The system and the transmitter
2.7 Scanner, qualitative description
Mismatch causes a standing wave in the feeder, VSWR rising as mismatch worsens. For example, when VSWR = 2, the associated voltage reflection coefficient, p, is 0.33 (Eq. (2.5a)) and transmission loss is 0.51 dB (Eq. (2.5b)).
Two-way, this throws away ~1.0dB (26 per cent) of radar performance. Reflec- tion loss is 9.65 dB, so as much as 11 per cent of the transmitter power reflects back into the receiver - a kilowatt or more, hugely bigger than the echoes it is designed to handle. Protection devices may prevent damage, but the receiver will have a bout of severe indigestion, taking several microseconds to recover full sensitivity after transmission, spoiling performance out to a kilometre or so range.
2.6.4 Ringing
Transmitter power reflected by mismatch introduces unwanted ringing clutter. As the receiver input is never perfectly matched, it re-reflects. The reflection bounces back and forth along the feeder before finally petering out, false echoes or ringing appearing each time. At the usual velocity of propagation near 200 m/|xs, a 50 m feed would deliver reflections to the receiver each 0.5 |xs or 75 m range equivalent. These successive strong reflected pulses can mask true echoes, spoiling the radar's short- range performance. Chapter 11, Section 11.8, details how they interact, depending on their spacing.
axis OQ. At any point X, the electric (E, V/m) and magnetic (H, AJm) alternating field components are mutually orthogonal to the direction of propagation, the ray energy being EH W/m2. The ratio E/H is set by the characteristic impedance of free space = VlW^o = y/47tc/\07 = 377 Q. Reversing the polarity of either field reverses the direction of propagation. In Figure 2.14(a) the electric field lies in the horizontal plane so the ray is horizontally polarised (HP).
Figure 2.14(b) shows a vertically polarised (VP) plane-polarised ray, having its nulls at the same instants as A. Addition of the E ox H vectors of A and B at any instant, say X and X', gives a resultant plane-polarised ray with slant polarisation, the slant angle depending on the relative powers of the components. Therefore, any slant-polarised ray can be resolved into HP and VP components.
Changing the relative phasing of B relative to A, so that a B null lies at an A peak, causes the resultant pair of E and H vectors to rotate spirally one revolution per wavelength. The ray is said to be circularly polarised (CP) if the component powers are equal. More generally, unequal components give elliptical polarisation.
The polarisation is said to be right-hand if an observer at the source looking along the direction of propagation sees the vectors rotating clockwise, like a nut on a con- ventional right-hand threaded screw. This is the usual (American) IEEE convention.
The (British) IEE convention was the opposite, a fruitful source of confusion. We avoid the term 'clockwise', partly because the scenario would imagine an observer within the clock, looking out along the direction of data flow, from which position the hands rotate anticlockwise! Changing the relative phasing of one component by TT rad (inverting its polarity) reverses the hand of polarisation to left hand. Thus any circular or elliptically polarised ray may be resolved into a pair of orthogonal plane components.
An isotropic radiator, Figure 2.17(a), is an antenna which radiates uniformly through the whole 360° x 360° (4n sr) of the surrounding space. The concept is useful, but because polarisation cannot be retained throughout the sphere it is impossible actually to make such a device. Placed at the centre of a large hollow sphere of radius R, the whole internal surface area 4nR2 would be weakly illuminated, an analogy being a torch's bare bulb suspended from wires with the reflector removed.
At some instant the radiator transmits with, say, the E field at a positive maximum, half a cycle later there is a negative maximum and so on. The phase fronts spread out at the speed of light in expanding spherical shells as they leave the radiator. Within the small solid angle occupied by a ray, fronts may be regarded as planar. The isotropic antenna can pick up signals from a source anywhere on the sphere, but it is necessarily insensitive, for it must 'watch' over the whole sphere all the time. Neither can it give any indication of direction.
