2. Horn Antenna
• The horn is widely used as a feed element for large radio astronomy, satellite
tracking, and communication dishes found installed throughout the world. In
addition to its utility as a feed for reflectors and lenses, it is a common element
of phased arrays and 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.
• The horn is nothing more than a hollow pipe of different cross sections, which
has been tapered (flared) to a larger opening. An electromagnetic horn can take
many different forms, four of which
3. Horn Antenna
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
6. E-Plane Horn
The E-plane sectoral horn is one whose opening is flared in the
direction of the E-field.
7. E-Plane Horn
The horn can be treated as an aperture antenna. To find its
radiation characteristics, the equivalent principle techniques can
be utilized.
To develop an exact equivalent of it, it is necessary that the
tangential electric and magnetic field components over a closed
surface are known. The closed that is usually selected is an
infinite plane that coincides with the aperture of the horn.
When the horn is not mounted on an infinite ground plane, the
fields outside the aperture are not known and an exact
equivalent cannot be formed. However, the usual approximation
is to assume that the fields outside the aperture are zero.
8. Aperture Phase Distribution
• It is assumed that, there exist a line source radiating cylindrical
waves at the imaginary apex of the horn. As waves travel in the
outward radial direction, the constant phase fronts are cylindrical
which do not coincide with aperture plane.
• At any point y’ at the aperture of the horn, the phase of the field
will not be the same as that at the origin.
• The phase difference because the wave travelled different
distances from the apex to the aperture.
• The difference in path of travel, designated as δ(y’), can be
obtained as follows
11. Aperture Fields
• When δ(y’) is multiplied by the phase constant k, the result is a quadratic phase
variation between the constant phase surface and the aperture plane.
It can be shown that if :-
(1) The fields of the feed waveguide are those of its
dominant TE10 – mode and neglecting the higher
order modes.
(2) The horn length is large compared to the aperture
dimensions, the lowest order mode fields at the
aperture of the horn considering the quadratic phase
variation are given by :-
13. • So the aperture fields become…..
Aperture Fields
14. Aperture Equivalent Currents
• To find the fields radiated by the horn, only the
tangential components of the E - and / or H – fields over
a closed surface must be known. The closed surface is
chosen to coincide with an infinite plane passing
through the mouth of the horn.
