Planar Inverted-F Antenna | Definition, Use, Radiation

Planar inverted- F antenna or PIFA is popularly used in developing mobile devices. The antenna gets its name from being shaped like the letter F but facing downwards. PIFA has been derived from the λ/4 rectangular microstrip.

This has allowed it to be small in size and hence extremely useful to make devices that can be carried around. Keep on reading this article to learn more about what is PIFA, its advantages, uses, radiation pattern, and other interesting facts.

What is Planar Inverted-F Antenna?

It’s a type of microstrip antenna that was created by the usage of a shorting pin for improved performance. Shorting pins are effective in reducing the size and increasing the efficiency of an antenna as well as achieving multi-frequency resonance, gain improvement, and desired radiation pattern.

These antennas have a dielectric material on top of a metal ground plane mostly made of aluminum. On one end of the dielectric material, a microstrip line is placed on top. Next to it goes the microstrip antenna, which is the feed.

A shorting sheet of λ/4 width is attached to the ground plane from the feed. Thus, giving it the F shape. The following figure shows the structure of PIFAs.

Planar Inverted-F Antenna
Planar Inverted-F Antenna

Why Are Planar Inverted-F Antennas Important?

PIFA is used to produce mobile phone handsets commercially because of its many advantages over other antennas. For example, PIFA is more compact than a whip, helix, or even rod antenna. So, they can fit easily inside the housing of a mobile phone.

There is also lesser backward radiation if PIFA is used, which means the Specific Absorption Rate or SAR is lower. Due to these facts, a PIFA performs better than other antennas when it comes to building wireless technologies.

Another advantage that sets PIFA apart is that their gain isn’t limited to vertical or horizontal polarization. Instead, you get medium to high gain in both directions. 

When a wireless communication system doesn’t have a specific antenna orientation and different positions in the environment contain reflectors, this characteristic of PIFA comes in very handy.

Due to all these superior qualities a PIFA possesses, it is widely used and is very important in wireless communication.

Basics of Planar Inverted-F Antennas

Here are some of the basic features of planar inverted-F antennas that make them such a popular choice when it comes to designing mobile phones.

  • PIFA is a quarter-wave monopole antenna with a shorting pin for enhanced performance.
  • You should find that the two adjacent sides of a PIFA only add up to λ/4 at the expected resonant frequency.
  • It is a low-profile antenna, which means that the antenna is small in width and height when viewed from the side. As mobile phones are getting slimmer every day, this is a great feature.
  • PIFA has a nearly omnidirectional radiation pattern.
  • These antennas show a low SAR or specific absorption rate.
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Development of Planar Inverted-F Antennas

Planar inverted-F antennas are an evolved form of quarter-wave monopole antennas or half-wave patch antennas. In 1960, the first inverted-L antenna was created by folding a monopole antenna of λ/4 length. It was the stepping stone for the invention of PIFAs.

When a shorting pin was first introduced as the next step, the antenna created was called a shunt-driven inverted L antenna. That name didn’t stick for too long. They soon became known as inverted-F antennas.

This change allowed the low impedance value of inverted-L antennas to change to a more convenient and applicable value. After that, for an improvement in bandwidth, a planar element was used to replace the conducting wire parallelly placed to the ground. Thus, the first PIFA was developed.

Advantages of Planar Inverted-F Antenna

There are many advantages to using a PIFA, which is what has made it indispensable in designing modern portable tech gadgets. Here are the most significant advantages.

  • PIFAs have low backward radiation, which results in a lower specific absorption rate or SAR compared to most other antennas.
  • You can get both horizontal and vertical polarizations by using PIFAs.
  • It’s lightweight, conformal, low cost, reliable, and easy to design.
  • Easy to incorporate into modern smartphone designs.

Planar Inverted-F Antenna Radiation Pattern

PIFAs give off medium to high bandwidth. Their radiation pattern is omnidirectional, which means they transmit signals in every direction. They also showcase mixed polarization.

PIFAs are also great at dealing with changes in their immediate environment due to their short circuit arm’s link with PCB and ground plane. It means that while other antennas suffer, PIFAs can maintain their radiation pattern. There’s no change in frequency or loss of radiation power in less-than-ideal environmental conditions.

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Take a look at the following figure to see how a PIFA gives off radiation.

Planar Inverted-F Antenna’s Resonant Frequency

The resonant frequency of PIFAs can be manipulated by changing the design here and there. For example, the addition of an open slot can effectively reduce the resonant frequency due to the flow of current at the outer edge of the slot. It reduces the obtained frequency bands as well as antenna size.

You can also obtain dual-frequency by the use of slots in the planar element. The width of the rectangular planar element and short circuit plate also has an impact on the resonant frequency. Thinner short circuit plates reduce the resonant frequency range.

The site of the feed point also effectively controls the resonant frequency. Calculating the minimum reflection coefficient helps in analyzing the resonant frequency and the bandwidth of the PIFA.

Getting Impedance Matching On Planar Inverted-F Antenna

Adjusting the value of input or output impedance to get a specific value is known as impedance matching. This is usually done to get the maximum power transfer possible and minimize the signal reflections.

Here are some impedance matching techniques when it comes to the planar inverted-F antenna.

  • Position the shortening pin with the single feed inside the shaped slot. Experiment with the distance between the two to see where you are getting the optimum impedance.
  • Don’t use any additional components for impedance matching. It’ll increase the size of the antenna and make its use pointless.

Planar Inverted-F Antenna Applications

As we already mentioned, the main use of PIFAs is manufacturing mobile phones. Due to its ability to produce dual-band frequencies, this antenna can conduct a variety of services on wireless devices.

A PIFA array antenna is useful to meet the specific needs of 5G. This antenna is now at the center of 5G mobile communication technology due to its low cost, high directivity as well as lightweight structure, and simple design. 

Inverted-F antenna gain is also higher than other antennas, which makes them a good choice. Needless to say, the use of PIFA is indispensable for designing wireless devices.

Tips To Design Planar Inverted-F Antennas

Our planar inverted-F antenna design guide should help you make an efficient antenna with optimal output. 

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Tip 1

The biggest limitation of PIFA is that it has a narrow operating bandwidth. Using a shorting pin close to the feed reduces the antenna size a lot but it also has a negative effect on PIFA’s impedance bandwidth. You can minimize these effects by implementing the following guidelines.

  • The proportion of the ground plane has an effect on the bandwidth. A smaller ground plan results in a broader bandwidth. If you want to lessen the quality factor and increase the bandwidth at the same time, you can go with the insertion of slits at the edges of the ground plane.
  • Using a tick air substrate is also an effective way to reduce the quality factor as well as broaden the bandwidth.
  • Get parasitic resonators that have resonant lengths similar to the main resonant frequency.
  • Changing the distance of the two shorting pins by switching their locations.
  • Utilize stacked components.

Tip 2

If you want an optimal radiation pattern, placing the feed close to the edges of the ground plane is a good idea. Adjusting the position of the feed with respect to the ground plane has significant effects on the antenna’s performance.

Read more

PIFA Antenna

Final Words

PIFAs are high-performing antennas that come in very useful to build wireless devices due to their low SAR, low backward radiation, high gain, small size, low profile, and cost-effectiveness. These antennas are small enough to fit into devices internally.

Experimenting with PIFA designs to get the correct impedance that you desire can be quite fun for someone for antenna enthusiasts. Its structure is pretty simple and it won’t cost you much to design one of your own.

The only downside to using a PIFA antenna is its narrow bandwidth. But you can overcome it to a great extent if you just follow the tips we shared.

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