Active Element Pattern
Understanding the Active Element Pattern
When designing an antenna (such as a microstrip patch or a dipole), an engineer typically simulates and measures its radiation pattern in a pristine, empty environment. This is known as the isolated element pattern. However, when that exact same antenna is placed into a dense grid to form a Phased Array—surrounded by dozens or hundreds of identical antennas just fractions of a wavelength away—its radiation pattern changes radically. This new, heavily distorted shape is the Active Element Pattern (AEP).
The distortion is caused by Mutual Coupling. When the central element radiates, the electromagnetic wave washes over all the adjacent, unpowered elements. Because those adjacent elements are resonant antennas themselves, they absorb some of that energy, ring up, and re-radiate it. These secondary "parasitic" radiations interfere constructively and destructively with the primary wave. The Active Element Pattern captures this entire chaotic electromagnetic interaction in a single, measurable far-field pattern.
Impact on Phased Arrays
The concept of the Active Element Pattern is the absolute cornerstone of modern phased array engineering. The fundamental theorem of array antennas states that the total radiation pattern of the entire array is simply the Array Factor (the math of phase shifting) multiplied by the Active Element Pattern. If the AEP has a "null" (a blind spot) at a specific angle due to severe mutual coupling, the entire massive phased array will be physically incapable of steering a beam to that angle, a catastrophic failure known as Scan Blindness.
Etotal(θ, φ) = AEP(θ, φ) × AF(θ, φ)
Where:
AEP(θ, φ) = Active Element Pattern (Includes all physics and mutual coupling)
AF(θ, φ) = Array Factor (Purely geometric math of element spacing and phase weights)
Crucial Note: You cannot multiply the Isolated Element Pattern by the Array Factor for a dense array; it will yield totally incorrect results.
Comparison
| Metric | Isolated Element Pattern | Active Element Pattern |
|---|---|---|
| Environment | Floating alone in free space | Embedded in the physical array grid |
| Adjacent Elements | None | Present and terminated in 50 ohms |
| Pattern Shape | Smooth, symmetric (usually) | Often rippled, asymmetric, narrowed |
| Use in Simulation | Good for single antennas | Mandatory for phased array prediction |
Frequently Asked Questions
How do you physically measure an Active Element Pattern in an anechoic chamber?
You build the entire physical phased array. You connect the central element to the transmit port of the Vector Network Analyzer (VNA). You must then physically attach high-quality 50-ohm termination loads to the RF ports of every single other element in the array. You then rotate the array on the positioner and record the far-field pattern of that single driven element.
Why are the adjacent elements terminated in 50 ohms instead of just left open or shorted?
In a real, operating active phased array, every element is connected to a Transmit/Receive (T/R) module that presents a 50-ohm impedance. Leaving the adjacent elements open or shorted would reflect 100% of the coupled energy back into space with the wrong phase, completely invalidating the measurement. Terminating them in 50 ohms perfectly simulates the real-world operational environment.
Do all elements in the array have the same Active Element Pattern?
No. The elements in the exact center of a massive array all 'see' the exact same symmetrical environment, so their AEPs are identical. However, elements near the physical edge of the array see antennas on one side, but empty space on the other. This asymmetrical mutual coupling heavily skews their AEPs. Array designers often add 'dummy elements' around the perimeter of the array to ensure the edge elements behave like center elements.