Inverted-F PCB Antenna Design

Key Takeaways

• Inverted-F PCB antennas are popular for wireless communication, offering compactness and impedance-matching capabilities.

• Planar inverted-F antennas (PIFAs) are commonly used in wireless circuitry and can integrate RF components.

• Meandered inverted-F antennas (MIFAs) allow for antenna implementation in constrained spaces.

Simple inverted f-antenna model

The inverted-F antenna is a popular choice for wireless communication applications, particularly at UHF and microwave frequencies. Especially in RF PCBs, printed antennas are highly favored due to their low-profile nature, allowing them to fit within the confines of various devices’ cases. 

Because inverted f antennae can be impedance matched by the designer to the feed circuit, they enable efficient power radiation without additional matching components. For this reason, inverted-F antennas are extensively used in handheld wireless devices where space is limited, such as mobile phones and tablet computers utilizing wireless technologies like GSM, Bluetooth, and Wi-Fi. Read on as we discuss inverted-f PCB antenna design.

Inverted-F PCB Antenna Basic Design Structure and Function

An Inverted-f antenna consists of a monopole antenna running parallel to a ground plane and grounded at one end. The antenna is fed from an intermediate point a distance from the grounded end.

Inverted-F antennas, in general, have narrow bandwidths. A wider bandwidth can be achieved by lengthening the antenna, which increases its radiation resistance. Another solution is to place two antennas in close proximity of each other. This works because coupled resonators have a bandwidth wider than the bandwidth of either resonator on its own. Most of the techniques for producing multi-band antennas are also effective at broadening bandwidth.

Inverted-F PCB Antenna Design Variations

The planar inverted-F antenna (PIFA) is a commonly used antenna in wireless circuitry, particularly in microstrip implementations. One of the major advantages of PIFA is its ability to implement necessary distributed-element RF components, such as filters while maintaining cost-effectiveness through mass production methods similar to printed circuit boards.

A printed inverted-F antenna is typically designed in the classic inverted-F shape, usually positioned to one side of the circuit board with the removal of the ground plane beneath the antenna. Another approach is the utilization of a modified patch antenna known as a shorted patch antenna. In this configuration, one edge of the patch or an intermediate point is grounded through grounding pins. This approach operates on the same principle as an inverted-F, with the F shape visible from a sideways perspective. However, the antenna element is significantly wider in the horizontal plane. The shorted patch antenna offers a wider bandwidth compared to the thin-line type due to its larger radiation area.

Both the thin-line and shorted patch antennas can be printed on the same printed circuit board as the rest of the circuitry. However, it is common for them to be printed on their own board or on a dielectric material attached to the main board. This arrangement allows the antenna to be suspended and effectively isolated from the ground plane, either by being at a greater distance or by utilizing a more suitable dielectric material for optimal RF performance.

In some configurations of the shorted patch antenna, the shorting pin is placed as close to one corner as possible, while the feed pin is positioned relatively close to the shorting pin. 

Another inverted-F pcb antenna design variation is the meandered inverted-F antenna (MIFA). When there is limited board space available to extend the antenna to the required length, the antenna can be meandered or folded to reduce its height while still preserving its designed electrical length. This allows for efficient antenna implementation even in constrained spatial conditions.

Inverted-F PCB Antenna Design Tips

The following are tips for optimal inverted-f PCB antenna designs:

• The feed arm of the antenna should be connected to a 50 ohm microstrip transmission line for proper impedance matching.
• The shorting arm of the antenna must be securely grounded, preferably through the use of ground vias.
• The antenna design requires the absence of a ground plane beneath it.
• It is recommended to cover the antenna with soldermask for protection and enhanced performance.
• Simulations have indicated that increasing the width of the shorting arm or mitering the bend in the shorting arm does not provide any benefits.
• There should be no copper pour below the antenna, allowing for nearly omnidirectional radiation around the longer leg of the antenna, where the current is non-zero.
• The ground plane should have a width equal to or greater than the length of the inverted-F antenna, and its height should be at least lambda/4. If the ground plane's height is smaller, the bandwidth and the efficiency of the antenna will decrease.
• The height (H) of the antenna itself should be a small fraction of a wavelength. The radiation properties and impedance are not significantly affected by this parameter.

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