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Chapter 5 213

It means that for a short electric dipole > /5. These rules could and should be applied to any
electrically small antenna that is the equivalent of a point-size source. Meanwhile, the broad
class of antennas is an alliance of multiple elementary
ones and as a whole is far from being elementary. For
example, the world’s next generation Square
Kilometer Array (SKA) radio telescope is a set of
thousands of different types of antennas separated by
over 3 000 kilometers. Each of the SKA core sets of 5
km in diameter includes multiple parabolic dishes
demonstrated schematically by the artist impression in
6
Figure 5.2.5a . That radio “… telescope aimed to be
built in Australia and South Africa. If built, it would
have a total collecting area of approximately one a)
square kilometer. It would operate over a wide range Figure 5.2.5a The SKA core set
of frequencies and its size would make it 50 times of antennas
more sensitive than any other radio instrument. It
would require very high performance central
z computing engines and long-haul links with a capacity
greater than the global Internet traffic as of 2013. It
P
should be able to survey the sky more than ten
thousand times faster than ever before. It would
exploit radio astronomy's ability to provide the highest
resolution images in all astronomy. The SKA would
y be built in the southern hemisphere, in sub-Saharan
x b) states with cores in South Africa and Australia, where
the view of the Milky Way Galaxy is best and radio
Figure 5.2.5b Two emitters’ interference least … The SKA will provide continuous
geometry
frequency coverage from 50 MHz to 14 GHz in the

first two phases of its construction. A third phase will then extend the frequency range up to 30
GHz.”
7

It is essential to adjust the far field zone interpretation for such or any other electrically sizable
antenna. Assume that only two independent sources and of EM waves are located over
0
1
said y-axis at the distance as shown in Figure 5.2.5b. Clearly, the phase shift between two

EM waves coming to the far away observation point P is equal to ∆ = ( − ). According
1
0

to the law of cosine, binomial theorem and the fact that ⁄ ≪ 1 at point P

0

2

2
2
= � + − 2 cos = �1 + � � − 2 cos
0
0
1
0

0 0 (5.25)
2 2
1
2
2
≅ �1 − cos + � � cos � = − cos + cos

0
0
0 2 0 2 0

6 Public Domain Image, with permission: http://www.skatelescope.org/
7 https://en.wikipedia.org/wiki/Square_Kilometre_Array