What is the active element pattern of a phased array and how does it differ from the isolated element pattern?
Active Element Pattern in Phased Arrays
The AEP concept is fundamental to accurate phased array performance prediction. Using the isolated element pattern instead of the AEP in the array pattern calculation leads to incorrect gain predictions, especially near scan blindness angles and for closely-spaced elements.
AEP Characteristics
- Broadside gain: AEP gain is typically 1-3 dB lower than isolated element gain at broadside; the mutually coupled power absorbed by neighboring terminations is a loss mechanism. This is called the "embedded element efficiency" and is typically 60-90% for well-designed arrays
- Edge elements vs. interior elements: Edge elements have fewer neighbors and therefore different mutual coupling. Their AEP differs from interior elements: typically higher gain (less coupling loss) but different pattern shape. Large arrays (> 10x10) can approximate all interior elements as having the same AEP
- AEP and array gain: The array gain is: G_array = N x G_AEP x eta_scan, where N is the number of elements, G_AEP is the active element gain, and eta_scan is the scan loss factor. This correctly accounts for mutual coupling losses
Array factor: AF = sum exp(j(k d_n sin(theta) + alpha_n))
Array gain: G_array = N × G_AEP (if all elements identical)
AEP gain: G_AEP = G_isolated × eta_coupling where eta_coupling = 60-90%
Scan loss: approximately cos(theta_scan) for well-behaved arrays
Frequently Asked Questions
How do I measure the AEP?
Excite one element with a signal source while terminating all other elements in 50-ohm loads. Measure the radiation pattern using a standard antenna range (far-field or near-field). This is straightforward for a built array. For design validation: simulate the AEP using a finite array EM simulation (HFSS, CST) with one element excited and all others terminated. For large arrays, simulate a central element in a 7x7 or larger sub-array to approximate the infinite-array AEP.
Why is AEP more accurate than isolated element pattern for array calculations?
The isolated element pattern ignores mutual coupling. In a real array: the current on element 1 induces current on element 2 (through mutual coupling), which re-radiates. This re-radiation modifies element 1's effective pattern. The AEP captures this effect automatically because all elements are present during the measurement/simulation. The error from using isolated element pattern can be 2-5 dB or more at specific angles, especially near scan blindness.
Does the AEP change with scan angle?
The AEP itself does not change with scan angle; it is a fixed property of the element in its array environment. What changes with scan angle is the array factor (AF). The scanned array pattern is the product of the fixed AEP and the scanned AF. However, the scan impedance (which affects the power delivered to the element) does change with scan angle, which effectively modifies the overall pattern. For rigorous analysis: the scan-dependent impedance mismatch loss should be applied separately.