FEEDHORNS, LNBS AND POLARIZERS
Every satellite dish has a relatively large surface
area which captures the incoming satellite TV
signal. A perfect, that is to say theoretical,
prime focus antenna would rely on the geometric
properties of its parabolic curve to reflect the
incoming satellite signal to a sharp, well defined
focal point located to the front and center of the
dish. In the real world, however, inevitable
deviations in the accuracy of this parabolic curve
result in the generation of a less-defined focal
cloud. It is the feedhorn's task to gather in this
imprecisely focused signal and pass it on to the
system's first stage of electronic amplification
with a minimum amount of loss.
The most common type of feedhorn manufactured today is called a scalar
feedhorn. This type of feed has a large circular plate with a series of three
or four concentric rings attached to its surface. The scalar rings conduct
the incoming signal from the outer edges of the focal cloud to the large
waveguide opening located at feed center. The scalar feedhorn primarily sees
or illuminates the inner portion of the antenna's surface area, while
attenuating the signal contribution from the outer portion of the dish by 8
to 22 dB, depending on whether the dish is deep or shallow in its
Molecular motion within the Earth itself generates random noise which
permeates the entire electromagnetic spectrum used for the transmission of
satellite signals. This random noise is many times stronger than the
satellite signals reaching any location. The attenuation or illumination
taper provided by the feed sharply reduces the reception of the Earth noise
which lies just beyond the antenna's rim. The outer area of the antenna's
surface therefore acts more as an Earth shield for the feedhorn than as a
contributor to the overall signal gain of the receiving antenna.
The distance between the center of the antenna surface and the feedhorn's
waveguide opening is called the focal length or (f). The focal length between
antenna surface and waveguide should be initially set to the distance
recommended by the antenna manufacturer. Adjustments of 1/8th inch or more in
or out from the recommended distance should be made while using a signal
meter or spectrum analyzer to determine the precise position required for
maximum signal acquisition. This is particularly important for antennas
composed of individual segments, especially those composed of mesh panels as
antenna surface irregularities due to careless antenna assembly can actually
shift the optimum position of the focal cloud from the value recommended by
the antenna manufacturer.
When adjusting the feedhorn in or out, be sure that the waveguide opening
remains precisely centered over the dish at all times. You can check this by
measuring from the antenna's rim to the outer ring of the waveguide opening
from four equidistant positions around the rim. All of these measurements
should be equal. In cases where the feedhorn is weighed down by two or more
electronic amplifiers, guy wires may need to be used to ensure that the
waveguide is precisely centered.
Small aperture Ku-band antennas often come with a fixed feed support bracket
which does not permit any adjustment of focal
length. In this case the system designer will
have to trust that the manufacturer has
selected the optimum focal length for its
product. Another satellite dish specification
which has an impact on feedhorn performance is
the antenna's focal length (f) to antenna
diameter (D) ratio, called the f/D. The
distance between the scalar ring plate and the
waveguide opening for many feedhorns can be
adjusted to a value that matches the f/D spec
of the dish. Making this adjustment allows the
feedhorn to achieve optimum illumination of the
antenna. Antenna f/D ratios range from .45 to
.25, with .4 the most commonly encountered.
The feedhorn will come with a plastic cap which
fits over the circular waveguide opening.
During assembly, be sure that this cap is
snugly in place. Otherwise wasps or other nasty critters may take up
residence in the waveguide and obstruct your reception.
Offset Antenna Feeds
Many of the small aperture Ku band dishes sold these days use an offset
antenna feedhorn design which places the focal point below the front and
center of the dish. This type of antenna, which is actually a small oval
subsection from a much larger parabolic antenna design, is oval in shape with
a minor axis (left to right) that is narrower than its major axis (top to
bottom). Because of its unique geometry, the offset fed antenna requires a
specially designed feedhorn which matches the antenna geometry precisely. For
this reason, the offset fed antenna and feedhorn are usually sold together as
a single unit.
Low Noise Block Downconverters
The incoming satellite signal passes through the feedhorn and exits into the
receiving system's first stage of electronic amplification called the low
noise block downconverter or LNB. A certain amount of noise is generated
within any electronic circuit. Any noise created by the LNB circuitry itself
will be amplified and passed on to succeeding stages. For best overall system
performance, this noise must be kept to a minimum.
The LNB sets the noise floor for your entire satellite receiving system. Less
noise here means that more signal will actually arrive at the indoor
receiver. Today's high performance LNBs use Gallium Arsenide (GaAs)
semiconductor and High Electron Mobility Transistor (HEMT) technologies to
minimize the noise level of the LNB.
