ProAnt PIFAs - Planar Inverted-F Antennas
In today’s electronics market, wireless connectivity has become an increasingly important feature. The result of this is an increased demand for components associated with wireless communication, including antennas.
In this blog, we will focus on one type of antenna, the Planar Inverted-F Antenna (PIFA), which can be used in a variety of wireless applications. The PIFA is a specialized monopole antenna (with a short circuit arm) originating from the inverted-F antenna (Figure 1). The PIFA (Figure 2) is a type of inverted-F antenna that has a top plate instead of a single wire.
Typical Inverted-F Antenna (left) and Typical PIFA Antenna (right) (Images source: ProAnt)
Patch antennas and PIFAs are similar in that they are both inherently narrow banded. However, incorporating different techniques, such as parasitic ground elements and cutting slots in the plate, can widen a PIFA’s bandwidth. Additionally, using these techniques allows for changing the size of the top plate which results in smaller antennas and less board space required.
Figure 3: Using a PIFA allows an RF-module and other components to be mounted underneath the antenna. (Image source: ProAnt)
You can also modify the size of the PIFA by using top loads. This does have consequences though. To reach lower frequencies, small capacitances can be added to the far end of the antenna. However, this will degrade the radiation capabilities. Using this method can help achieve an optimal balance between size and performance for a certain application.
With the antenna being positioned parallel and above the circuit board, where the mounting pins are positioned is the only board space required. This is a big advantage of using PIFAs. Additionally, there is no required clearance toward the ground or other metal. This means other components can be mounted underneath the antenna on both layers of the PCB (Figure 3).
The short circuit arm, previously mentioned, acts to impedance match the antenna to the rest of the circuitry. Monopoles, being naturally capacitive, require an inductance to be added so that the antenna can be matched to the optimal impedance (usually 50 Ω). The short circuit arm provides this impedance matching inductance. The result of this is that fewer passive components are required to match impedance. With fewer components, there are less parasitic losses and thus a higher radiation efficiency.
Figure 4: PIFA Radiation Pattern and Polarization Examples (Image source: ProAnt)
Depending on the application, PIFAs tend to exhibit a moderate to high bandwidth and a radiation pattern that is omnidirectional with mixed polarization (Figure 4). Additionally, PIFAs are inherently robust in regards to environmental changes due their strong connection to the PCB GND plane through the short circuit arm. The result of this is that PIFAs will tend to sustain their efficient radiation patterns while other antenna types show a tendency to drift in frequency and lose radiation capabilities when encountering sub-optimal environmental changes (e.g., when operating near metal).
Antennas are required for a myriad of applications, both fixed and mobile, with a variety of communication protocols and frequencies. Antenna technology choice can be critical for stable wireless connectivity.
While certain antenna types are typically only good for applications where radiation direction is constant and always known (e.g., ceramic patch antennas), PIFAs offer more flexibility. PIFA technology offers a very omnidirectional radiation pattern with a lot of mixed polarization. This proves to be very useful in fixed applications, but can prove particularly beneficial with mobile devices that are used in ever-changing environments, such as wearable devices, due to the fact that PIFAs stay very stable no matter what the surrounding environment may be.
1 – ProAnt Technical note 2016-08-23 – Planar Inverted-F Antennas (PIFA)