Patch antenna
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A patch antenna is a popular antenna type, which gains its name from the fact that it basically consists of a metal patch suspended over a ground plane. The assembly is usually contained in a plastic radome, which protects the structure from damage (as well as concealing its essential simplicity). Patch antennas are simple to fabricate and easy to modify and customize. They are closely related to microstrip antennas, which are just patch antennas constructed on a dielectric substrate, usually employing the same sort of lithographic patterning used to fabricate printed circuit boards.
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[edit] Patch Antenna Configuration
The simplest patch antenna uses a half-wavelength-long patch and a larger ground plane. (Large ground planes give better performance but of course make the antenna bigger. It isn’t uncommon for the ground plane to be only modestly larger than the active patch.) The current flow is along the direction of the feed wire, so the vector potential and thus the electric field follow the current, as shown by the arrow in the figure labeled E. 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.
[edit] Patch Antenna Gain
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.
[edit] Patch Antenna Impedance Bandwidth
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), Zo is the impedance of free space, and Rrad 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 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%, or about 120 MHz.
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. Real patch antennas often use ground planes only modestly larger than the patch, which also reduces performance. The details of the feed structure affect bandwidth as well.
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.
[edit] Circular Polarization
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. In this fashion, when (say) the vertical current flow is maximized, the horizontal current flow will be 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. Note that, while circular patches can be used for these techniques, a circular patch does not necessarily radiate circularly-polarized waves! A symmetric circular patch with a single feed point will create linearly-polarized radiation. Finally, a nearly-square patch can be driven at the corner; if the length is just a bit less than resonant and the height a bit more (or vice versa) a circularly-polarized wave will result.
[edit] 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 – with high gain, customizable beam and return loss properties, and other unique features, at low cost.
[edit] See also
- Choke ring antenna, another antenna for GPS applications.
[edit] References
Antenna Theory (3rd Edition), C. Balanis, Wiley 2005
Antenna Engineering Handbook, ed. R. Johnson, McGraw-Hill 1993