From consideration of conservation of energy, it follows that when a source radi- ates power P watts from an isotropic radiator in unobstructed free space, at range R metres the signal power gives a flux density d watts per square metre of transverse cross section, which is the power divided by the surface area of the sphere:
rf= i w/m2 (2 - 6)
Figure 2.17 Antenna radiation patterns. Diagrammatic plans (upper row) and side elevations of (a) theoretical iso tropic radiator, covering whole sphere;
(b) active device antenna covering 360° azimuth x^25° elevation;
(c) radar scanner concentrating energy into ~ i ° x 25° fan beam.
Transmit mode. Receives within same solid angles
The isotropic radiator is used as a reference against which practical antennas are compared, and is defined as having unity gain, G = 1. In decibel form, G = 0 dB (usually written 0 dBi, gain relative to isotropic). Its radiation in azimuth and elevation patterns are shown as circles in Figure 2.17(a).
2.7.2 Directional radiation
Fitting a paraboloidal reflector to the torch bulb concentrates its light on a small part of the surface of the sphere, which it brightly illuminates at the expense of the remainder, now dark. A small solid angle has gained light, although of course the total output remains unaltered, or diminishes if the reflector is lossy. Placing a reflector behind the isotropic radiator has a similar effect; a beam is formed within which energy density is stronger than before, Figure 2.17(c). The radiator/reflector combination forms a directional antenna, with gain of power density within the beam. The gain is high when the beam is narrow in azimuth and elevation, covering a small solid angle.
The annulus of sphere wall at the horizon can be scanned by slowly rotating the bulb
Plan views Radiator Positive phase fronts
Negative fronts
Electromagnetic axis = OQ Narrow beam scanned in azimuth
Fronts travel at speed of light
Omnidirectional azimuth
Enlargement At long range rays are almost parallel and fronts are planar Omnidirectional azimuth
Whole sphere illuminated weakly Theoretical only Elevations
Horizon
Broad elevation beam
Broad elevation beam Small solid angle illuminated strongly (c) Radar scanner
High gain Horizon illuminated at moderate strength
(b) Racon, SART and RTE Some gain Omnidirectional elevation
(a) Isotropic radiator Unity gain
or antenna, gradually building up a picture of the whole horizon. The beamwidth is the angular width of the main lobe of the beam, measured between its half-power (—3 dB) points, where electric field strength is 1/V2 (71 per cent) that at beam axis.
Figure 2.1 l(b) shows an intermediate arrangement with only elevation directivity and low gain, used for racons, etc.
Unless otherwise stated, the gain of a scanner always means the one-way gain on beam axis, denoted by Gmax or plain G. Gain is a pure number, the ratio of actual power density on beam axis to power density at the same range from an isotropic radiator (gain 1.0 by definition). There are ATT steradians in a sphere so gain of a lossless scanner is given by:
Crmax - ^
where 0, </> are the azimuth and elevation beamwidths in radians. Putting 0(f> = £2 sr, the solid angle of the beam,
Gmax = -^-• (2.7a)
To cover the surface of a sphere needs 41 253 tiles, each 1° square, so when beamwidths are expressed in degrees and the solid angle illuminated is £2deg =
#deg0deg>
Gmax = — • (2.7b)
£2deg
A handy rule of thumb for typical actual scanners is:
r 2 5 0 Q 0 n 7 ^ Gmax ~ — • (2.7c)
S2deg
This is often expressed in decibel form:
Gmax = 44 - 10 log ftdeg dBi. (2.7d)
The ratio 25 000/41 253 = 0.606 represents a typical loss of ~2.2 dB (one-way) within the scanner, accounting for unavoidable resistance, mismatch, etc. Loss tends to be high when antennas have particularly good sidelobe levels.
We shall see in Section 2.8.1, Eq. (2.11b) for a slotted array having constant power density across the aperture width, a, that beamwidth, O9 in the plane of the aperture is
0 = ^ = 0 . 8 8 6 * . 2 a a
The factor ^/TT/2 arises from the power lost to sidelobes. A hypothetical uniformly illuminated antenna without sidelobes would have
a
For an aperture of a x b, mouth area A, solid beamwidth angle Q = X2 /A.
Substituting in Eq. (2.7a)
m a x~ ft ~ X2 ' { }
The gain, aperture area A, wavelength and directivity D (the ratio of the energy per unit area transmitted relative to that from an isotropic radiator at the same range, neglecting losses), are therefore related.
