Mallesh Antenna
Mallesh Antenna
Mallesh Antenna
SUBMITTED BY
A.D.MALLESWARARAO
I /II MTECH-CESP
168W1D8001
INTRODUCTION:
A mobile satellite communication usually provides so-called bidirectional
communication, such as voice, data, and telex and facsimile for mobiles .The other
type of services offered is navigation and broadcasting. For each service the
frequency bands allocated by the Radio Regulation are often designated with
alphabetical symbols such as C, L, Ku and Ka bands, as shown in Table 1 .Some
examples of typical mobile satellite services are as follows.
Communication
Navigation
A typical system is the GPS, in which L-band (1.6 and 1.3 GHz) frequencies are
used from a satellite to the earth.
Broadcasting
This service is to broadcast TV and radio programs from a satellite to the earth.
Although the present systems are designed for fixed terminals, not for mobiles, an
antenna system for trains and ships has been developed to receive TV programs
from the broadcasting satellite (BS) satellite in Japan.
proposed systems such as IRIDIUM and ODYSSEY, the s-band and L-band
are allocated in WARC 92.
Figure 6.4 shows the relationship between polarization mismatch loss and
axial ratios of receiving antenna, in which ARa and ARb denote axial ratios
of transmitting and receiving antennas.
4. Figure 6.8 shows the fading depth for antenna gain of 24, 20, 15 and 8 dBi
as a function of elevation angle with fully developed incoherent component.
5. Values estimated from the simple method give around the mean value of
those in Figure 6.7.
6. Mean value of fade duration (Td) and the fade occurrence interval (Ti) for a
given threshold level as a function of time percentage can be estimated from
the fading spectrum.
1. Blocking is caused by Ship Superstructure such as the mast and various type
of other antennas.the geometry
2. For blocking by a mast is shown in Figure 6.10. Attenuation due to blocking
depends on various parameters such as diameter of the column, distance
between antenna and column, and size of antenna. Based on experimental
data reported so far, attenuation due to blocking caused by a column type
structure is given in Figure 6.11 for antenna gain of 20 and 14 dBi.
For Low and medium gain systems the effect of multipath fading due to sea
surface reflection is a severe problem, especially at low elevation angles.
Fading Reduction Techniques applicable to these SES (ship earth station)
antenna are discussed in the following sections.
4. With this technique, the greatest Reduction effect is expected when the
correlation of the signal between the two antennas is near zero or lower.
6. If the frequency bands become widened in the future this method could be
applicable
1. For reflections from the sea Surface at 1.6/1.5 GHz, the horizontally
polarized wave is almost perfectly reflected, while the vertically polarized
wave is reflected with a large attenuation at grazing angle below 20 deg.
2. The phase is adjusted by a phase a shifter inserted in one of the ports of the
short backfire antenna feed, as shown in Figure 6.14(a).
6. In the figure, data denoted by ON are the depth received by the 40-cm
modified SBF mentioned below, with optimally controlled polarization
given by additional phase as shown in Figure 6.15(b).
9. The electrical constants of sea water are chosen to be r =70 for dielectric
constant, =5.5 S/m for conductivity and f =1.54 GHz for frequency.
10.The phase difference between the both components is almost constant and is
independent of the elevation angles.
1. In the method, the fading reduction effect will be decreased when the fading
with very slow fluctuation is dominant, as in very calm sea conditions.
2. The method is exceptionally useful in a calm sea when the coherent
component of the reflected wave is far stronger than the incoherent
component, and it has a considerable degree of fading reduction on the up
link path as well as the down link path with a very simple configuration, this
overcomes the disadvantages generally associated with diversity systems.
There are two typical mounts of the two-axis configuration: one is the EL/AZ
mount and the other is the Y/X mount. Simplified stick diagrams of both mounts
are shown in Figure 6.24.
In the Y/X mount, a full steerable function is achieved by permitting the rotation
angle from -90 to +90 deg to the X-axis and Y-axis.
In general, the ship rotation consists of seven elements: turn, roll, pitch, yaw,
heave, surge, and sway. Turn is change of headway. The components are
illustrated in Figure 6.25.
Three axis mount of an X/Y/X type is two-axis Y/X mount system with the X-
axis on it to remove the gimbal lock at the horizon.
In the mount, stabilized platform is made by the X-axis and Y-axis to take
out roll and pitch, and the two axis mount of the El/Az is settled on the
stabilized platform.
The axis configuration is shown in figure 6.27.
It includes:
Manual Track
Step Track
Program Track
2. At first the operator acquires the signal and moves the antenna around one
axis of the mount.
3. If the signal level increases, the operator continues to move the antenna in
the same direction.
4. If the signal level decreases, operator reverses the direction and continues to
move the antenna until the signal level is maximized.
5. The same process is repeated around the second axis and the antenna is held
after both axes are optimized.
6. The operator will try again to find the optimum points for both axes when
the received signal level decreases.
1. The principle of the step track system is same as that of manual track. The
only difference is that an electric controller plays the role of an operator in
the manual track.
3. The signal levels are compared before and after the antenna have been
moved by a present angular step.
5. Wrong decisions on the comparison of levels generally arise from S/N ratio,
the level changes due to the multipath fading and the stabilization error.
3. In the program track, the antenna is steered to the point of the calculated
direction based on the position data of the navigation equipment .Since the
Satellite direction changes because of roll , pitch and turn , a function to
remove the rapid motions is required in the program tracking.
4. The error of the program track system mainly depends on the accuracy of
sensors for roll, pitch and turn (i.e., the stabilization error).
5. An adequate sensor for the program track system is a vertical gyro, because
it is hardly affected by the lateral acceleration.
6. The controller calculates the direction of satellite to compensate for the ship
motions.
REFERENCES