Microstrip Patch Antennas for Mobile Devices: Design and Implementation

Microstrip patch antennas are commonly used in mobile handsets due to their simple design, low cost, small size, and light weight. These antennas don’t require much extra space and can even be etched directly onto the mobile device’s PCB.

The image above shows a typical mobile microstrip patch antenna design. Microstrips are constructed using a dielectric substrate, with a ground plane on one side and metal conducting strip lines on the other.

Because a microstrip has air on one side and a dielectric material on the other, a pure Transverse ElectroMagnetic (TEM) mode doesn’t exist.

These patch antennas can be manufactured using photo-lithographic techniques, after which other RF and microwave components (both active and passive) can be assembled.

Patch antennas are widely used in cellular mobile terminals for GSM, CDMA, and LTE networks. Let’s look at the design of a DCS band microstrip patch antenna used in mobile devices.

Mobile Microstrip Patch Antenna Design Example

Let’s design a patch antenna with the following specifications:

• Center Frequency (Fo): 1800 MHz
• Dielectric Constant (εr): 11.9
• Substrate Height (h): 1.5 mm

Here’s how we can calculate the dimensions:

• Calculate the Width (W):

  Using the equation: W = c / 2*Fo*((εr +1)/2) 0.5 (where c ≈ 3 x 10^8 m/s)

  This gives us: W = 0.03293 meters = 32.93 mm

• Calculate the Effective Dielectric Constant (εreff):

  Using the equation: εreff = ((εr +1)/2)+((εr -1)/2)*[1+12*h/W] -0.5

  This gives us: εreff = 10.8323

• Calculate the Effective Length (Leff):

  Using the equation: Leff = c/ 2*Fo*εreff 0.5

  This gives us: Leff = 0.025 meters = 25 mm

• Calculate the Length Extension (ΔL):

  Using the equation: ΔL= 0.412*h* (εreff + 0.3) (W/h + 0.264) / (εreff - 0.258) (W/h + 0.8)

  This gives us: ΔL = 6.3528 x 10^-4 mm

• Calculate the Actual Length (L):

  Using the equation: L = Leff - 2*ΔL

  L = 25 mm - 2 * (6.3528 x 10^-4 )

  L = 24.9987 mm

• Calculate Ground Plane Dimensions (Lg and Wg):

  Lg = 6h + L = 6 * (1.5) + 24.9987 = 33.9987 mm

  Wg = 6h + W = 6 * (1.5) + 32.93 = 41.93 mm

• Calculate Feed Point Position (Xf, Yf):

  The center of the patch is typically taken as the origin. The feed point is determined relative to this origin. The feed point should be positioned where the input impedance is approximately 50 Ohms at the resonant frequency.

  Finding this location typically involves trial and error, testing various points. The point where the return loss is the most negative is considered the optimum feed point. In this specific patch antenna design, Yf is set to zero, and Xf is determined through this trial and error process.

Conclusion

This antenna design can be simulated and optimized using RF and microwave antenna design software like IE3D, AN-SOF, and CST. This mobile microstrip patch antenna design can also be adapted for other frequency bands using a similar process.