Thursday night is New Tech night at The Nines hotel in Portland. At the last meeting, I realized that all the hardware engineers were building wireless Bluetooth and GPS devices. One can’t help but marvel at how a small strip of copper on a PCB allows us to connect so many different devices and solve such unique problems.
Inverted-F antennas are used in many mobile devices and a popular choice compared to chip antennas. Applications include mobile phones and tablet computers that communicate via GPS, GSM, Bluetooth, ZigBee, and Wi-Fi. An inverted-F antenna can also be designed as either a wide-band or multiband antenna, giving your device multiple capabilities.
Compared to chip antennas, microstrip inverted-F antennas have higher efficiency and linear gain than a ceramic chip antenna. Their bandwidth is also tunable based on their geometry and dielectric contrast. This allows an inverted-F antenna to be carefully designed to meet a specific wireless application and communication protocol.
Wireless card with an integrated antenna module
Inverted-F antennas are typically planar and printed directly on the PCB. The resonant frequency is determined by the microstrip dimensions, the antenna aspect ratio, and the contrast between the substrate and microstrip dielectric constants. A larger dielectric constant redshifts the resonant frequency in 2.4 GHz antennas. Matching the length of the long leg to a quarter or half wavelength creates a monopole or dipole antenna, respectively.
Another variation is the meandered inverted-F antenna (MIFA). This antenna is very useful when there is a small amount of available board space. The long leg of the antenna can be meandered to reduce its footprint while still using the required conductor length. This antenna is the smallest inverted-F antenna solution for applications requiring wireless communication at 2.4 GHz.
Designing and Printing An Inverted-F Antenna
Adjusting the geometry of the inverted-F antenna allows you to select the resonant frequency, the radiation pattern, and its bandwidth. Each of these parameters is very important in different applications. For example, Bluetooth and Bluetooth Low Energy (BLE) require antennas to have very narrow bandwidth, as specified by industry standards.
Access the cloud and more with your wireless device
In addition to antenna dimensions, the resonant frequency of a planar inverted-F antenna also depends on the dielectric constant of the board material. The board material can also affect the efficiency and gain at some frequencies. FR-4 is a common PCB substrate material, but it may be the best choice for applications that operate at 2.4 GHz. Some radiated and received power can be lost to the substrate, which will limit the operating range of the device.
Some wireless devices may need to operate in extreme environments with very high or low temperatures. The antenna will expand and contract as the temperature changes, which changes the geometry of the antenna. While geometry changes are unavoidable, the chances of blistering and delamination can be reduced by finding the closest match between the volume expansion coefficients and the thermal conductivities of the metal traces and the PCB substrate material.
Power and Ground
The two arms of the inverted-F antenna form the power and ground connections. The lower arms of the “F” is connected to the power plane, and the upper arm provides the short to the ground plane using vias.
The ground plane should run up to the edge of the two arms of the antenna with a via fence placed along the boundary of the ground plane. The ground connection can then be made directly through the via fence at the edge of the ground plane. Placing a via fence along the entire edge of the antenna ground plane can eliminate radially propagating RF waves from the edge of a board.
The size and shape of the ground plane will affect the radiation pattern.PCB trace antennas are ground plane dependent, as they must have a ground plane to be effective. Impedance matching and the PCB antenna performance are affected by the geometry of the ground plane relative to the antenna.
The last step in placing an inverted-F antenna is impedance matching using a capacitor and resistor. For a single band antenna, the required capacitance and inductance for matching can be calculated rather easily. Matching a dual-band inverted-F antenna requires an iterative process or a simulation tool. Matching one band affects the matching of the other band, and properly matching both bands at once requires some prior experience.
PCB design software like Altium Designer makes it easy to add wireless capabilities to your device and design the right antenna for your application thanks to its range of layout tools. With smart design rule checking, auto-interactive routing potential, and analysis and simulation tools, your designs can come out flawless.
To learn more about how Altium Designer can help you design your next wireless device, talk to an Altium expert today.
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