The Feed Assembly
While
most people think of a satellite reflector (or dish) as an "antenna", the actual
antenna portion of the system is contained within the feed assembly. The feed
assembly is located at the focal point of the reflector where the received signals
are focused for maximum gain. A small probe within the throat of the assembly
gathers the high frequency signals and routes them to the LNB
(Low Noise Block
amplifier) which amplifies and converts the received signal for use by the satellite
receiver. The feed assembly also contains a servo motor which, controlled by
the satellite receiver, rotates the probe to allow for reception of both horizontally
and vertically arranged signals.The feed assembly must be VERY accurately positioned
to allow for a maximum transfer of signal from the reflector to the LNB.
The feed assemblies shown are manufactured by Chaparral Communications. The feed on the left is the PR1-E/A, a C-Band feed with adjustable scalar ring. The feed pictured on the right is the Corotor II PLUS for both C and Ku-Band operation.
The
LNB is the electronic workhorse of the satellite system. It must amplify the
incoming signal by an incredible amount and convert it to the lower frequency
used by the receiver while introducing almost no electronic noise of it's own.
C-Band LNBs are rated in degrees Kelvin from 20 deg(BEST) to 100 deg(ANTIQUE).
Ku-Band LNBs are rated in decibels from .6dB(BEST) to 2.0dB(ANTIQUE). The choice
of the LNB is dependent on dish size, satellite strength and programming quality
desired. Generally, a 30 deg C-Band LNB and a 0.8dB Ku-Band LNB are considered
nominal.
The
feed assembly connects to the indoor receiver via a multi-conductor ribbon cable
specially designed for this purpose. Within it are the 2 necessary coaxial cables
for the C & Ku Band LNBs, the 3-conductor servo control cable and the 4/5-conductor
actuator control cable. Cable lengths of 100 - 300 ft are acceptable but the
rule of thumb is "da shorter - da better". The radio frequency losses
in the coaxial cable and the voltage drop to the actuator can cause reception
and operation problems. In some cases a line amplifier can be used to amplify
the RF signals on the coaxial cable.
The Reflector
The Feed Assembly
The Actuator
The Receiver
FCC Fact Sheet
For additional Full-Arc Satellite Systems information: sodac@sodac.com