How does the element pattern affect the scan performance of a phased array?
Element Pattern Effects
The embedded element pattern (EEP) is the radiation pattern of one element measured while all other elements are present and terminated in matched loads. It differs from the isolated element pattern due to mutual coupling, array lattice effects, and finite ground plane effects. The EEP is the most important characteristic for predicting the scanned array performance.
For arrays on thick, high-εr substrates: the embedded element pattern can have deep nulls at specific scan angles due to surface wave excitation. At these angles, the element radiates into a surface wave mode rather than into free space, causing scan blindness. Scan blindness typically occurs at angles where kd sinθblind = kd - ksw, where ksw is the surface wave propagation constant.
Wide-scan element design requires: thin substrates (suppress surface waves), low εr (reduce substrate mode coupling), and wide element patterns. Techniques include: probe-fed patches with shorting walls, tightly-coupled dipole arrays (connected arrays that use mutual coupling constructively), and magneto-electric dipole elements that maintain broad patterns to ±60°.
Frequently Asked Questions
What is scan blindness?
Scan blindness is a phenomenon where the array gain drops to near-zero at a specific scan angle due to total internal reflection of the TM₀ surface wave. It manifests as a sharp spike in the active VSWR (return loss) at the blindness angle. It can be eliminated by using thin substrates (h < 0.05λ) or by adding surface wave suppression structures.
How do I measure the embedded element pattern?
Excite one element in the array while terminating all other elements in matched loads. Measure the far-field radiation pattern. This measurement includes all mutual coupling and array structure effects. Repeat for elements at different positions in the array (center, edge, corner) because the EEP varies with position in a finite array.
What about tightly coupled arrays?
Tightly-coupled or connected arrays (such as the current sheet antenna or the fragmented aperture) operate on the principle that strong mutual coupling between elements can be exploited rather than suppressed. These arrays achieve very wide bandwidth (4:1 or more) and wide scan capability (±60° or wider) by designing the coupling to maintain a uniform active impedance across frequency and scan angle.