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피드혼... FEEDHORNS
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 피드혼
조정 Feedhorn Adjustments
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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.
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| Low Noise Block Downconverters
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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
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LNB/Dish Trade Offs
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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.
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| LNB Install Tips
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 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.
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Linear Polarization
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원형편파
및 선형편파
편파란 위성에서 송출하는 방식을
말하는 것으로서 국내에서 수신가능한 대부분의 해외위성방송은 선형편파
( 수평/수직 - horizontal/vertical)이다. 
원형편파로는 국내의 무궁화위성 3호의
KBS는 좌편파(LHCP)를 사용하며 일본의 NHK는 우편파(RHCP)를 사용한다.
또한 러시아의 이동위성(STATSIONAR - Gorizont)은 위의 두 원형편파를
모두 사용한다. 이원형편파는 전용 피드혼이 필요하다. 즉 KBS는 방송의
세기가 워낙 강해서 보통의 LNB로 클립을 사용하지 않고도 시청이 가능하나,
아날로그 NHK의 경우는 편파기를 넣어 주어야만 깨끗한 방송을 시청할
수가 있다. 하지만 러시아의 이동위성에서 방송하는 오스탄키노(Ostankino)나
레떼르(RTR)같은 방송은 위성자체도 이동위성이며 편파 또한 원형 편파를
사용해서 LNB를 선형으로 사용할 경우 화면의 질이 좋지 않다.
국내에서 대부분 시청하는 위성( Palapa,
AsiaSat, Apstar, PanAmSat)은 대부분 선형을 사용한다
물론 운영자의 해석이 틀린 것일 수도
있다.. 이때는 주저말고 운영자에게
메일을...
우편파 ( Right hand
circularly polarized - RHCP)
좌편파 ( Left hand circularly
polarized - LHCP)
수평파 ( Horizontal polarized - HP)
수직파 ( Vertical polarized - VP)
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Circular Polarization
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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 lost.
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
Teflon.
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| Polaris's
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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.
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| Hybrid Feedhorns
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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.
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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 
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.
