Different parameter estimations for Horn Antennas

1. MEASUREMENTS
HORN ANTENNA
Presented By:
Ayushi Gagneja

2.  The horn is nothing more than a hollow pipe of
different cross sections, which has been tapered (flared)
to a larger opening. The type, direction, and amount of
taper (flare) can have a profound effect on the overall
performance of the element as a radiator.
 An electromagnetic horn can take many different
forms, four of which are:
(a) E-plane (b) H-plane
(c) Pyramidal (d) Conical
What is HORN Antenna?

4. The horn is widely used as a
 feed element for large radio astronomy
 satellite tracking
 communication dishes installed throughout the world
 feed for reflectors and lenses
 it is a common element of phased arrays
 serves as a universal standard for calibration and gain
measurements of other high-gain antennas.
Its widespread applicability stems from its simplicity
in construction, ease of excitation, versatility, large
gain, and preferred overall performance.

5. IMPEDANCE
 Horns input impedance is slowly varying over a wide frequency
range.
 Antenna impedance is represented as ratio of voltage to current at
antenna’s terminal.
 In order to achieve maximum energy transfer input impedance of
antenna must identically match characteristics impedance of
transmission line.
 If the two impedance don’t match, reflected wave will be
generated and travel back towards energy source. This reflection
of energy results in overall antenna efficiency.
 Impedance of an antenna, with no load attached, is defined as:
ZA=RA+jXA

6. ZA= antenna impedance (ohms)
RA= antenna resistance (ohms)
XA= antenna reactance (ohms)
Resistive part is RA=Rr+RL
where Rr=radiation resistance of antenna
RL=loss resistance of antenna
Input impedance of antenna is function of frequency. Thus
antenna will be matched to interconnecting transmission
line and other associated equipment only within a
bandwidth.
In addition, impedance depends on factors like geometry, its
method of excitation, and its proximity surrounding
objects.

8. POLARIZATION
Polarization of antenna is orientation of electric
field (E-field) of the radio wave with respect to
Earth’s surface and is determined by physical
structure of antenna and its orientation.
Straight wire antenna will have one polarization
when mounted vertically, and different
polarization when mounted horizontally
Reflection generally affects polarization. For
radio waves most important reflector is
ionosphere.-signals which reflect from it will
have their polarization changed.

9. • Polarization of radiated wave is defined as the property of
electromagnetic wave describing time varying direction
and relative magnitude of electric field vector.
• It is described by the geometric figure traced by the
electric field vector upon a stationary plane perpendicular
to direction of propagation, as waves travel through the
plane.
• Three different types of polarization are shown:

10. Vertical and horizontal polarization are simplest
forms and fall under the category of Linear
Polarization.
Circular Polarization occurs when two or more
linearly polarized waves add together, such that the
E-field of net wave rotates. It is advantageous that it
helps to overcome effects of propagation anomalies,
ground reflections and effects of spin that occur on
many satellites.
Elliptical Polarization occurs when there is mix of
linear and circular polarization

11. RADIATION FIELD
The waves of the signal will propagate down the horn antenna
towards the aperture. As they travel along the flared opening, the
waves travel as spherical wavefronts, its apex is referred to as the
phase centre of the horn antenna.
As the phase front progressing along the horn antenna are spherical,
the phase increases smoothly from the edges of the aperture plane to
the centre.
The difference in phase between the centre point and the edges is
called the phase error. This increases with the flare angle reducing
the gain, but increasing the beamwidth(radiation pattern).
As a result horn antennas have wider beamwidths when compared
to similar-sized plane-wave antennas like parabolic reflectors.

12. In order to provide a narrow beamwidth a longer horn is
required, i.e. having a smaller angle of flare, to keep the phase
angle constant.
However the phase error issues mean that horn sizes are
practically limited to around 15 wavelengths otherwise larger
sizes would require a much longer antenna.
This limits gain of practical horns to about 30dBi and minimum
bandwidth to about 5-10degrees.
The radiation pattern at 2 GHz
is shown

13. For example: Assume a=3.69
inches, b=1.64 inches, A=30
inches, and B=23.8 inches.
This horn is somewhat large,
and will work well above
roughly 2 GHz.
Horns made for higher
frequencies are smaller. This
horn antenna, with a
waveguide feed is shown

14. GAIN
 Horn antennas have very little loss, so
the directivity of a horn is roughly equal to its gain.
 For a horn to be realizable, the following must be true:
 It can be shown that

16.  The optimum-gain condition in the E-plane
 There is only one physically meaningful solution
 Similarly, the maximum-gain condition for the H-plane
 Since must be fulfilled

17.  Substituting in the expression for the horn’s gain,
gives the relation between A, the gain G, and the aperture
efficiency

19. Any Queries??