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A patch antenna (also known as a rectangular microstrip antenna) is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane. The assembly is usually contained inside a plastic radome, which protects the antenna structure from damage. Patch antennas are simple to fabricate and easy to modify and customize. They are the original type of microstrip antenna described by Howell in 1972; the two metal sheets together form a resonant piece of microstrip transmission line with a length of approximately one-half wavelength of the radio waves. The radiation mechanism arises from discontinuities at each truncated edge of the microstrip transmission line. The radiation at the edges causes the antenna to act slightly larger electrically than its physical dimensions, so in order for the antenna to be resonant, a length of microstrip transmission line slightly shorter than one-half a wavelength at the frequency is used. A patch antenna is usually constructed on a dielectric substrate, using the same materials and lithography processes used to make printed circuit boards.
The simplest patch antenna uses a patch which is one-half wavelength long, mounted a precise distance above a larger ground plane, sometimes using a spacer made of a dielectric between them. Electrically large ground planes produce stable patterns and lower environmental sensitivity but of course make the antenna bigger. It isn’t uncommon for the ground plane to be only modestly larger than the active patch. When a ground plane is close to the size of the radiator it can couple and produce currents along the edges of the ground plane which also radiate. The antenna pattern becomes the combination of the two sets of radiators.
The current flow is along the direction of the feed wire, so the magnetic vector potential and thus the electric field follow the current, as shown by the arrow labeled "E" in the figure below. A simple patch antenna of this type radiates a linearly polarized wave. The radiation can be regarded as being produced by the ‘’radiating slots’’ at top and bottom, or equivalently as a result of the current flowing on the patch and the ground plane.
The gain of a rectangular microstrip patch antenna with air dielectric can be very roughly estimated as follows. Since the length of the patch, half a wavelength, is about the same as the length of a resonant dipole, we get about 2 dB of gain from the directivity relative to the vertical axis of the patch. If the patch is square, the pattern in the horizontal plane will be directional, somewhat as if the patch were a pair of dipoles separated by a half-wave; this counts for about another 2-3 dB. Finally, the addition of the ground plane cuts off most or all radiation behind the antenna, reducing the power averaged over all directions by a factor of 2 (and thus increasing the gain by 3 dB). Adding this all up, we get about 7-9 dB for a square patch, in good agreement with more sophisticated approaches (see Balanis, p. 841, for more details).
A typical radiation pattern for a linearly-polarized 900-MHz patch antenna is shown below. The figure shows a cross-section in a horizontal plane; the pattern in the vertical plane is similar though not identical. The scale is logarithmic, so (for example) the power radiated at 180° (90° to the left of the beam center) is about 15 dB less than the power in the center of the beam. The beam width is about 65° and the gain is about 9 dBi. An infinitely-large ground plane would prevent any radiation towards the back of the antenna (angles from 180 to 360°), but the real antenna has a fairly small ground plane, and the power in the backwards direction is only about 20 dB down from that in the main beam.
The impedance bandwidth of a patch antenna is strongly influenced by the spacing between the patch and the ground plane. As the patch is moved closer to the ground plane, less energy is radiated and more energy is stored in the patch capacitance and inductance: that is, the quality factor Q of the antenna increases. A very rough estimate of the bandwidth is:
where d is the height of the patch above the ground plane, W is the width (typically a half-wavelength), is the impedance of free space, and is the radiation resistance of the antenna. The fractional bandwidth of a patch antenna is linear in the height of the antenna. The impedance of free space is approximately 377 ohms, so for the typical radiation resistance of about 150 ohms, a simplified expression can be obtained:
For a square patch at 900 MHz, W will be around 16 cm. A height d of 1.6 cm will provide a fractional bandwidth of around 1.2(1.6/16) ≈ 12%, which gives a Bandwidth of 108 MHz at the center frequency.
A patch printed onto a dielectric board is often more convenient to fabricate and is a bit smaller, but the volume of the antenna is decreased, so the bandwidth decreases because the Q increases, roughly in proportion to the dielectric constant of the substrate. Patch antennas utilized by industry often use ground planes which are only modestly larger than the patch, which also alters their performance. The details of the feed structure affect bandwidth as well.
The negative return loss for a pair of representative commercial patch antennas is shown below; both antennas are nominally designed to operate in the US Industrial, Scientific, and Medical (ISM) band from 902-928 MHz. Antenna B uses a 16-mm patch height above ground, and the measured bandwidth of about 150 MHz at 10 dB return loss is rather close to that estimated above. However, this antenna also uses a very large (30x30 cm) ground plane. Antenna A delivers similar bandwidth but at about 20x20 cm is considerably smaller and more convenient to mount and position. Commercial antennas vary widely in performance, often due to poor centering of the band even when theoretical bandwidth is achieved.
Rectangular (non-square) patches can be used when it is desired to produce a fan beam: a radiated wave whose vertical and horizontal beamwidths are substantially different. Circular patches can be used instead of square patches; fabrication is straightforward though calculating the current distribution is more involved.
It is also possible to fabricate patch antennas that radiate circularly-polarized waves. One approach is to excite a single square patch using two feeds, with one feed delayed by 90° with respect to the other. This drives each transverse mode and with equal amplitudes and 90 degrees out of phase. Each mode radiates separately and combine to produce circular polarization. This feed condition is often achieved using a 90 degree hybrid coupler. When the antenna is fed in this manner, the vertical current flow is maximized as the horizontal current flow becomes zero, so the radiated electric field will be vertical; one quarter-cycle later, the situation will have reversed and the field will be horizontal. The radiated field will thus rotate in time, producing a circularly-polarized wave.
An alternative is to use a single feed but introduce some sort of asymmetric slot or other feature on the patch, causing the current distribution to be displaced. A square patch which has been perturbed slightly to produce a rectangular microstrip antenna can be driven along a diagonal and produce circular polarization. The aspect ratio of this rectangle is chosen so each orthogonal mode ( and modes) are both non-resonant. At the driving point of the antenna one mode is +45 degrees and the other -45 degrees to produce the required 90 degree phase shift for circular polarization.
Note that, while circular patches can be used for these techniques, a circular patch does not inherently radiate circularly-polarized waves. A circular patch with a single feed point will create linearly-polarized radiation. If the circular patch antenna is perturbed into an ellipse and fed properly it can produce circularly polarized electromagnetic waves.
Variants and elaborations
When fabricated using printed-circuit techniques on a dielectric substrate, it is straightforward to create complex arrays of patch antennas – a microstrip antenna
- "Microstrip Antennas," IEEE International Symposium on Antennas and Propagation, Williamsburg Virginia, 1972 pp. 177-180
- "Radiation from Microstrip Radiators," IEEE Transactions on Microwave Theory and Techniques, April 1969, Vol. 17, No. 4 pp.235-236
- "Microstrip and Printed Antenna Design" (2nd Edition) Randy Bancroft SciTech Publishing Inc. 2009 page 106
The RF in RFID: UHF RFID in Practice (2nd Edition), D. Dobkin, Newnes n.d., pg. 256, Web. 05 Jan. 2015. <GOOGLE_BOOKS>
Antenna Theory (3rd Edition), C. Balanis, Wiley 2005
Antenna Engineering Handbook, ed. R. Johnson, McGraw-Hill 1993
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