ul

[Emmanuel Des Meaux]

Microstrip patch antennas and arrays are probably the simplest antennas to design next to monopole and dipole antennas. These antennas are also easy to integrate into a PCB, so much so that they are commonly used in advanced systems like 5G antenna arrays and radars. These antenna arrays also follow a simple set of design equations in the fundamental mode and in higher order modes, so you can even design them without using a simulation tool.

Microstrip patch antennas are essentially open resonators. The antenna is placed above a ground plane, and field confinement between the patch antenna and the ground plane determines a set of eigenmodes in which the antenna can operate (similar to non-TEM transmission lines). The eigenmodes correspond to specific modal field distributions inside the resonant cavity created by the antenna, although these antennas are typically operated in the fundamental mode. An illustration of the field distribution around a patch antenna on a PCB is shown below.

Because this is an open resonant structure, it can radiate strongly when a mode is excited. Just like other resonant structures, the frequency of operation is easily tuned by adjusting the length and width of the patch antenna, as well as the height above the ground plane. The input impedance is then equal to the ratio of the electric and magnetic fields around the patch antenna.

The design of a microstrip patch antenna relies on the following equations. First, we have an effective dielectric constant for a given PCB substrate Dk value, which then determines the width and length of the patch for a given operating frequency. The design process goes as follows:

The main set of design equations for L and W assume we are operating in the (i, j, k) = (1, 0, 0) mode. The next highest order frequency involving L* determines a cutoff for the patch antenna due to its excitation from the edge.

By substituting the above equation for L into this equation, you can get a more complex equation relating frequency and W with h as a parameter. This can then be solved by hand, by graphing intersections, or using a random search application like differential evolution.

As I mentioned above, the input impedance looking into the antenna is equal to the ratio of the electric and magnetic fields. In the fundamental mode, the fields are nearly constant along the feedline width right at the edge, and the input impedance looking into the antenna is given by:

The calculator tool shown below will provide input impedance and the dimensions of a microstrip patch antenna given a desired operating frequency, substrate dielectric constant (Dk), and distance to the reference plane through the substrate (h).

Once the input impedance is known, the designer will need to match the input impedance at the feedline connection to the patch. Typical guides show the use of a quarter-wave impedance transformer, but these feedline sections will be comparable to the size of the antenna, so this might make the system unnecessarily large.

Because these patch antennas can have moderate Q-values, they can radiate efficiently over bandwidths up to about 10% of the carrier frequency as long as impedance transformers are not used for impedance matching. For broadband matching with bandpass filtering, a higher-order LC filter may be needed; this will be a topic for a future article.

One option for impedance matching is to use an inset, as shown in the image below. The line inset is designed to set the input impedance looking into the edge of the patch to a target impedance. This works by taking advantage of coplanarity between the antenna and the feedline, which produces some capacitance along the input section of the feedline. The feedline dimensions are shown below:

The inset feedline design relies on the equation below, which is used to determine the depth of the inset into the antenna patch. The inputs are a target input impedance, which will be equal to the impedance of the feedline into the patch antenna (typically 50 Ohms). The feedline will reach a certain depth into the antenna, and the depth to spacing ratio (D/S) will affect the input impedance. The required design equation relating the inset depth, antenna impedance, and feedline impedance is:

Note that there is a cos^4 dependence, which is contrary to most microstrip antenna inset calculators. Most calculators will list a cos^2 dependence, but this is a point of confusion as the cos^2 dependence applies to a probe-fed antenna. It only applies to an inset-fed antenna when D/L is large.

These antennas are very easy to design and implement, but they are also limited in their usage by available board area. Microstrip patches can be rather large because they rely on resonant excitation between the patch and the reference plane. This means that the microstrip patch size will be proportional to the wavelength of the signal that is being broadcast/received by the antenna.
A smaller alternative may be a printed microstrip antenna, such as a printed trace antenna or an inverted F antenna. The inverted-F is used in some popular MCU boards or modules, for example the ESP32 Ai-Thinker module shown below.

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 2500+ technical articles on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA). He previously served as a voting member on the INCITS Quantum Computing Technical Advisory Committee working on technical standards for quantum electronics, and he currently serves on the IEEE P3186 Working Group focused on Port Interface Representing Photonic Signals Using SPICE-class Circuit Simulators.

This page looks at the PCB Editor's support for differential pair routing - a design technique employed to create a balanced transmission system able to carry differential (equal and opposite) signals across a printed circuit board

This page looks at the PCB Editor's support for printed electronics - the ability to print the electronic circuit directly onto a substrate, such as a plastic molding that becomes a part of the product

This tutorial guides you through the complete design process using Altium Designer and a connected Workspace, from idea to generated manufacturing output released to the Workspace. Also explores features related to project management and collaboration

This page acts as a checklist to help you work out if your board is ready for routing, including how to find nets on the board, making sure relevant design rules are defined, and setting up the routing layers

This page looks at the Glossing and Retrace features in the PCB Editor. Where Glossing attempts to shorten the overall route length and reduce the number of corners, Retrace re-applies the preferred width and clearance requirements to an existing route

Low-profile, low-cost antennas support the operation of many modern communication systems. Microstrip patch antennas represent one family of compact antennas that offers the benefits of a conformal nature and the capability of ready integration with a communication system's printed circuitry. By using a straightforward transmission-line model, it is possible to accurately model and analyze microstrip-line inset-fed patch antenna designs. In addition, by applying a curve-fit formula, it is possible to locate the exact inset length needed for a 50-Ω input impedance.

The feed mechanism plays an important role in the design of microstrip patch antennas. A microstrip patch antenna can be fed either by coaxial probe or by an inset microstrip line. Coaxial probe feeding is sometimes advantageous for applications like active antennas, while microstrip line feeding is suitable for developing high-gain microstrip array antennas. In both cases, the probe position or the inset length determines the input impedance.

The input impedance behavior for a coaxial probe-fed patch antenna has been studied analytically by means of various models, including the transmission-line model and the cavity model, and by means of full-wave analysis.1-3 Experimentally and theoretically, it has been found that a coaxial-probe fed-patch antenna's input impedance exhibits behavior that follows the trigonometric function:

On the other hand, it has been found experimentally4 that on low-dielectric-constant materials, the input impedance of an inset-fed probe antenna exhibits fourth-order behavior following the function:

Fortunately, a simple analytical approach has been developed using the transmission-line model to find the input impedance of an inset-fed microstrip patch antenna. Using this approach, a curve-fit formula can be derived to find the inset length to achieve a 50-Ω input impedance when using modern thin dielectric circuit-board materials.

Figure 1 is a graphical depiction of an inset-fed microstrip patch antenna. The parameters εr, h, L, W, wf, and y0, respectively, are used to denote substrate dielectric constant, thickness, patch length, patch width, feed-line width, and feed-line inset distance. The input impedance of an inset-fed microstrip patch antenna depends mainly on the inset distance, y0, and to some extent on the inset width (the spacing between the feed line and the patch conductor). Variations in the inset length do not produce any change in resonant frequency, but a variation in the inset width will result in a change in resonant frequency. Hence, in the following discussion, the spacing between the patch conductor and feed line is kept constant, equal to the feed line's width; variations in the input impedance at resonant frequency with respect to inset length will studied as a function of various parameters.

c01484d022