Front-Side Microstrip Line Feeding a Raised Antenna Patch

Relative to prior patch antennas, this is simpler, thinner, and less expensive.

An improved design concept for a printed-circuit patch antenna and the transmission line that feeds the patch calls for (1) a microstrip transmission line on the front (radiative) side of a printed-circuit board based on a thin, high-permittivity dielectric substrate; (2) using the conductor covering the back side of the circuit board as a common ground plane for both the microstrip line and the antenna patch; (3) supporting the antenna patch in front of the circuit board on a much thicker, lower-permittivity dielectric spacer layer; and (4) connecting the microstrip transmission line to the patch by use of a thin wire or narrow ribbon that extends through the thickness of the spacer and is oriented perpendicularly to the circuit-board plane. The thickness of the substrate is typically chosen so that a microstrip transmission line of practical width has an impedance between 50 and 100 . The advantages of this design concept are best understood in the context of the disadvantages of prior design concepts, as explained below.

One prior design concept involves the use of a probe feed, in which the coupling between a transmission line behind the ground plane and an antenna patch on the front side of the circuit board is effected by a small wire that passes through a coaxial hole in the ground plane and the printed-circuitboard substrate. In another prior design concept, a transmission line on the back and an antenna patch on the front are coupled magnetically via slots. In yet another prior design concept, the antenna patch and its microstrip feed line are both formed on the front surface of the printed-circuit board along with the rest of the printed circuitry.

Probe- and slot-coupling designs exhibit a tendency toward leakage of radiation from the transmission-line region behind the antenna. Leakage can be reduced by shielding the microstrip or making it asymmetrical, but this undesirably increases the thickness of the antenna and causes excitation of parallel-plate modes that have a zero cutoff frequency, contribute spikes to the return-loss spectrum, and sometimes make the antenna “blind” in some directions. Special mode-suppression pins, or vias, can be used in probe coupling to suppress the parallel-plate modes, but this significantly increases the cost, complexity, and size of the antenna. In addition, fabrication of the feed line at the thickness and permittivity that optimize connection to the patch antenna tends to make coupling to the next layer behind the feed circuit very difficult.

The prior design concept in which the microstrip feed is printed on the same surface as that of the antenna patch is seldom implemented and is fraught with drawbacks. First, the thickness and permittivity needed for a microstrip transmission line tend to result in a very narrow- band and inefficient microstrip radiator. To increase the radiative bandwidth of the patch, one must increase the substrate thickness, but this increases the percentage of power launched into the lowest order transverse magnetic surface wave mode, resulting in greatly reduced efficiency and loss of control over the radiation pattern. A second drawback is that the patch takes up so much space on the circuit board that there is very little area available for placement of feed lines. The only alternative is to place the feed lines close together and close to the patch, leading to undesired cross coupling between lines and patches, scattering of radiation, and consequent loss of control of the radiation pattern.