more data, and the wireless operators have adopted LTE as a standard, since spectral efficiency permits more data in the same amount of frequency. Using LTE raises two issues: 1. Antenna efficiency needs to be very good to take advantage of the spectral efficiency. 2. The antennas must cover the LTE bands, which are both above and below the 3G bands (you need to cover a very wide frequency bandwidth).
The equation shows the relationship of antenna volume (a3) to wavelength (λ), efficiency (η), and fractional bandwidth (∆f/f). Given how precious real estate is inside today’s smartphones, there is no room to design anything but narrow band antennas. So how can wide frequency bands be covered with a narrow band antenna?
To address this issue, there are two important trends. The first one is tunable impedance matching, which takes a narrow band antenna and changes the match to lower the VSWR when various frequency bands are used. This technique improves the power transfer into the terminals of the antenna, but does nothing to help the antenna radiate more efficiently. A better use of tunable impedance matching is to keep the VSWR within a smaller range regardless of the environmental factors of changing usage models, which is a much more difficult problem.
The second emerging trend is antenna frequency tuning, which changes the resonance by tuning in the antenna aperture so that the peak radiation efficiency is at the frequency currently being used. Ironically, the best broadband antenna solution is a narrow band antenna with the ability to tune the resonance when needed. The antenna doesn’t need a broadband response all of the time. By narrowing the instantaneous bandwidth, the efficiency of the antenna can be improved and its size will fit today’s form factor for mobile devices.
By applying frequency tuning techniques, radiation efficiency can be improved and the additional LTE frequencies can be covered.
This article originally appeared in the November/December print issue. Click here to read the full issue.