Research Concepts RC2500 - Manual (Page 112)

RC2500 Antenna Controller Appendix H Polarization Sensing with a Potentiometer 105
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas 66218-9680 • USA www.researchconcepts.com
Configuring Controller to Display Polarization Angle
To interface the controller to the polarization position sense potentiometer and calibrate the controller to
display the polarization in an angular format, use the following procedure …
1. Power down the controller. Ensure that the potentiometer will not encounter an internal physical limit
before the feed’s physical limit of rotation is reached or the polarization limit switch becomes active.
This is sometimes referred to as ‘centering the pot’. An ohm meter may be useful here.
2. Connect the potentiometer to the controller using the interface diagram. It is best to use shielded
cable with a bare drain wire. Connect the pot’s wiper to the Sig terminal of the interface board. Make
the other two pot connections to the Ref and RTN terminals of the interface board. Connect the drain
wire at the back of the interface board’s RTN (return) terminal. The drain wire should not be
connected to anything at the antenna nor should the drain wire or shield be allowed to come in
contact with earth ground (or any metal connected to earth ground - like the antenna mount). If the
cable is spliced be sure to splice the drain wire. By not allowing the shield or drain wire to come in
contact with earth ground ground loops will be avoided. Ground loops can lead to noise pickup and
unreliable polarization position readings.
3. Connect the polarization motor drive lines.
4. Power up the controller. Make sure that the Pol Display Offset and Reverse Pol Sensor Direction
CONFIG mode items are set to zero.
5. From MANUAL mode, jog the controller in polarization using the Pol CW and Pol CCW keys. When
the Pol CW key is jogged the displayed polarization position must increase. Note that if the pol jog
keys are depressed and the controller does not sense antenna movement or if the movement is in the
wrong direction a Pol Alarm message will flash on the bottom row of the controller’s LCD display.
The jog keys must be depressed for at least two seconds before the alarm will be sensed. This alarm
can be reset by either using the controller’s RESET mode or by cycling power to the controller.
If the movement is in the wrong direction either reverse the wires connected to the Ref and RTN
terminals, reverse the motor’s direction of rotation by reconfiguring the motor drive line, or use the
Reverse Pol Sensor Direction CONFIG mode item.
6. The reference position for polarization position angle display is usually vertical polarization of a
satellite located at a longitudinal position equal to that of the earth station. Note that for C band
satellites the polarization orientation of adjacent satellites are well coordinated. For Ku band
satellites this is often not the case. Align the antenna with a satellite and peak up vertical polarization
on that satellite. Use a volt meter to measure the voltage between the interface board’s Sig and RTN
terminals. Record this voltage along with the satellite longitude position. Call this SAT1.
7. Repeat step 6 for a different satellite. Call this SAT2.
8. Use the following relationship to calculate the satellite polarization angle …
Pol_Angle = - ATAN( sin(L) / tan(A) ) (Equation 2)
where L is the difference in longitude between the satellite and earth station, and A is the earth
station latitude. All angles are in degrees. Treat westerly longitudes as negative numbers. The sign
of L is important.
9. Calculate the ratio …
R = (Pol_Angle_SAT1 – Pol_Angle_SAT2) / (Pol_Voltage_SAT1 – Pol_Voltage_SAT2)
(Equation 3)
If R is a negative number change the sign to make it a positive number. Round to the nearest integer
value and enter the value into the controller as the Pol Display Scale Factor CONFIG mode item.
10. Go to the controller’s MANUAL mode. With the antenna aligned with SAT2 and peaked on vertical
polarization note the displayed polarization angle (designated Disp_SAT2). Calculate the following
quantity ..
Pol_Display_Offset = Pol_Angle_SAT2 – Disp_SAT2 (Equation 4)