Feed horn
Encyclopedia
In satellite dish
and antenna design, a feedhorn (or feed horn) is a small horn antenna
used to convey radio wave
s between the transmitter
and/or receiver
and the reflector
, particularly in parabolic antenna
s. In transmitting antennas, it is connected to the transmitter
and converts the radio frequency
alternating current
from the transmitter to radio wave
s and feeds them to the rest of the antenna, which focuses them into a beam. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which converts them to a tiny radio frequency voltage which is amplified by the receiver. Feed horns are used mainly at microwave
(SHF) and higher frequencies
in parabolic antenna
s.
The feedhorn also selects the polarization of the waves to be received, which helps to attenuate unwanted signals from adjacent channel
s and transponder
s, and from other communications satellite
s at nearby orbital positions. This can be either horizontal
or vertical
if the polarization is linear
, or clockwise or counterclockwise (also called left- and right-handed) if it is circular
. Certain devices can also allow a feedhorn to accept both linear and circular, though these cause a slight insertion loss
to all incoming signals.
When used with a parabolic reflector
or lens antenna, the phase center
of the horn is usually placed at the focal point
of the reflector, with the 3 dB point
s of the horn's radiation pattern
set to be at the edge of the reflector. On a satellite dish, the feedhorn is what is mounted at the end of a mast
from the center of the dish, or on tripod
legs mounted to the edge of the dish.
In satellite dishes, part of the receiver electronics is usually mounted in a box just behind the feed horn. This converts the high satellite microwave downlink frequencies to a lower frequency so it can be more easily sent through the coaxial cable
feedline to the receiver inside the building. This is called the low-noise blockconverter (LNB), and the unit consisting of the horn plus the converter is called the LNB with feedhorn (LNBF).
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
and antenna design, a feedhorn (or feed horn) is a small horn antenna
Horn antenna
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct the radio waves. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz...
used to convey radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s between the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and/or receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
and the reflector
Reflector (antenna)
An antenna reflector is a device that reflects electromagnetic waves.It is often a part of an antenna assembly.The most common reflector types are...
, particularly in parabolic antenna
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
s. In transmitting antennas, it is connected to the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and converts the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
from the transmitter to radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s and feeds them to the rest of the antenna, which focuses them into a beam. In receiving antennas, incoming radio waves are gathered and focused by the antenna's reflector on the feed horn, which converts them to a tiny radio frequency voltage which is amplified by the receiver. Feed horns are used mainly at microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
(SHF) and higher frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
in parabolic antenna
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
s.
The feedhorn also selects the polarization of the waves to be received, which helps to attenuate unwanted signals from adjacent channel
Adjacent channel
In broadcasting an adjacent channel is an AM, FM, or TV channel that is next to another channel. First-adjacent is immediately next to another channel, second-adjacent is two channels away, and so forth. Information on adjacent channels is used in keeping stations from interfering with one...
s and transponder
Transponder
In telecommunication, the term transponder has the following meanings:...
s, and from other communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
s at nearby orbital positions. This can be either horizontal
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
or vertical
Vertical direction
In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i.e., with the direction of the gravitational force at that point...
if the polarization is linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
, or clockwise or counterclockwise (also called left- and right-handed) if it is circular
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....
. Certain devices can also allow a feedhorn to accept both linear and circular, though these cause a slight insertion loss
Insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels ....
to all incoming signals.
When used with a parabolic reflector
Parabolic reflector
A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
or lens antenna, the phase center
Phase center
In antenna design theory, the phase center is the point from which the electromagnetic radiation spreads spherically outward, with the phase of the signal being equal at any point on the sphere. Apparent phase center is used to describe the phase center in a limited section of the radiation pattern....
of the horn is usually placed at the focal point
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
of the reflector, with the 3 dB point
Half power point
The half power point of an electronic amplifier stage is that frequency at which the output power has dropped to half of its mid-band level. Other names include the cutoff frequency....
s of the horn's radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
set to be at the edge of the reflector. On a satellite dish, the feedhorn is what is mounted at the end of a mast
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...
from the center of the dish, or on tripod
Tripod
A tripod is a portable three-legged frame, used as a platform for supporting the weight and maintaining the stability of some other object. The word comes from the Greek tripous, meaning "three feet". A tripod provides stability against downward forces, horizontal forces and moments about the...
legs mounted to the edge of the dish.
In satellite dishes, part of the receiver electronics is usually mounted in a box just behind the feed horn. This converts the high satellite microwave downlink frequencies to a lower frequency so it can be more easily sent through the coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
feedline to the receiver inside the building. This is called the low-noise blockconverter (LNB), and the unit consisting of the horn plus the converter is called the LNB with feedhorn (LNBF).
See also
- Horn (telescope)Horn (telescope)A horn is a stationary radio telescope. Horns were originally built to observe specific objects in the sky, and preceded the more commonly known movable dishes used today.-See also:* Feed horn* Horn...
- Focal cloudFocal cloudA focal cloud is the collection of focal points of any imperfect lens,whether optical, electrostatic or electromagnetic. This includes Parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography.In this focal cloud...