Losses reduce efficiency below unity:
G
efficiency = —. (2.7g) When calculating detectability, the scanner is often assumed to have a rectangular radiation pattern; full gain at all angles between the half-power points and zero else- where, allowing for the reduced gain at the edges of this rectangle by introduction of a hypothetical beamshape loss, see Section 2.7.15.
2.7.3 Beam characteristics
Ideally, an antenna would direct all its energy within the desired beamwidths, but this is impossible on theoretical grounds. If the energy distribution across the aperture is uniform, called uniform illumination, a proportion is transmitted in unwanted subsidiary beams at other bearings, called sidelobes. The loss of energy within the main beam is not usually very important. But the sidelobes may illu- minate some scatterer, its reflection being in turn picked up by the sidelobe in receive mode and falsely displayed as a target at main beam azimuth. Sidelobes are minimised in practical scanners by shaping or weighting the energy density or illumination so that the edges of the aperture receive less than the centre, the aperture not being fully filled. There are penalties - for a given effective aperture size, the physical size rises, beamwidths go up and gain goes down. Cross- polarisation effects may cause butterfly lobes, sidelobes at 45° to the principal planes.
When both beamwidths are fairly small and approximately equal, radiating a near- circular cone, as in a few VTS scanners, the beam is called a pencil beam. A fan beam antenna has elevation beamwidth sharply larger than azimuth to cater for roll and pitch and radiates an elliptical solid angle (Figure 2.5(a); radiation as Figure 2.17(c)).
As stabilised mounts are not used, to cater for roll all marine radars must have fan beams, the vertical beamwidth (sometimes called divergence) being an order of mag- nitude larger than the horizontal. A few VTS scanners use inverse cosec2 beams, discussed later, others being intermediate in shape between fan and inverse cosec squared. Unfortunately, narrow beams demand great physical size. The beam elec- trical centreline is called the beam axis or boresight, dating from WW2 fire-control service, when radar axis was referenced to the gun-barrel using a sight inserted in the bore.
All scanners need narrow azimuth beamwidth for the following reasons.
• To obtain enough gain for adequate signal to noise ratio from weak distant targets.
• To minimise illumination of sea and precipitation clutter.
• For precision determination of target position, particularly for track prediction on ARPA or ATA.
• To reveal target aspect, particularly in VTS service.
• To discriminate between close-spaced targets, revealing presence of tugs, etc.
near another ship and to prevent track-seduction (swapping) when two targets manoeuvre close together. This also demands low sidelobes.
2.7.4 Rotation
The scanner is continuously rotated clockwise, typically driven from a three phase induction motor through spur gears or toothed belts. Speed may fluctuate a few per cent with wind load. Typical speeds are:
• Marine radar: 20 rpm IMO minimum, with a tendency toward rather smaller scanners rotated rather faster, typically 25 rpm.
• High speed craft (HSC): 40 rpm (IMO minimum) to 60 rpm for fast display update, the penalty being few sweeps per scan.
• VTS: 10-20 rpm, low speed helping to decorrelate sea clutter from one scan to the next, also reducing the drive motor load of these large structures. Windage may be further reduced by aerofoils.
Instantaneous bearing of the beam and hence of currently received targets is reported to the signal processor by a shaft encoder having much better resolution than the beamwidth. VTS radars and auxiliary docking radars on some ferries often also use bearing data to inhibit transmission within blind arcs or blanking angles containing obstructions or persons. The feeder or transceiver is connected to the radiator by a rotating joint forming part of the scanner and drive motor assembly.
2.7.5 Size and beamwidth
Table 2.3 lists some parameters of typical scanners. Apertures are still sometimes expressed in feet nominal width, lft = 0.3048 m. Nine gigahertz band marine scan- ners have azimuth apertures between 0.45 m (1.5 ft, yachts) and 3.7 m (12 ft, biggest ships) giving azimuth beamwidth, 6, between about 5° and 0.7°, respectively. Three gigahertz band has | the number of wavelengths, hence 3 x 0 for the same aper- ture, which is generally here 2.7^.Om (9-13 ft), 0 ~ 2°. In each case elevation beamwidth <p ~ 20°-25° to cope with rolling. Typically, the effective azimuth aper- ture, a, is about 75 per cent of the nominal overall width of the scanner, the scanners of Table 2.3 varying between 67 and 85 per cent. Typical radiation patterns are given in Figures 2.29-2.31. Losses are discussed in Section 2.7.15.
