Reflector antennas use a reflecting surface to direct the radiation pattern of a feeding element. Parabolic reflectors provide highly directional beams by reflecting waves from a feed at the focus into a parallel beam. Reflectors can have different shapes like flat sheets, corners, parabolas, ellipses, and hyperbolas. Parabolic reflectors are widely used in applications like television, communication, and radio astronomy due to their ability to produce a narrow beam. The feed is a key component and common options include dipoles, horns, and Cassegrain feeds which place the feed behind the reflector. Design factors like the focal length to diameter ratio determine properties like beamwidth and efficiency.

1. REFLECTOR ANTENNAS
R.Ramalakshmi
Assistant Professor
Ramco Institute of Technology
Rajapalayam

5. • Reflector antennas are widely used to modify
the radiation pattern of a radiating element.
• For example, the backward radiation from an
antenna may be eliminated with a plane sheet
reflector of large dimensions.
• In general, a beam of predetermined
characteristics may be produced by means of
a large, suitably shaped and illuminated
reflector surface.

6. Reflectors of Various Shapes

9. • Figure (a) has a large flat sheet reflector near a
linear dipole antenna to reduce backward
radiation. Reflector element is relatively
insensitive to small frequency changes.
• The desirable properties of the sheet reflector
may be largely preserved with the reflector
reduced in size as in fig (b).
• In fig(c), the sheet has degenerated into a thin
reflector element. This reflector element is highly
sensitive to frequency changes.

10. • With two flat sheets intersecting at an angle α
(α<180) as in fig (d), a sharper radiation pattern
can be obtained. This arrangement is called an
active corner reflector antenna.
• A corner reflector without an exciting antenna
can be used as a passive reflector. Corner angle is
always 90 for passive reflector.
• Reflector with this angle have the property that
an incident wave is reflected back towards its
source as in fig(e), the corner acting as a retro
reflector.

11. • The parabolic reflectors can be used to
provide highly directional antennas.
• A parabolic reflector shown in fig(f) reflects
the waves originating from a source at the
focus into a parallel beam, the parabola
transforming the curved wave front from the
feed antenna at the focus into a plane wave
front.

12. • The elliptical reflector as in fig(g) produces a
diverging beam with all reflected waves
passing through the second focus of the
ellipse.
• Examples of the reflectors of other shapes are
the hyperbolic and circular reflectors shown in
fig (h) & (i).

13. Applications of Corner Reflector
• Television
• Point to Point Communication
• Radio Astronomy Applications

14. Parabolic Reflectors
• A parabola may be defined as the locus of a point
which moves in such a way that its distance from
the fixed point (called focus) plus its distance
from a straight line is constant.
• A parabola is a two dimensional plane curve.
• A parabola with focus F and vertex O is shown in
below figure.

15. Parabolic Reflectors

16. • OF = Focal length = f
• F = focus
• O = Vertex
• OO’ = Axis of parabola
• By the definition of parabola,
FP1 + P1P1’ = FP2 + P2P2’ = FP3 + P3P3’ = FP4 + P4P4’
= Constant (say K)

17. • The open mouth D of the parabola is known
as the aperture.
• The ratio of the focal length to aperture size
i.e f/D is known as f over D ratio. Its value
usually varies between 0.25 to 0.50

18. • Focusing or beam formation action of
parabolic reflector can be understood by
considering a source of radiation at the focus.
• Let a ray starts from the focus F at an angle ө
with respect to parabolic axis (oo’). The curve
strikes at point P1.
• Let a tangent be drawn at P1 on the curve.
According to law of reflection, the angle of
incidence and angle of reflection will be equal.

19. • This results that the reflected ray is parallel to
the parabolic axis, regardless of the particular
value of Ө. i.e., All the waves originating from
focus will be reflected parallel to the parabolic
axis.
• This implies that all the wave reaching at the
aperture plane are in phase.
• So a very strong and concentrated beam of
radiation is there along the parabolic axis.

20. • In fact, parabola converts a spherical wave
front coming from the focus into a plane wave
front at the mouth of the parabola as in fig (a).
• The part of the radiation from the focus which
is not striking the parabolic curve appears as
minor lobes.
• Obviously this is waste of power. This can be
minimized by partially shielding the source as
in fig (b).

