§5.15 Antenna characteristics

Main lobes, side lobes and back lobes

An antenna never radiates its energy perfectly in one direction. The actual radiation pattern consists of:

• Main lobe: the desired, strong beam in the look direction. Its width (beamwidth, measured between the -3 dB points) determines the angular resolution (Angular resolution).
• Side lobes: weaker, unwanted beams at an angle beside the main beam — an unavoidable physical by-product of every finite antenna. Good antennas suppress them to 25–40 dB below the main lobe.
• Back lobe: the (weak) radiation straight backwards.

Side lobes are operationally much more important than their low power would suggest:

• False targets: a very strong reflector (a mountain, a supertanker) in a side lobe can exceed the threshold and then appears on the screen in the direction of the main lobe — in the wrong place.
• Jamming entry point: a jammer does not have to be in the main beam; with enough power it gets in through the side lobes at every antenna position. Countermeasures: side-lobe suppression/blanking and adaptive nulling (steering a "null" in the pattern towards the jammer).
• ESM detection: the radar also leaks its signal all around through its side lobes — so a sensitive receiver hears the radar almost all the time, not only when the beam passes.

Antenna shapes

The shape of the antenna determines the beam shape, and with it the task of the radar:

The phased array deserves extra attention because it dominates the modern radar landscape. Hundreds to thousands of small radiators transmit the same signal with a phase that can be set per element. Choosing the phases produces constructive interference (EM waves in phase, in antiphase and out of phase) in exactly one direction — the beam — and that direction can be shifted electronically, within microseconds, without moving parts. An AESA (Active Electronically Scanned Array) moreover has a small transmitter/receiver of its own per element: reliable (no single point of failure), flexible in power per direction, and able to carry several beams and frequencies at once — which also makes it usable for EW tasks (directed jamming). The two designs — PESA and AESA — and how they work are explored in depth below (Phased array: electronic beam steering to PESA versus AESA — summary).

Phased array: electronic beam steering

Before we separate PESA and AESA, first the shared principle. A phased array replaces the moving dish with a flat panel of hundreds to thousands of fixed small radiators (elements). The beam is no longer aimed mechanically but electronically — by controlling the phase of each element individually.

The mechanism follows directly from interference (EM waves in phase, in antiphase and out of phase). If all elements transmit exactly in phase, the wavefront adds up constructively straight ahead: the beam points perpendicular to the panel (boresight). Give each successive element a small, increasing phase difference, and the combined wavefront "tilts" — and the beam points at an angle, without anything moving.

The consequences are far-reaching and explain why phased arrays dominate the modern radar landscape:

• Beam agility: the beam jumps in microseconds instead of the seconds a rotating antenna takes. A single array can therefore search, track several targets and guide a missile at the same time — it becomes a multifunction radar (cf. Target tracking by radar: the Target Tracking Radar).
• No moving parts: more reliable, and suitable for flush mounting in a fuselage, mast or ship's superstructure.
• Pattern freedom: the same electronics that aim the beam can also steer deep nulls towards a jammer (Main lobes, side lobes and back lobes) or distribute the power over time.

There are physical limits too. The elements are about half a wavelength apart; if they are too far apart, unwanted extra main beams appear (grating lobes). And the further the beam is steered from boresight, the more the effective aperture "shortens" — the beam gets wider and the gain drops (scan loss). That is why a single flat array usually covers up to ±60°; for all-round coverage three or four faces are used (like the fixed panels on modern frigates).

PESA — Passive Electronically Scanned Array

A PESA (Passive Electronically Scanned Array) is the first generation of phased array. Its hallmark: there is one central transmitter — typically a powerful tube (TWT or klystron, Components of the radar system) — whose power is spread over all elements through a distribution network. Each element contains only a phase shifter that steers the beam; the element itself amplifies nothing.

Advantages over a mechanical antenna: all the beam agility and multifunctionality of Phased array: electronic beam steering, at lower cost and complexity than an AESA, because there is only one transmitter.