2.7.6 Marine radar scanners
Azimuth beamwidth is required by IMO for ships within the SOLAS convention to be less than 2° at —3 dB points and less than 5° at —20 dB points, with sidelobes less than
Table 2.3 Typical scanner parameters, all horizontally polarised
Ref. Aperture, a Band Type Shape Beamwidth, ° Gain First ITM , ^ , (GHz) G sidelobe
(Metres) (Feet) 0 <t> ( d B i ) ( d B
Azimuth Elevation , . down) Small craft
A 0.45 1.5 9 Slot array Fan beam 4.7 25 24 20 B 1.2 4 9 Slot array Fan beam 2 25 27 26
Deep-sea ships
C 2.7 9 9 Slot array Fan beam 0.7 20 33 31 D 3.6 12 3 Slot array Fan beam 2 27 26 23
VTS service
E 3.6 12 9 Slot array Fan beam 0.6 19 32 21 F 5.5 18 9 Slot array Fan beam 0.41 20 37 35 G 5.4x0.64 17.7x2.19 Reflector Inv. cosec2 0.4 4.5 39 28
—23 dB within ±10° of beam axis and less than —30dB further out (all relative to main beam). Elevation beamwidth has to be sufficient for ±10° ship's roll. Horizontal polarisation has to be available.
Originally separate small 'hoghorn' (waveguide horn) radiators illuminated transmit and receive parabolic reflector strips which were supported by horizon- tal flat plates, aptly called cheeses. Today's marine radars combine transmit and receive functions in a single slotted waveguide array (SWG) scanner, Figures 2.18(a) and 2.19(a)-(c), necessitating only a single rotating joint and feed, and giving better performance. The radiator is a horizontal waveguide fed from one end.
Aperture, a, is somewhat less than the physical width. Up to 80 or so slots form the radiating elements, alternately cut at opposite slants and spaced S metres apart.
Some smaller vessels cannot accommodate scanners whose turning circles exceed about 1.3 m, giving undesirably wide azimuth beamwidth even at 9 GHz.
Using the concept of wavelets introduced by Huygens for optics in the seventeenth century, the simplified geometrical description of operation of a slotted array scanner is as follows. The slot elements disturb the electric and magnetic field patterns on the inner waveguide wall, causing radiation in a similar manner to radio dipole or loop antennas. Wavelets of energy radiate from each element, increased slot width and obliquity increasing the coupling, the proportion of power extracted. What little energy remains at the far end of the guide is absorbed in a built-in resistive plug or load, preventing end-reflections from spoiling the wavelet phasing. If each element radiated the same power, so the aperture was uniformly illuminated, an excellent main beam would form at the expense of poor sidelobes. Elements near the ends are therefore slanted less to make them radiate less, tapering the illumination across the aperture to improve sidelobe performance. If S were kg/2, beam axis would be
(b) Vertical polarisation Broad-face slots (c) Cosine on pedestal illumination
Figure 2.18 Slotted waveguide scanner. Perspective view. Horizontally polarised fan beam, narrow in azimuth, broad in elevation. Slot radiation set by inclination and width. Flare mouth height sets elevation beamwidth.
Slotting for vertical polarisation shown at (b)
normal to the mechanical axis. Sidelobe performance is improved by making S rather less, typically 0.4A,g, squinting the beam ~3° from the mechanical axis. Care is taken to control the inevitable coupling which always exists between each element and its neighbours, to prevent unwanted phase and amplitude variations, often by stub filter waveguides, visible in Figure 2.19(c). If 5 exceeded A.o/2, the smooth pattern would be disrupted by grating lobes, as with optical gratings.
2.7.7 Radiation patterns
Variation of gain with angle within a single plane (azimuth or elevation) is described graphically by the radiation pattern in that plane. Combination of patterns of both planes in a map-like representation gives a target pattern map (TPM). In azimuth, each slot forms a small antenna, transmitting in a broad beam, shown in Figure 2.20(a).
