Research Concepts RC2000A - Manual (Page 108)

108 RC2000A Dual Axis Antenna Controller Appendix I 2.6x Addendum
Here is an example of how fading can lead to a bad peakup decision. Assume that during a peaking
operation the controller takes a step away from the true satellite position. If during that movement a
cloud that had previously been positioned between the antenna and the satellite ‘moves out of the way’
the controller will record a stronger signal even though the antenna has been moved away from the
satellite. To reduce the probability of this bad decision occurring the position step size can be made
larger or the received signal averaging period can be made shorter to cause the peakup to occur more
quickly. The controller’s signal averaging interval is fixed at three seconds. Note that a reduction of the
signal averaging time interval leads to an increased probability of a bad peaking decision due to noise in
the receiver AGC circuits as described in the next section.
AGC Noise
The controller’s signal strength input is an analog signal typically generated by a receiver’s AGC output
circuits. A certain amount of noise is present in any analog signal. The noise voltage is summed with
the signal. The noise typically has an average value of zero volts. To minimize the affect of noise the
controller’s averaging interval can be increased. Note that an increase of the signal averaging time
interval leads to an increased probability of a bad peaking decision due to signal fading as described in
the previous section.
Mechanical Hysteresis
Mechanical hysteresis, or slop, is looseness in the antenna drive system. Hysteresis is typically caused
by backlash in a linear actuator or chain drive or looseness in the actuator attachments bolts or pivot
points. When the step sizes used for peaking result in antenna movements smaller than or equal to the
antenna’s mechanical hysteresis, a step may not result in a change in the antenna’s absolute pointing
angle. If this occurs, any change in measured signal strength will be due to fading or noise in the
receiver’s AGC circuits and the controller will make a ‘bad decision’ regarding the peakup.
Here is a test to quantify the amount of mechanical hysteresis in a given mount. Better results will be
obtained if the winds are calm when the test is performed.
1. Jog the antenna far off of the satellite in one direction.
2. Jog the antenna back towards the satellite. Initially approach the satellite moving the antenna at
high speed.
3. When the antenna is close to the satellite jog towards the satellite at slow speed moving 1 or 2
position counts at a time. Only jog the antenna in one direction – do not back up. Record the signal
strength at each position. Continue until well past the peak of the antenna pattern.
4. Next position the antenna far off of the satellite in the other direction and repeat steps 2 and 3 as
you approach the satellite from the other direction.
5. Using a spreadsheet, plot signal strength vs. position for movement in both directions. Compare
the plots, any difference between the peak positions is probably due to slop in the antenna.
Note that the test is more accurate for larger antennas and higher frequencies (which result in a
narrower antenna beamwidth). Sometimes this test will not fully characterize slop about the elevation
axis due to the weight of the antenna. Elevation slop can still affect a heavy antenna in gusty winds.
3.5 Step Size Selection
The controller’s peakup step size is specified in position counts via the following CONFIG mode items:
C Band El Step Size, C Band Az Step Size, K Band El Step Size, and K Band Az Step Size. Section
3.5.5 tabulates recommended step sizes (in position counts for a number of antennas.
3.5.1 Azimuth and Elevation Step Size Calculation
The controller’s peaking accuracy can be no better than one half of the step size used for peaking.
Section 3.4 described the problems that occur when the step size is made too small. It has been found
that peakup step sizes that results in a 0.5 to 1.0 dB drop in signal strength (assuming that the antenna
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