22. Paraboloidal Reflector or Microwave Dish
• A parabola is a two dimensional plane curve. A
parabolic reflector or paraboloid is a three
dimensional curved surface.
• A paraboloid is formed by rotating a parabola
about its axis (oo’).
• It produces a parallel beam of circular cross
section, because the mouth of the paraboloid
is circular.

25. • The radiation pattern of an antenna
employing paraboloidal reflector has a very
sharp major lobe accompanied by a number
of minor lobes which are smaller in size.
• The narrow major beam is in the direction of
paraboloid axis as in fig (b).
• The three dimensional pattern is a figure
obtained by revolving the figure (b) about oo’.

26. • Paraboloidal reflector can be designed by
keeping the mouth diameter fixed and varying
the focal length f.
• There are 3 possible cases. (i) f<D/4 (ii) f= D/4
(iii) f>D/4

27. • In the first case, the focal length is small such that
the focus lies well inside the mouth aperture. In
this case it is difficult to get a source giving
adequately uniform illumination over such a wide
angle.
• In the second case, the focus lies in the plane of
the open mouth. The focal length is equal to one
fourth of open mouth diameter. (D/4)
• In the third case, when the focal length is large
such that the focus lies beyond the open mouth,
it becomes difficult to focus all the radiation from
the source on the reflector.

28. • The radiation beam from an antenna
employing paraboloid reflector should be
theoretically a pencil – shape.
• The most widely used antenna for microwave
is the paraboloidal reflector antenna. It
consists of a primary antenna such as dipole
or horn situated at the focal point.
• The mouth or the physical aperture of the
reflector is circular.

29. • If the feed or primary antenna is isotropic,
then the paraboloid will produce a beam of
radiation.
• Assuming the circular aperture is large, the
beamwidth between first nulls is given by,
BWFN = 140λ/D degree
Where λ – free space wavelength in m.
D – diameter of aperture in m i.e mouth
diameter.

30. • The beamwidth between first nulls for a large
uniformly illuminated rectangular aperture is
given by,
BWFN = 115 λ/L degree
where L is the length of aperture in λ.
• Half power beamwidth for a circular aperture
is given by,
HPBW = 58 λ/D degree

31. • Directivity D of a large uniformly illuminated
aperture id D = 4πA/ λ2
Area of the aperture – A
• In practice, the primary antenna is not isotropic
and thus does not radiate uniformly which
introduces distortion.
• This results in less capture area which is smaller
than actual area i.e Ao = KA
• Ao – capture area; A – actual area of mouth; K –
constant depends on type of antenna used = 0.65
for dipole antenna.

32. • The power gain of circular aperture paraboloid with
respect to half wave dipole is given by,
• Gp = 4πAo/λ2 = 4π KA/ λ2
• Gp = (4πK/ λ2) (πD2/4) = 0.65 (πD2/4) = 0.65 * (3.14)2 *
(D/λ)2
• Gp = 6.389 (D/λ)2 = 6 (D/λ)2 ; D- Diameter of circular
aperture.
• The gain is a function of aperture ratio of the
paraboloid.
• The effective radiated power (EIRP) of an antenna is
multiplication of input power fed to antenna and its
power gain.

33. Spill Over & Back Lobe
• In addition to the desired radiation, some of the desired
rays are not captured by the reflector and these constitute
spill over (overflowing or spreading into another area).
• While receiving, spill over increases noise pickup which is
particularly troublesome in satellite ground stations.
• Some radiations from the primary radiator occur in the
forward direction in addition to the desired parallel beam.
• This is known as backlobe radiation as it is from the
backlobe of the primary radiator.

34. • Backlobe radiations are not desirable as it can
interfere destructively with the reflected
beam and hence practical radiators are
designed to minimize this.

35. • The directivity of the paraboloidal reflector is a
function of the primary antenna directivity and
the ratio of the focal length to reflector diameter.
• This focal length to diameter ratio is known as
aperture number.
• The aperture number determines the angular
aperture of the reflector.
• This in turn determines how much of the primary
radiation is intercepted by the reflector.

36. • The effective area is given by, Ae = ᶯ A
• Where ᶯ - aperture efficiency
• A – physical area of the reflector = πD2/4
• When the aperture number f/D >λ/4, the focal
point is outside the reflector and spill over loss
occurs.
• However the primary radiation is not much
reduced and the reflector illumination
approaches a uniform value. This increases the
aperture efficiency.

