Demonstrating antenna diversity, Part 2: The PIFA

Antennas come in a broad range of sizes, styles, and configurations to meet frequency, bandwidth, directivity, and many other objectives; the PIGA and Yagi antenna are two different yet widely used versions.

Part 1 of this three-part series set the stage by explaining how many of today’s antennas are based on one of two concepts. Part of the solution to the antenna dilemma, whether single or multiple, is to look at configurations that offer new options for designers which are based on monopoles or dipoles but are not constrained by their basic form factor.

One such antenna design is the PIFA. What is a PIFA? Start with the inverted-F antenna (IFA) – proposed in 1958 – which is the third step in the evolution away from the basic monopole antenna and a variant of the patch antenna (Figure 1). (From the side, it looks somewhat like the letter “F,” hence the name.)

In this arrangement, the monopole element runs parallel to a ground plane and grounded at one end, and the antenna has a low impedance on the order of a few ohms (in contrast, the classic base-fed λ/4-wavelength monopole has an impedance of 36.5 Ω.)
The antenna feed is placed at an intermediate point a short distance from the grounded end. By adjusting the placement of the feed and other “tweaks and trims” its impedance can be made to match the power amplifier (PA) feed, so it is an efficient radiator without the need for additional matching components.

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