This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: Dec 2012

Parabolic Reflector Antenna with offset feed

• In this the radiation pattern of the horn is offset so that it illuminates only the upper portion of the reflector. The feed horn and its support can be placed well clear of the main beam so that no blockage occurs.
• With the centre fed arrangement, the blockage results typically in a 10% reduction in efficiency and increased radiation in the sidelobes. The offset arrangement avoids this.
• When the antenna is in the transmit mode the energy from HPA is given to the feed phase centre where the horn antenna is present. The horn antenna then illuminates the reflector by radiating the energy towards it. The reflector then radiates to form parallel beam of energy.
• When the antenna is in the receive mode the reflector captures energy from all directions and converges it to the focal point, from there it is fed to the LNA via OMT.
• The main disadvantages of the offset feed are that a stronger mechanical support is required to maintain the reflector shape and because of the asymmetry, the cross-polarization with a linear polarized feed is worse compared with the centre fed antenna.
• Polarization compensation can be introduced into the primary feed to correct for the cross-polarization, or a polarization-purifying grid can be incorporated into the antenna structure
• The advantages of the offset feed are sufficiently attractive for it to be standard on many satellites.
• It is also used with double-reflector earth station antennas, and is being used increasingly with small receive-only earth station antennas.

Cassegrain Antenna:

1. The basic Cassegrain antenna consists of a main paraboloid reflector and a subreflector which is a hyperboloid.
2. The subreflector has two focal points, one of which is made to coincide with that of the main reflector and the other with the phase center of the feed horn.
3. The Cassegrain system is equivalent to a single paraboloidal reflector of focal length $f=\frac{eh+1}{eh-1} fs$ where eh is the eccentricity of the hyperboloid and f is the focal length of the main reflector.
4. The eccentricity of the hyperboloid is always greater than unity and typically ranges from about 1.4 to 3. The equivalent focal length, therefore, is greater than the focal length of the main reflector.
5. When the antenna is in the transmit mode the energy from the high power amplifier (HPA) is radiated to the real focal point of the sub reflector by the feed, which illuminates the convex surface of the sub reflector. The sub reflector reflects back the energy to the paraboloid reflector. The paraboloid reflector then radiates the energy to form parallel energy signal.
6. In the receiving mode, signal energy is captured by the paraboloid reflector which transfers it to the real focal point of the main reflector. Then the hyperboloid reflector captures the energy and then reflects it to the phase centre where feed is located. This energy is then given to the low noise amplifier (LNA) via orthogonal mode transducer (OMT).

7. Advantages-

a. Has better gain and good mechanical strength.

b. Feed line loss is less due to reduced feed line length.

c. Has low noise temperature.

d. Blocking problem is less.

8. Disadvantages-

a. Expensive than single parabolic reflector antenna.

b. Integration of the main reflector, sub reflector and feed is difficult.

Problem:

$Given-Efficiency(η) =0.6$

$Diameter (D) =0.1 meter$

$Frequency(f) = 6 GHz$

$Gain = η \frac{4 π A_e}{λ^2}$

Where

$A_e= area of the dish antenna = π r^2= π\frac{D}{4^2} ;r= radius of the dish antenna.$

$And λ = wavelength$

Therefore,