Initializing The Antenna Scale Factors - RESEARCH CONCEPTS RC2000C Manual

RC2000C Az/El Tracking Antenna Controller
There are four places in a satellite receiving system that may have gain flatness problems that can
affect the operation of the tracker. These are the LNB, the coaxial cable connecting the LNB to the
receiver, line amps (or bullet amps) inserted in the 950 - 1750 MHz IF, and the receiver's IF or AGC
stages. If a spectrum analyzer is connected into the receiver's block IF line, the gain flatness of the
LNB and any line amps present can be observed. When the antenna is pointed away from any satellite,
the spectrum analyzer displays the received noise, which should be constant with frequency. If the
display is not a horizontal line then some gain variation with frequency is present.
To understand how a poor gain flatness characteristic can cause problems, remember that the purpose
of the AGC input is to let the controller determine whether a satellite signal is present or not, and to
provide relative signal strength information when peaking the antenna. A signal is assumed to be
present whenever the AGC input is above the threshold level for whichever band - C or Ku or L - has
been selected. When the receiver is tuned to various transponders, gain flatness problems could cause
the AGC signal to be above the threshold, when in fact no satellite signal is present. For certain
transponders, gain flatness problems could also cause the controller's AGC input scaling network to be
saturated when the antenna is aligned with a strong satellite, making it impossible for the controller to
detect changes in signal strength when attempting to peak the antenna.
To test the gain flatness of the satellite receiver, perform the following procedure. The procedure
assumes a single receiver, single frequency band system.
1. Position the antenna well off of any satellite. Tune the receiver to each transponder. Make sure
that the signal strength reading is below the threshold assigned via CONFIG mode.
2. Align the antenna with a strong satellite. Tune the receiver to each active transponder on the
satellite. Make sure that the signal strength is well above the threshold and below 999.
If the system fails either of the tests above, then the user can either attempt to correct the problem by
readjusting the GAIN and OFFSET pots (using the procedure outlined earlier), or by correcting the gain
flatness problem. Most gain flatness problems can be traced to a problem with the coaxial cable
connecting the antenna to the receiver (sometimes called a 'suck-out') or to bullet amplifiers. Some
bullet amplifiers have been observed to have really terrible gain flatness characteristics.
To perform the above tests on a dual frequency band system, repeat the test for each frequency band.
On a dual receiver system, simply perform the test on each receiver. On a dual frequency system the
tests should be performed with the controller in LIMIT mode, so that there is no confusion as to which
AGC input channel the displayed signal strength reading corresponds to.
4.4.2

Initializing the Antenna Scale Factors

The azimuth and elevation scale factors refer to the mapping of antenna azimuth and elevation position
counts to antenna azimuth and elevation angles. The elevation scale factor is entered by the user. The
azimuth scale factor can either be specified by the user, or it can be derived from the position and
longitude data of adjacent geostationary satellites. The antenna scale factors are specified via the Azim
Constant and Elev Constant CONFIG mode items.
The Azim Constant and Elev Constant items are specified in position counts per radian. A radian is a
unit of angular measure. 2 times PI radians equals 360 degrees where PI is equal to 3.1415... . To
convert degrees to radians, multiply the degree value by 0.0174532. To convert radians to degrees,
multiply the radian value by 57.295. Note that the RC2000 counts both rising and falling edges of the
position pulses so that a single position pulse generates 2 position counts.
NOTE: The scale3.exe program described in section 3.7 – Angle display of Az/El position data
calculates the azimuth and elevation scale factors
Elevation Scale Factor
This section describes the method used to determine the elevation scale factor needed to initialize the
Elev Constant CONFIG mode item. The method is based on moving the antenna in elevation over a
known angle (as determined by an inclinometer) and obtaining the number of counts which corresponds
to the angular movement. A hand calculator is then used to determine the Elev Constant.
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