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Figure 4.6
Gain of a directional antenna in decibels for all angles with respect to the peak of the
main beam indicating the worst-case envelope of sidelobes.
Figure 4.7
A comparison of the radiation (directivity) pattern of a real 10m C-band antenna with
the ITU sidelobe envelope.
characteristics of Earth station antennas when one is considering transmit or receive
interference. Figure 4.7 includes a smooth curve, called a sidelobe envelope, that
defines a specification of maximum sidelobe gain. Two such envelopes are shown,
the lower being a tighter specification that applies between 1
°
and 20
°
off the peak
of the main beam.
4.2.3.3
Isolation
The directive property of an antenna dertermines how effective it will be for getting
signal power from the source to the receiver. However, any link can be degraded
by signals on the same frequency that enter the receiving antenna from a direction
118
Microwave Link Engineering
other than along the main beam. Likewise, a transmitting station can cause degrada-
tion to other systems by sidelobe radiation. Any undesired signal that can potentially
degrade reception is RFI. There is a whole field of engineering study that focuses
on the identification of sources of RFI, the establishment of criteria for acceptable
operation in the presence of RFI, and the development of techniques for countering
its effects. Techniques such as beam shaping, cancelation, and shielding are effective
in that regard.
The presence of RFI is a consequence of the fact that all radio communications
systems reuse frequencies, that is, there will be more than one radio station operating
on any particular frequency at any particular time. What keeps the RFI within
acceptable limits is isolation that is either natural (e.g., geographical or angular/
orbital separation) or artificial (e.g., measures such as RFI shielding or beam
cancellation). Satisfactory operation of independent microwave users on the same
frequency often requires cooperation (called frequency coordination) because ‘‘one
user’s radio link is another user’s RFI.’’ Several satellites can operate in the geosta-
tionary arc in the same frequency band because each directional ground antenna
can focus on one particular satellite, suppressing the RFI produced by adjacent
satellites (see Figure 1.26).
For example, if we assume the receiving user’s antenna has the characteristic
given in Figure 4.6 and the interfering satellite is located 45 degrees away from
the satellite that we wish to receive (the desired satellite), then the RFI is suppressed
by at least 20 dB. Actual ground antennas used in FSS and BSS links provide 40
to 60 dB of peak gain with suppression at 45 degrees of 50 to 70 dB. At closer-
in angles, the isolation may amount to only 20 to 30 dB, which is adequate to
allow satellite spacing as small as 2 degrees. The first sidelobe, in particular, is
normally the strongest, being typically 15 to 20 dB down from and within 1 degree
(more or less) of the peak gain. The control of those sidelobes demands careful
design and installation of such antennas. A useful specification in that regard is
the sidelobe envelope (shown in Figure 4.7), which defines the worstcase potential
for RFI. A standardized formula, which was adopted by the ITU as Recommenda-
tion S.465-5, provides a common standard for the larger ground antennas used at
C and Ku bands. In it, gain at a particular off-axis angle is specified in the direction
of a potentially interfering (or interfered-with) satellite by:
G(
⌰
)
≤
29
−
25 log
10
⌰
,
dBi
where
⌰
is the offset angle between the direction of the main beam and that toward
the interfering or interfered-with satellite, for angles between
±
1 degree and
±
37 degrees. Furthermore:
G(
⌰
)
≤
8 dBi, for angles between
±
7 degrees and
±
9.2 degrees
For angles between
±
9.2 degrees and 48 degrees:
G(
⌰
)
≤
32
−
25 log
10
(
⌰
), dBi
Lastly, G(
⌰
)
≤ −
10 dBi for
⌰
>
48
°
.
4.2
Propagation on the Earth-Space Link
119
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