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Polar Diagrams

A polar diagram is used to show the radiation or reception pattern of an aerial. It is simply a line joining all points of equal signal strength and is generally a plan view perpendicular to the plane of radiation or reception. From here on we will talk about radiation only, but the same principle applies to reception.

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A dipole aerial radiates most energy at right angles to the aerial with signal strength decreasing towards the ends of the aerial, where there is no radiation. A three dimensional representation of radiation from such an aerial would be a torus, centred on the centre point of the aerial:

VERTICAL PD

COMPOSITE PD

Figure 4.3 3-D Polar Diagram (PD)

Clearly such diagrams would be cumbersome so a plan view of the plane of radiation is used:

HORIZONTAL PD

VERTICAL PD

Figure 4.4 Plan view polar diagram

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Directivity

Many systems require the directional emission or reception of energy, for example; radar, ILS, MLS and many more. How this directivity is achieved depends on the frequency and application.

The simplest way to achieve directivity is to add parasitic elements to the aerial. If we place a metal rod 5% longer than the aerial at a distance of quarter of a wavelength from the aerial and in the same plane as the aerial, it will act as a reflector.

Figure 4.5 Directivity using reflector

This reflector re-radiates the energy 180° out of phase, the resulting polar diagram is shown above, with no signal behind the reflector and increased signal in front of the aerial.

This process can be taken further by adding other elements in front of the aerial. These elements are known as directors and are smaller than the aerial itself.

Figure 4.6 Improved directivity using reflector and directors

All will recognize this as being the type of aerial array used for the reception of television signals. The directors have the effect of focussing the signal into (or out of) the aerial, giving a stronger signal than that which would be generated by a simple dipole.

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However, directivity comes with its own price. As can be seen from the diagram, we have produced a strong beam along the plane of the aerial, but have also produced many unwanted side lobes which would receive (and transmit) unwanted signals. Signals received in these side lobes produce characteristic ghosting on television pictures, usually caused by reflections from buildings etc. These side lobes give major problems which have to be addressed in SSR and ILS, and also produce problems in primary radars.

The Instrument Landing System (ILS) uses an extension of this idea to produce the narrow beams (or lobes) of energy required to guide aircraft along the runway centre line: the ILS ‘localizer’ antenna which produces this is an array of 16 or 24 aerials placed in line with half wavelength spacing. There is some modification to the way the signal is fed to the aerials but the end result is that two narrow beams of energy are produced which are symmetrical, close to the centre line of the runway as shown in Figure 4.7.

Figure 4.7 ILS localizer lobes.

In the Automatic Direction Finder (ADF) a loop aerial is used to detect the direction of an incoming signal.

LOOP

NULL

NULL

Figure 4.8 Loop aerial ‘Figure-of-eight’ polar diagram

When the loop is aligned with the incoming signal then there is a phase difference between the signals in each of the vertical elements of the loop and there will be a net flow of current from the loop. If the loop is placed at right angles to the incoming signal then the induced currents will be equal and will cancel each other out giving a zero output.

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The resulting polar diagram will have two distinct nulls which can be used to determine the direction from which the radio wave is coming. How this principle is utilized will be discussed in detail in Chapter 7.

Radar Aerials

Radar systems operate in the UHF and SHF bands; the transmission of such frequency energy requires the use of ‘waveguides’ rather than cables. The parabolic dish is widely used as a ‘reflector’: the open end of a waveguide (see Figure 4.9) is positioned at the focal point of the parabola (the centre of curvature, designated by point F in Figure 4.10) and directs the RF energy towards the dish. The energy from the open waveguide is reflected by the dish as parallel rays; the path length FXB, FYA etc. will therefore be equal and the transmitted wavefront will be made up of parallel rays that are all in phase.

Figure 4.9 Horn feed to Parabolic Reflector

Figure 4.10 Principles of the Parabolic Reflector

In principle a very narrow pencil beam should be produced as shown below, but apart from the region very close to the antenna, the beam, in fact, diverges. In effect, the parabolic reflector converts a point source of energy (the open waveguide) at the focal point into a plane wavefront of uniform phase.

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