Annular Ring Patch
Understanding the Annular Ring Patch Antenna
The standard microstrip patch antenna is a solid rectangular or circular piece of copper etched onto a dielectric circuit board. It is cheap, low-profile, and rugged. However, standard patch antennas are fundamentally governed by a half-wavelength (λ/2) resonance rule. At low frequencies (like 900 MHz RFID or 1.5 GHz GPS), a solid patch antenna must be physically massive—often too large to fit inside a handheld device. The Annular Ring Patch is a brilliant geometric modification designed to solve this size problem.
By removing the entire conductive center of a circular patch, leaving only a hollow copper ring (an annulus), the electromagnetic resonant cavity is radically altered. The electrical currents are forced to travel around the longer outer circumference of the ring rather than bouncing straight back and forth across the center. Because the current path is artificially lengthened, the fundamental resonant frequency (the TM11 mode) drops significantly compared to a solid patch of the exact same outer diameter. This allows engineers to achieve extreme antenna miniaturization.
Dual-Band Capabilities
Beyond miniaturization, the annular ring is highly prized for its multi-band characteristics. A solid circular patch has a set harmonic sequence, but an annular ring's higher-order resonant modes (like TM12 or TM21) can be independently tuned simply by changing the ratio of the inner radius to the outer radius. This allows a single, compact ring antenna to be perfectly tuned to operate simultaneously on two completely different frequencies, such as GPS L1 and L2 bands, without requiring two separate antennas.
fr ≈ c / [ π × (a + b) × √(εeff) ]
Where:
c = Speed of light
a = Inner radius of the ring
b = Outer radius of the ring
εeff = Effective dielectric constant of the PCB substrate
Comparison
| Feature | Solid Circular Patch | Annular Ring Patch |
|---|---|---|
| Physical Size at 1 GHz | Large (~ 150 mm diameter) | Much Smaller (~ 80 mm diameter) |
| Impedance Bandwidth | Narrow (2% - 4%) | Extremely Narrow (1% - 2%) |
| Dual-Band Tuning | Difficult (Requires physical slots) | Easy (Adjust ratio of inner/outer radii) |
| Feed Location Sensitivity | Moderate | Extremely High (Requires precise probing) |
Frequently Asked Questions
What is the primary trade-off of using an Annular Ring for miniaturization?
Bandwidth. The fundamental rule of antenna physics (the Chu-Harrington limit) states that if you forcefully shrink an antenna below its natural resonant size, its Q-factor explodes. An annular ring patch is much smaller than a solid patch, but its impedance bandwidth shrinks to almost nothing (often less than 1%). It must be manufactured with extreme precision, or it will completely miss the target frequency.
How do you feed an annular ring antenna?
Because the center is hollow, you cannot use a standard center-fed coaxial probe. You must use a coaxial probe that punches through the PCB substrate exactly on the metallic ring itself, precisely located to match the 50-ohm impedance point. Alternatively, engineers often use aperture coupling, placing a microstrip feed line on a different layer below the ground plane and coupling the energy up through a slot into the ring.
Can an annular ring generate Circular Polarization (CP)?
Yes, and this is why they are heavily used in GPS receivers. By strategically adding small notches or 'perturbation tabs' to the inner or outer edge of the ring, the fundamental mode is split into two orthogonal modes that are 90 degrees out of phase, perfectly generating Right-Hand Circular Polarization (RHCP).