2.6.1 Single Feed CP Rectangular Microstrip Antenna Design

There are essentially two methods used to create rectangular circularly polarized microstrip antennas. The first is to feed the patch at a single point and perturb its boundary, or interior so that two orthogonal modes exist at a single frequency which have identical magnitudes and differ in phase by 90 degrees. The second is to directly feed each of two orthogonal modes with a microwave device which provides equal amplitudes and a 90 phase difference (e.g. 90 branchline hybrid). This section addresses the first type of design.

In Figure 2-15 we see four common methods used to create circularly polarized radiation from a rectangular microstrip antenna with a single driving point. The first method (I) is to choose an aspect ratio a/b such that the TM ₁₀ and TM ₀₁ modes both exist at single frequency where their magnitudes are identical and their phases differ by 90 degrees. The two orthogonal modes radiate independently and sum in the far field to produce circular polarization.

Figure 2-15: Four methods for generating circular polarization from a rectangular microstrip antenna using a single feed. (I) Using the aspect ratio of a patch to generate two orthogonal modes with equal amplitude and 90 degrees out of phase. (II) Use of indentations and/or tabs (III) Cutting off corners to create orthogonal modes. (IV) Introduction of a diagonal slot.

The second method presented in Figure 2-15 (II) is...