But the central transmitter is also the weakness:

• Single point of failure: if the tube fails, the whole radar is down.
• Losses: the power has to reach the elements through long feeds and passive phase shifters — a lot is lost along the way, both when transmitting and when receiving.
• Limited flexibility: in practice one frequency and one beam at a time; no freely distributable power per direction.

PESA was the breakthrough (well-known examples: older long-range and naval radars and the radar of various fighters from the 1980s–1990s), but for new systems it has largely been replaced by the AESA.

AESA — Active Electronically Scanned Array

An AESA (Active Electronically Scanned Array) moves the transmitter into the elements. Behind each radiator sits its own T/R module (Transmit/Receive): a small transmit/receive module with its own power amplifier, receive amplifier (LNA) and phase shifter. A large AESA thus contains hundreds to thousands of independent little transmitter-receivers — often built in GaN or GaAs.

Because the power is generated and received on the spot, the feed losses of the PESA disappear and a series of advantages arises that has made the AESA the standard:

• Graceful degradation: no central transmitter. If a few T/R modules fail, performance drops by a fraction — the radar keeps working. Very reliable.
• Multiple beams and frequencies at once: subsets of elements can work independently — searching in one direction while a target is tracked in another, on different frequencies.
• Power management per direction: more energy and dwell time towards a suspect sector, or spreading it out for LPI (see below).
• Wide bandwidth & agility: ideal for frequency agility (PRI modulation) and high range resolution (Range resolution).

The price: complexity, cooling (all those modules produce heat) and cost. But the advantages are so great that almost every new military radar — from fighter to frigate to ground-based air defence — is an AESA.

PESA versus AESA — summary

The essentials in one table:

What they share: both steer the beam electronically through phase (Phased array: electronic beam steering), both have no moving parts, and both can work as multifunction radars. The difference lies entirely in where the power is generated: centrally (PESA) or distributed over active modules (AESA). That one difference explains every other row in the table.

Scan patterns of phased arrays

A mechanical antenna can do only one thing: go round in circles. A phased array (Phased array: electronic beam steering) shifts its beam electronically in microseconds and can therefore program its "looking behaviour" — the scan pattern — freely, and even run several patterns interleaved. The main forms:

• Raster / search scan: the beam systematically covers a sector (row by row) to find new targets.
• Sector scan: searching only a defined area, quickly and repeatedly (e.g. the direction from which a threat is expected).
• Track-While-Scan (TWS): the beam keeps searching, but in between keeps jumping briefly to known targets to update their tracks — searching and tracking at the same time.
• Spotlight: the beam "dwells" on one target or sector for maximum dwell time — more energy and a better measurement (and burn-through, Overpowering the jamming signal).
• Agile / interleaved: random-looking jumps between tasks; searching, tracking, guiding a missile and operating in LPI mode at the same time.

Cassegrain, monopulse feed and duplexer

Three components around the antenna that recur in almost every radar:

• Cassegrain antenna: a parabolic dish with a small, convex subreflector in front of the focus. The feed sits in the centre of the main dish and radiates towards the subreflector, which reflects the energy back across the main dish. Advantage: the heavy feed and the waveguide sit behind the dish, short and compact — ideal for tracking radars and for antennas that have to move fast.
• Monopulse feed: instead of one horn, four sit side by side in the focus. Adding and subtracting their signals produces a sum channel (Σ, for detection and range) and two difference channels (Δ, for the error in azimuth and elevation). The angle error thus follows from a single pulse — the basis of monopulse tracking (Techniques of the target tracking radar) and the reason these radars are insensitive to the amplitude-modulated jamming that does deceive conical scan.
• Duplexer and diplexer: the duplexer (Components of the radar system) lets transmitter and receiver share the same antenna. Classically with a gas-discharge tube (TR tube) that fires at the transmit pulse and short-circuits the receiver; nowadays with a ferrite circulator and a limiter. A diplexer does something else: it combines two transmitters on different frequencies onto one antenna, as with frequency diversity (Losses and fluctuation).