On the elementary geometrical theory of operation, the element wavelets merge to give the fully formed beam at the Rayleigh distance, defined as that giving 45°
variation of phase across the beam. This range, ~2a2/k, a few tens of metres for the smallest marine scanners and a kilometre or so for the largest VTS scanner, separates the near-field, Fresnel or focussing region, from the far-field or Fraunhofer region of most interest to us. Here the phase fronts are planar overall and previously- unfocussed transmitted energy combines to become a single coherent energy wave.
Rays hitting a target in this region are considered as parallel. Fronts, TU in the figure,
Coupling depends on slot width and offset
Aperture not fully filled Volts
Azimuth aperture, a Boresight horizontal or depressed -1°
(a) Horizontal polarisation
Window Transceiver
Rotating joint
Slotted waveguide Elevation
aperture, b
internal load End slots narrower or less slanted to radiate least Rotates about centreline
Metal flares
From rotating joint via hockey-stick shaped waveguide
Centre Edge
Figure 2.19 Conventional slotted waveguide scanner. Elevation beamwidth con- trolled by flares. All reproduced by permission of Kelvin Hughes Ltd, Ilford UK (a) Outer view. Wing plates improve aerodynamics. Weath- erproof window; turning gear, rotating joint and transceiver below, (b) Cross section, 3 GHz band, (c) Internal view, showing 9 GHz end- fed slotted waveguide, horn plate flares and a few of the slots and their
associated filters. Aluminium construction
are approximately parallel to the waveguide face. Direction of propagation along the boresight OQ is of course normal to the front.
If the frequency and therefore the waveguide wavelength change, the wavelets no longer emerge quite at the same phase, so the beam axis in the horizontal plane is somewhat frequency dependent or dispersive (centre-fed designs are not dispersive).
Radiation
Throttle Slotted waveguide
Horn plate
Figure 2.20 Radiation from slotted array. Plan views, (a) At design frequency, wavelets from many elements add coherently on boresight to form a beam but suffer destructive interference when sin O ~ a/k, defining the position of first nulls and so the beamwidth. (b) When frequency is offset, Xg changes and changes squint angle; a problem for offset- frequency racons and some VTS radars. Reception uses reciprocal processes
This differential squint, shown in Figure 2.20(b), is typically about 1° per 100 MHz in the 9 GHz band, more or less independent of aperture. It may necessitate realignment of the display after renewing the magnetron by one of different frequency. Differential squint also places limitations on frequency stagger and on operation of frequency offset racons, considered in Chapter 8. Few ships' scanners exceed 4.0 m (13 ft) aperture. To improve azimuth resolution, VTS scanners routinely reach 5.5 m (18 ft), some more.
Apertures as big as 11 m have been produced, but require very stiff support to prevent sag and attendant performance loss. Because the slots are driven in series, problems arise with frequency dispersion and propagation time when transmitting short (~50 ns) pulses. Velocity of propagation is about 0.2 m/ns (two-thirds the speed of light), so for the 27 ns it takes the leading and trailing edges to traverse a 5.5 m aperture, only some of the slots are energised, reducing gain, introducing squint and spoiling sidelobe performance (see next section); 11m apertures are unsuited to pulselength <70 ns. At pulse edges, rate of change of power is high, depending on the rise and fall times of the modulator/magnetron combination, so spectrum is broad.
The upper and lower frequency components are dispersed by the frequency-dependent nature of Xg, further degrading the radiation patterns. Only during the central part of the pulse can the scanner meet its designed performance.
Slotted waveguide
Up to 100 slots, each radiating broad arc At nulls, wavelets interfere with zero resultant
Phase front
R. First null
Q. Beam axis P. Mechanical axis
R'. First null Differential
squint angle New beam axis
Azimuth aperture, a
Load
Energy from transmitter
Radiated field alternately +ve and -ve
Phase fronts
On beam axis, all wavelets are in phase and add
Frequency changed
Slots no longer radiate same phase
(a) Slotted array, design frequency (b) Slotted array, offset frequency