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편파
; Polarization
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 선형편파
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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...
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| 원형편파
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우회선파 ( Right hand
circularly polarized - RHCP)
좌회선파 ( Left hand circularly
polarized - LHCP)
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 dual-band
feed is a technical compromise that sacrifices a small amount of
C-band signal in order to place both the C- and Ku-band feed openings
at the focus of the dish. For those situations where the installer
can't afford to sacrifice ANY Ku-band signal, a second C-band feed
and LNB can be attached to one side of the Ku-band feed along the
plane of the dish that is perpendicular to the mount's polar axis.
The reverse is also possible if the C-band signals in your area
are marginal. Another option is to install a second dish that is
permanently pointed at a single satellite, or satellites collocated
at a single orbital position.
