Q3Antenna And Wave Propagation

Phased Array Antenna

A phased array antenna consists of multiple individual radiating elements arranged in a specific geometric configuration (linear, planar, or conformal), each fed with an independently controllable phase (and, in more general designs, amplitude) via electronic phase shifters, allowing the composite array's overall radiation pattern — and specifically the direction of its main beam — to be electronically steered simply by adjusting the relative phase excitation applied to each element, without requiring any physical rotation or mechanical repositioning of the antenna structure itself. This electronic beam-steering capability, achieved purely by controlling the inter-element phase progression, makes phased arrays extremely valuable for applications requiring rapid, agile beam scanning (such as tracking radar systems, satellite communication ground terminals, and modern 5G/mmWave beamforming base stations), since electronic phase control can reposition the beam essentially instantaneously, in sharp contrast to the comparatively slow mechanical repositioning required by a conventional rotating dish or mechanically-steered antenna.

Applications of Phased Arrays: Radar and 5G Beamforming

In radar systems, phased arrays enable electronically scanned array (ESA) radars capable of switching the beam direction from one look angle to another within microseconds, allowing a single radar face to simultaneously (or in rapid time-multiplexed sequence) track multiple targets, perform wide-angle search scans, and maintain fine-track illumination on a selected target, all without any mechanical antenna movement — this capability underlies modern multi-function military and air-traffic-control radars, and also permits graceful performance degradation, since the failure of a small number of individual transmit/receive modules in a large array only slightly reduces overall gain and pattern quality rather than causing complete system failure, unlike a single-feed reflector antenna. In 5G and other modern cellular/mmWave wireless systems, phased-array beamforming is used at both the base-station and (increasingly) the user-equipment side to concentrate radiated energy toward a specific served user (rather than broadcasting omnidirectionally), improving received signal-to-noise ratio, extending effective range at the high mmWave frequencies where path loss and atmospheric attenuation are severe, and enabling spatial multiplexing (simultaneously serving multiple users on the same time-frequency resource using distinctly steered, mutually low-interference beams) — the array's beam can additionally be rapidly re-steered to track a moving mobile user or to switch between multiple served users, again exploiting the same fundamental electronic phase-control principle used in scanning radar.

Difference Between BSA and EFA

The fundamental distinguishing parameter between a Broadside Array (BSA) and an End-Fire Array (EFA) is the value of the progressive phase difference, psi, deliberately introduced between successive, uniformly-spaced array elements.

Broadside Array (BSA): all elements are fed in phase (psi=0, zero progressive phase difference between adjacent elements), causing the individual elements' radiated fields to add constructively (in phase) specifically in the direction perpendicular (broadside) to the line of the array, since in this direction, the additional path-length delay between successive elements exactly equals zero, meaning all elements' contributions arrive in phase regardless of the zero inter-element excitation phase difference — the array's maximum radiation therefore points broadside (at 90 degrees) to the array axis, with correspondingly reduced or null radiation directly along the array axis itself.

End-Fire Array (EFA): the progressive phase difference between successive elements is deliberately set equal (in magnitude) to the spatial phase delay corresponding to the element spacing, psi=-betad (for radiation maximum along the +z axis) or psi=+betad (for radiation maximum along the -z axis), where beta=2*pi/lambda is the free-space phase constant and d is the inter-element spacing — this specific phase setting ensures that the elements' individual excitation phase delays exactly compensate for the additional path-length delay a wave experiences traveling along the array's own axis, causing all elements' contributions to add constructively specifically along the array's own end-fire (axial) direction, giving the array its maximum radiation directly along its own physical axis (rather than broadside to it, as in the BSA case).

Normalized Field Pattern for 8-Element Array, Spacing lambda/2

For a uniform linear array of N elements with spacing d and progressive phase difference psi, the normalized array factor (field pattern) is given by the standard array factor expression:

For the Broadside Array (BSA) with N=8, d=lambda/2 (so betad=pi), and psi=0: psi_t=picos(theta). The array factor is maximum (AF=1) when psi_t=0, i.e., cos(theta)=0, giving theta=90 degrees, confirming the main beam points broadside, perpendicular to the array axis, with the pattern being symmetric about theta=90 degrees and exhibiting nulls at angles where N*psi_t/2 is a non-zero integer multiple of pi.

For the End-Fire Array (EFA) with N=8, d=lambda/2 (betad=pi), and psi=-betad=-pi (ordinary end-fire condition, for maximum along theta=0): psi_t=pi*cos(theta)-pi=pi(cos(theta)-1). The array factor is maximum when psi_t=0, i.e., cos(theta)=1, giving theta=0 degrees, confirming the main beam points along the array's own axis (end-fire direction), with a correspondingly narrower main lobe (approximately) but generally higher side-lobe levels compared to the broadside case for the same N and spacing.

Comparing the two patterns, the BSA produces a fan-shaped (bidirectional, broadside-pointing) main lobe perpendicular to the array's physical line, symmetric on both sides of the array, while the EFA produces a main lobe directed along the physical line of the array itself (unidirectional if using the Hansen-Woodyard or simple ordinary end-fire condition with appropriate design, or bidirectional along both +z and -z for the simplest ordinary end-fire case without additional directivity-enhancing modification) — this fundamental difference in main-beam direction relative to the physical array axis, controlled entirely by the choice of inter-element phase difference psi, is the key distinguishing and defining characteristic separating broadside and end-fire array operation.

Grating Lobes and Their Avoidance

A grating lobe is an additional, spurious lobe of full main-lobe intensity that appears in an array's radiation pattern at some angle other than the intended main-beam direction, arising when the inter-element spacing is made too large relative to the wavelength — mathematically, grating lobes occur whenever the total phase term psi_t=betadcos(theta)+psi passes through an additional multiple of 2*pi (beyond the one intended value corresponding to the desired main-beam direction) for some other real, physically realizable angle theta, which becomes possible once the element spacing d exceeds a certain fraction of the wavelength. For a broadside array, grating lobes appear once d exceeds one wavelength; for scanned or end-fire arrays, grating lobes can appear at even smaller spacing values, since the required progressive phase shift for beam-steering effectively shifts the whole visible-space mapping of psi_t and brings a repeat lobe into physically realizable angle-space sooner. Grating lobes are highly undesirable because they radiate (or, in receive mode, accept) power in unwanted directions with the same efficiency as the main lobe, wasting transmitted energy, creating ambiguous target directions in radar applications, and causing unwanted interference pickup — they are avoided in practical array design by restricting the element spacing to at most half a wavelength (d<=lambda/2) for arrays required to scan across a wide angular range, or by using somewhat larger spacing only when the maximum scan angle is limited enough that no grating lobe can yet enter the physically visible region, a standard trade-off consideration in phased-array antenna engineering between element count (cost, complexity) and spacing-driven grating-lobe risk.