The noise performance of C band LNBs is quantified as a noise temperature
measured in degrees Kelvin (K), while Ku band LNB noise performance is
expressed as a noise figure measured in dB. Today's C band LNBs commonly
achieve a noise temperature of 40 K or less, while Ku band noise figures of
less than 1 dB are commonly available. In either case, the lower the LNB's
noise performance rating, the less noise introduced into the LNB by its own
circuitry. The conversion chart presented below shows the relationship
between these two commonly used LNB noise measurement systems:
Ku band LNBs
Three distinct frequency sub bands or spectrums
are available from various Ku band satellites:
the 10.75 to 11.7 GHz, the 11.7 to 12.5 GHz,
and the 12.5 to 12.75 GHz frequency spectrums.
Care should be taken to ensure that your system
uses the LNB which matches the Ku band
frequency spectrum or spectrums used by the
satellites which you desire to view.
Wideband or universal Ku band LNBs are now
available which can switch electronically
between any of the above frequency spectrums to provide complete coverage of
the entire Ku band frequency range. The receiver or IRD sends a switching
voltage (13 or 17 volts d.c.) to the LNB which automatically changes the LNB
input frequency range to the desired frequency spectrum (10.70 to 11.75 GHz
or 11.7 to 12.75 GHz). Keep in mind that the universal LNB has an IF output
frequency range of 950 to 2150 MHz and can only be used effectively with a
receiver or IRD which also has a comparable IF input frequency range.
LNB/Dish Trade Offs
A satellite transponder's effective isotropic radiated power (EIRP) is
expressed in dBW (for decibels referenced to 1 Watt of power). The various
footprint maps for satellites serving the Middle East (such as those shown in
The World of Satellite TV) allow system designers to determine the
transponder EIRP likely to arrive at the site location. The southern beam
transponders on AsiaSat 1, for example, deliver an EIRP of 33 dBW to Kuwait,
while AsiaSat 2 transponders deliver an EIRP of 39 dBW. This information can
be used to determine the appropriate combination of antenna size and LNB
noise temperature to receive a signal that will exceed the minimum threshold
requirements of the indoor satellite TV receiver.
The tables below can be used to compute the combination of antenna size
(given an antenna efficiency of 65 percent) and LNB noise temperature needed
to produce a signal that exceeds an analogue receiver threshold of 7 dB
(diagonal blue line). If the intersection of the EIRP line (from left to
right) and the antenna line is above the blue diagonal line for the LNB in
use, then the resulting signal level would be above receiver threshold. keep
in mind that sparkle free reception of analogue (non-digital) TV signals
requires at least a margin of 2 dB above receiver threshold (7 to 10 dB
without threshold extension, depending on manufacturer).
Table 2 below allows the reader to determine the appropriate combination of
antenna aperture and LNB noise temperature for dishes ranging from 3.9 to 7.6
meters in diameter.
Table 3 (below) illustrates the equivalent effect of combining a 2.5 dB or
1.2 dB noise figure Ku band LNB with antenna apertures ranging from 60cm to
2.4m in diameter. Any improvement due to a drop in Ku band LNB noise figure,
however, is only achievable under clear sky conditions. Unlike C band
signals, which are unaffected by changing weather conditions, Ku band signals
are adversely affected by the presence of moisture in the Earth's atmosphere.
The presence of rain, or even rain clouds, will dramatically raise the noise
temperature of the sky and therefore raise the noise temperature of the
receiving system as well.
Another LNB specification commonly encountered is the amount of amplification
or gain provided by each unit. This is also measured in dB. Generally,
consumer LNBs produce 50 to 60 dB of gain, multiplying the received signal by
as much as 1,000,000 times. This provides the receiver with the necessary
amount of amplification for efficient operation. In most cases, the gain
figure itself matters less than the LNB's noise contribution.
LNB Install Tips
A rectangular flange on the back of the
feedhorn mates with a similar flange located at
the front of the LNB. A neoprene gasket is
inserted between these to flanges to prevent
any moisture from entering this junction.
During assembly, be sure that this gasket is
properly seated as any moisture entering
through this critical junction will degrade
signal reception and possibly damage either or
both of these components. Many manufacturers
now make a combination of a feedhorn and LNB
called an LNF. This combination product
eliminates the junction between feed and LNB as
a source of potential moisture problems.
The IF output connector on the back of every LNB is another potential source
for the ingress of moisture. After attaching the coaxial cable to this
connector, the junction should be sealed from the weather, either by using a
special waterproofing compound such as coax seal which wraps tightly around
the outside of the connection, or by flooding the inside of the coax's F
connector with a waterproofing silicon sealer. If you elect to flood the F
connector, be sure to first unplug the receiver and wait for the compound to
dry before plugging the receiver back into an a.c. power source.