37. • However if f/D is very much greater than λ/4, the
spill over loss gets increased and the aperture
efficiency decreases.
• When f/D < λ/4, then the focal point is inside the
reflector, no spill over loss occurs but the
illumination of the reflector becomes non-
uniform and this again reduces efficiency.
• The relationship between aperture number and
angular aperture is given by, f/D = ¼ cot (θ/2) =
0.25 cot(θ/2)

38. Feed Systems of Reflector Antennas
• The antenna placed at the focus of a paraboloid is
known as feed radiator or primary radiator or simply
feed and its radiation pattern is known as primary
pattern.
• The parabolic reflector is known as secondary radiator.
• An ideal feed would be that radiator which radiates
towards reflector in such a way that it illuminates the
entire surface of the reflector and no energy is radiated
in any other directions. But it is not available in
practice.

39. Various Feeds Used in Reflector
• Dipole Antenna
• Horn Antenna
• End fire Antenna
• Cassegrain Feed

40. Feeding Structures

41. Dipole Antenna Feed
• It is not very much suitable but occasionally used.
• The simplest and generally used is a dipole with
parasitic reflector (i.e Yagi uda) or small plane
reflector which is fed with a coaxial line.
• Typically spacing between driven element and
parasitic element is 0.125λ and for a plane
reflector, it may be around 0.4λ.

42. End Fire Feed
• The double dipoles are so spaced and phased
such that end fire pattern is produced which
illuminates the paraboloid reflector.

43. Horn Feed
• The most common feed radiation for paraboloid
reflector antenna is waveguide horn. The horn
feed is a waveguide feed.
• Horn antenna is pointing the paraboloid and the
direct radiation from the horn antenna is
minimum.
• If circular polarization is required, conical horn or
helix antenna can be used as feed at the focus of
paraboloid.

44. • For getting maximized beam pattern along the
parabolic axis, feed is placed at the focus.
• But if the feed is moved laterally from the
focus i.e., perpendicular to axis, limited beam
motion can be obtained.
• If the feed is moved along the axis, the pattern
is broadened.
• This important position of feed is the focus.

45. Cassegrain Feed
• Here the primary feed radiator is positioned
around an opening near the vertex of the
paraboloid instead of at focus i.e., horn antenna
is placed at the vertex and secondary feed is the
sub reflector usually hyperboloidal reflector.
• The focus of hyperboloid coincides with the focus
of paraboloid. In this system, the paraboloid is
the main reflector. The hyperboloid is the sub
reflector.

46. • The radiations emitted from primary feed
radiator reach sub reflector. The sub reflector
reflects and illuminates the main parabolic
reflector. The main reflector reflects the rays
parallel to the axis.
• It becomes important to minimize the length
of the transmission line or waveguide
connecting the feed radiator with receiver or
transmitter. This is needed specially to avoid
losses.

47. Advantages of Cassegrain Feed
• Reduction in spill over and minor lobe
radiation
• Simple in construction
• Quite inexpensive
• Widely used in fixed point to point micro wave
communication.
• Satellite reception and tracking
• Ability to place feed in a convenient location.

48. Disadvantage of Cassegrain Feed
• The disadvantage is that some of the radiation
from the paraboloid reflector is obstructed
because of the presence of sub reflector along
the path of parallel rays.
• This is tolerable in greater dimensions
paraboloid but problem in small size
paraboloids.

49. Offset Paraboloidal Reflector
• The aperture blocking defect can be avoided
by using an offset reflector which is applicable
to focal point feed.
• An offset dish antenna or off-axis dish antenna
is a type of parabolic antenna. It is so called
because the antenna feed is offset to the side
of the reflector.

50. Offset Paraboloidal Reflector

51. REFERENCES
• John D Kraus,” Antennas for all Applications”,
3rd Edition, McGraw Hill, 2005.
• Edward C.Jordan and Keith G.Balmain”
Electromagnetic Waves and Radiating
Systems” PrenticeHall of India, 2006
• Constantine.A.Balanis “Antenna Theory
Analysis and Design”, Wiley Student Edition,
2006.

52. Thank You