Circular Patch
Understanding the Circular Patch Antenna
The standard microstrip patch antenna is a simple rectangle. It is easy to calculate and perfectly radiates linear polarization. However, for aerospace applications—like GPS receivers, satellite radio (SiriusXM), and drone telemetry—linear polarization is dangerous because the signal drops out when the vehicle rolls or banks. These systems strictly require Circular Polarization (CP). While you can force a rectangular patch to generate CP by cutting specific corners off, the Circular Patch Antenna is vastly superior due to its perfect, continuous geometric symmetry.
A circular patch is simply a solid copper disc etched onto a dielectric substrate over a ground plane. Because the circle has no harsh corners, the electrical current distributes incredibly smoothly across the surface. The fundamental resonant mode is the TM11 mode, where the current flows straight across the diameter of the circle. By carefully adjusting the radius of the disc, the engineer tunes the antenna to the exact desired frequency (e.g., 1.575 GHz for GPS).
Generating Circular Polarization
To generate Circular Polarization, the antenna must simultaneously radiate two separate signals that are orthogonal (90 degrees apart in space) and exactly 90 degrees out of phase in time. The perfect symmetry of the circular patch makes this easy. Engineers simply use a "Dual-Feed" system—placing two coaxial probes exactly 90 degrees apart on the edge of the circle. When fed through a 90-degree hybrid coupler, the disc excites two perfectly orthogonal TM11 modes that merge to create an exceptionally pure, spinning corkscrew of RF energy.
fr = (1.8412 × c) / ( 2π × aeff × √εr )
Where:
c = Speed of light
aeff = The effective radius of the patch (Physical radius 'a' plus a small mathematical fringing extension because the electric fields bow outward past the edge of the copper).
εr = Dielectric constant of the PCB.
Comparison
| Feature | Rectangular Patch | Circular Patch |
|---|---|---|
| Resonance Math | Simple (L = λ/2) | Complex (Bessel Functions) |
| Physical Footprint | Moderate | Slightly smaller than a square patch |
| Circular Polarization Purity | Good (Requires corner truncation) | Excellent (Perfect geometric symmetry) |
| Array Packing Density | High (Squares pack tightly) | Moderate (Circles leave gaps) |
Frequently Asked Questions
How can you get Circular Polarization with only a single feed probe?
Using two feed probes and a heavy coupler takes up too much space. To achieve CP with a single probe, engineers use 'Perturbation'. They take a perfect circular patch and cut two tiny, identical notches on opposite sides, or add two tiny copper tabs. This intentionally ruins the perfect symmetry. When the single probe excites the patch, the geometric defect forces the energy to split into two separate modes that resonate at slightly different frequencies, generating a spinning CP wave. It is cheaper, but the CP bandwidth is very narrow.
Why does the formula use 1.8412?
In a rectangular patch, the electric fields behave like simple sine waves. In a circular cavity, the fields propagate as Bessel functions (specifically, cylindrical harmonics). The value 1.8412 is the first mathematical root of the derivative of the first-order Bessel function, J'1(x) = 0. It represents the exact mathematical boundary where the TM11 electromagnetic mode perfectly fits inside the circular boundary.
What is an Annular Ring Patch?
If you take a solid circular patch and punch a massive hole out of the center (turning it into a donut), you get an Annular Ring. This forces the electrical current to travel a much longer path around the rim rather than straight across the middle. This lowers the resonant frequency dramatically, allowing for massive miniaturization compared to a solid circular patch.