Some digital satellite TV systems, primarily those which transmit on a
relatively narrow carrier, require an ultra-stable LNB that uses a phased
lock loop circuit to keep its local oscillator frequency from drifting too
far away from its nominal value. Other digital satellite TV systems require
an LNB with low phase noise performance. Be sure that you check with the
digital satellite service operator to determine the specific LNB requirements
for receiving their satellite services.
The signals transmitted by satellites such as EUTELSAT, Astra, PAS 4, Arabsat
1 D, INTELSAT (Ku band only) and AsiaSat are sent by an antenna aboard the
satellite that positions the microwave energy in either a relatively vertical
(straight up and down) or horizontal (lying flat) polarization. For best
reception of these signals, the pick up probe at the back of the waveguide
must be oriented in the same plane, horizontal or vertical, as that of the
desired satellite transponder. If the orientation of this pick up probe is
not exactly matched to the satellite transponder's polarization, some of the
signal will be lost.
When a feedhorn has been adjusted for best reception of a horizontally
polarized transponder, the overlapping vertically polarized transponders
cannot be seen. If the feedhorn inadvertently is set somewhere between
vertical and horizontal polarization, more than one TV channel can be seen at
the same time, an unsatisfying viewing experience at best.
A slight adjustment of polarity is usually necessary when you switch from
bird to bird. The amount of variance from satellite to satellite is referred
to as the polarization skew.
C band satellites such as the INTELSAT (C band only), Arabsat 1 C, Gorizont
and Express spacecraft use an alternate polarization format known as circular
polarization. For the best possible reception of circularly polarized
satellite transmissions, you will need to use a feedhorn that has been
constructed to receive these signals.
Instead of beaming the microwave energy along a linear plane, whether
vertical or horizontal, circular polarization is transmitted in a helical
rotating pattern, with right hand circular rotating in a clockwise direction
as seen from the satellite, and left hand circular signals rotating in a
counterclockwise direction. Although standard linear feedhorns can still pick
up any circular polarized signal, half of the available signal power will be
There are several manufacturers that offer special feedhorns that can receive
both the linear and circular polarization formats. Many linearly polarized
feedhorns also can be modified to receive circularly polarized signals with
the addition of a rectangular insert made from a dielectric material such as
Most communications satellites maximize their use of the limited frequency
spectrums assigned for satellite communications by overlapping the
transponders, with their polarization switching from one sense of
polarization to the opposite sense every other transponder. This allows twice
as many channels in the same amount of space. In order to select the correct
polarization, most feedhorns incorporate a small probe that is rotated until
best reception is obtained.
The probe is rotated by means of a small servo-motor which is powered by the
indoor receiver or IRD. By sensing the strength of the incoming signal, some
receivers can select the correct polarization setting automatically. However,
most receivers are programmed during the installation process to recall the
correct polarization format for each individual satellite stored in memory. A
few manufacturers use a ferromagnetic device which electronically adjusts
feedhorn polarization, instantaneously and silently. This introduces a small
amount of signal loss, typically 0.1 to 0.2 dB, which for most applications
is negligible. Ferromagnetic Polaris's have no moving parts that can cause
maintenance problems in the future.
Dual band hybrid feedhorns place both the C and Ku band waveguide openings
directly over the focal cloud of the antenna. This type off feedhorn will
give the satellite receiver direct access to all of the TV services carried
on dual band satellites such as PAS 4 or INTELSAT 704. The placement of both
the C and Ku band feed openings in such close proximity to each other,
however, will reduce the level of C band satellite TV signals over what a
good C band only feed can achieve. This may be an important consideration for
system designers who wish to use the smallest dish possible to receive C band
satellite TV services.
An alternative design approach to receiving dual band satellite signals is to
attach an optional Ku band feedhorn to one side of an existing C band feed
which illuminates an antenna with an f/D greater than .35. Several
manufacturers make add on Ku band feeds for this purpose which have a bracket
that mates with existing mounting holes on their C band feedhorns. The add on
Ku band feed is positioned so that its waveguide opening is on a plane that
is 90 degrees from the plane of the polar axis of the dish.
So called shallow dishes with an f/D of .35 to .45 can generate multiple
focal points spaced at intervals from the main focal point of the antenna.
The add on Ku band feedhorn is mounted so that it can pick up one of these
secondary focal points.
If used on a large C band antenna, the add on Ku band feed will capture
enough signal to exceed the threshold rating of the receiver even though the
secondary locations immediately adjacent to the main focal point are of
lesser intensity. To receive C and Ku band signals from the same satellite,
the operator will have to change the antenna's pointing direction along the
Clarke Orbit to compensate for the switch to the secondary focal point.