Astronomy Principles and Practice Fourth Edition.pdf

Antenna design 359
Figure 21.7. A four-element collinear array and its polar diagram. (The axes of the antennas are collinear with
the y-axis.)
contributions arrive in phase at the receiver. The polar diagram drawn in figure 21.8(b) is that which
is found in the xz-plane. In the yz-plane, the polar diagram is the same as that of a single half-wave
dipole.
By a combination of the collinear and broadside arrays, the directivity of the two-dimensional
array is reduced in both the x and y directions, leaving the main narrow lobes pointing in either
direction along the z-axis. One of these directions can be eliminated by a wire mesh reflector put
at the correct distance in a plane parallel to the two-dimensional array. An array of 64 half-wave
antennas gives a beamwidth of about 14 ◦ . The absolute power gain is equal to the number of dipoles
in the array multiplied by 2 × 1·64 (the factor of two is obtained by using the back reflector) and, in
this case, the array has an absolute power gain equal to 210.
By mounting the whole system on an adjustable rig, the highly-directional antenna can be pointed
to any source on the celestial sphere. Such a system is one of the forms of a radio telescope.
Similar highly-directional antennas can be devised by using an array of parasitic antennas. Again,
the absolute power gain is obtained by multiplying the absolute power gain of a single antenna by the
number of antennas in the array.
It may be noted that the contributions from the elements in an array may be connected in different
ways and are referred to as phased arrays. In the simplest design of collinear array, each signal
flows into a single line whose centre feeds the receiver (see figure 21.9(a)). In the Christiansen
interferometer,orChristmas Tree array, the signals are added together in pairs in a progressive way
so that the total length of feed between each antenna and the receiver is the same. This is illustrated in
figure 21.9(b). The collinear system has smaller power losses in the feed than the Christmas Tree array