A COMPACT FOAM-BASED UHF PIFA Shashank Kulkarni and Sergey MakarovECE Dept, WPI, 100 Institute Rd., Worcester, MA 01609

IntroductionIntroduction Modeling work is presented on the design of a low-cost linearly-polarized broadband PIFA operating in the UHF band (420-480 MHz). Extensive parametric simulations

using Ansoft HFSS are performed in order to optimize the antenna’s performance. High-density polystyrene foam is employed as a dielectric substrate and support for all antenna prototypes. The metal sheets are made of copper foil. To minimize the antenna dimensions, a tapered patch with slots and capacitive loading is used. The fabricated antennas have bandwidth measured in excess of 17%.

PIFA (Planar Inverted-F Antenna) OverviewPIFA (Planar Inverted-F Antenna) Overview PIFA is a quarter-wavelength open-short microstrip resonator with a dominant quasi-TEM mode.

Typical impedance bandwidth of a conventional PIFA is about 4% for a small ground plane and reaches 8% when the length of the ground plane is on the order of a wavelength [1].

Some methods to increase the impedance bandwidth whilst keeping the antenna small include tapering the patch [2], capacitive loading of the open end [3], and using slots for a longer current path [4].

[1] M. C. Huynh and W. Stutzman, “Ground plane effects on planar inverted-F antenna performance,” IEE Proc.-Microw. Antennas Propag., vol. 150, no. 4, pp. 209-213, Aug. 2003.

[2] B. Kim, J. Park, and H. Choi, “Tapered type PIFA design for mobile phones at 1800 MHz,” Vehicular Technology Conference, vol. 2, pp. 1012-1014, April 2005.

[3] C. R. Rowell and R. D. Murch, “A capacitively loaded PIFA for compact mobile telephone handsets,” IEEE Trans. Antennas and Propagation, vol. AP-45, no. 5, pp. 837-842, May 1997

[4] B. Kim, J. Hoon, and H. Choi, “Small wideband PIFA for mobile phones at 1800 MHz,” Vehicular Technology Conference, vol. 1, pp. 27-29, May 2004.

Design a UHF PIFA with the following characteristics: Large bandwidth (> 10%) centered around 440 MHz Low gain and widest possible beamwidth (both E/H-planes) Linear polarization Low phase distortion over the band with respect to antenna orientation Small size and conformal shape (wearable) Low cost and simple prototyping

Problem statementProblem statement

Reducing PIFA sizeReducing PIFA size

Use a scaled tapered PIFA as a starting point.

Capacitively load the open end of the antenna resonator as described in [3]. The capacitive load is formed by folding the open end of the patch toward the ground plane and adding a plate (parallel to the ground plane) to create a parallel-plate capacitor.

Introduce cutting slots as described in [4] to increase the electrical length.

Initial approach – non-optimized PIFAInitial approach – non-optimized PIFA Use a scaled antenna prototype adopted

from the family of GSM PIFA - a tapered PIFA at 1.8 GHz from [4] as a starting point.

Fine tune feed/shorting stub positions and the antenna geometry to a proper impedance bandwidth (Ansoft HFSS).

Build and test antenna prototype(s) - measure return loss.

These non-optimized dimensions are too large for a wearable antenna.

Even though the desired impedance bandwidth (12%) is achieved, the size of the top plate is still too large (~17 cm at 440 MHz).

The ground plane is even larger (~30 cm).

Modeling setupModeling setup

Simulations are carried out using Ansoft HFSS v.10.1 on fine meshes.

A fully parameterized model is set up in order to allow for efficient control of the antenna configuration.

Ansoft Optimetrics is used to determine the best design fit in the multivariable optimization task.

The design variables are patch length, slot length and width, number of slots, capacitive loading dimensions, and position of the feed.

Antenna prototyping and MeasurementsAntenna prototyping and Measurements

Patch, shorting plate, and ground plane are made of copper foil.

Support is provided by a high density polystyrene foam from Dow Chem. Co. (εr=1.06; 3 pcf).

Feed is mounted without solder using a screw, nuts and a washer.

Simulation resultsSimulation results

Optimized PIFA Optimized PIFA ConclusionConclusion A reduced-size PIFA with a bandwidth of about 17% and a patch length of about

0.165 has been presented at 440 MHz.

Multiple PIFAs manufactured on the foam substrate show acceptable repeatability despite potential fragility and uncertainty of shape. Measurements and simulations are in good agreement.

Optimized reduced-size PIFA

Size reduction is about 40%

This metal enclosure is used for housing the hardware. Thus the excess ground plane space is properly utilized

This project is a part of work supported by DoJ Grant 2003-IJ-CX-K025, PI – Prof. John A. Orr, WPI

Bandwidth Full impedance bandwidth is 17% (415-492 MHz)

Radiation pattern Nearly omnidirectional (H-plane)

Maximum gain 2.9 dB along the z-axis

Polarization isolation Low, about 10-15 dB

Optimized PIFA at 600 MHz Optimized PIFA at 600 MHz