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), 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.
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