Page 255 - Maxwell House

Chapter 5 235

inverse issues: how to create a coherent whole from a restricted number of primary objects.
Imagine that you bought a LEGO construction set with a variety of basic bricks. The diversity
of objects you are able to create is limited with such a set, but it is possible to construct the final
objects using different combinations of LEGOs. We encounter a similar situation in antenna
synthesis: in most instances, a unique solution does not exist, and there are multiple solutions
with different techniques and functional qualities. In some cases, we can succeed in getting the
answer but later find that a technically sound realization is physically unrealistic, prohibited by
enormous cost, contains an excessive number of radiating elements, is too bulky, etc. If so, the
real starting point of building any antenna in modern engineering practice must be with looking
through a list of the well-known and practically tested antennas, choosing something that
closely fits a newly developed system, and then applying some modifications to satisfy the
system requirements as completely as possible. Such an approach replaces the thorny problem
of synthesis with the much more straightforward task of interactive numerical optimization [12,
13]. Note that the exemplary radiation patterns shown in Figure 5.4.1 typically can be created
by relatively simple discrete or continuous assemblies of elemental radiators.
Antennas with a “donut shaped” pattern in Figure 5.4.1a are omnidirectional in the azimuth
plane and highly directional in elevation. Their typical applications are TV and AM-FM
broadcast stations as well communication systems arranging the information exchange between
mobile devices like cell and cordless phones, computers, moving cars, bus, and trains, network
routers and base stations, etc. They are also a critical element of Wireless Local Area Networks
(WLANs) that wirelessly links two or more devices inside buildings or another limited area.
Figure 5.4.1b illustrates a pencil beam required in a broad diversity of systems like surveillance
and weather radars, ground-based and satellite communications, radio astronomy, etc. The
beam steering (scanning) is achieved by mechanical or electronic means (see Figure 5.4.1c).

In high-resolution monopulse radars, capable of detecting and tracking targets even per single
pulse, the antenna system typically produces two differently shaped and steered in tandem
receiving beams: the first one is a pencil beam as shown in Figure 5.4.1b customary called sum-
or Σ-beam and the second one is called difference- or Δ-beam consisting of two narrow lobes
of opposite polarity with very steep beam slope as shown in Figure 5.4.1d. Here the picture in
picture graph illustrates the fact that the Σ-beam peak and Δ-beam deep null occurs in the
direction of a target. Subsequently, the strong signal received in the Σ-beam channel is used
mainly for target detection and preliminary angular estimation. Meanwhile, the ratio of signals
in Δ-beam and Σ-beam channels delivers much more precise angular estimation based on the
Δ-beam null position measurement. Evidently, the target movement leads to a high signal
variation in the Δ-beam channel and its polarity provides dynamic target tracking. There are
more details in Section 8.3.7 of Chapter 8.

Figure 5.4.1e demonstrates the idealized elevation pattern of 12-pencil-beams stacked into so-
2
2
called cosecant squared pattern. Antennas with such pattern provide a gain of ()~ /
where is the maximum radar range and is the distance to a reflective object hovering
with a constant height h within the beam. It can be an aircraft, bird, thunderstorm front, missile,
or something else moving in the direction to antenna. Why we need antennas with so unusual
patterns? To answer this question let refer to (5.67) describing the power of signal scattering