ANTENNA VSWR TESTS AND TROUBLESHOOTING
VSWR TESTS - Figure 5-3 illustrates the test setup required to perform VSWR tests on the antenna. To
obtain true VSWR of the antenna under test, the VSWR as indicated should be corrected for the loss due to the length of
RG-87/U cable used.
TROUBLESHOOTING - Troubleshooting of the antenna is performed by measuring the VSWR at the input
(back) of a parabola. A VSWR of 3: 1 or less indicates proper operation. If VSWR exceeds 3: 1, move along the feed
line, measuring wherever a connector permits a break in the line, until an acceptable reading (less than 3: 1) is obtained.
The component measured just prior to the acceptable measurement should be the faulty component.
GENERAL DESCRIPTION AND THEORY OF OPERATION - The polarization unit for the antenna consists
of a 28-volt DC drive motor, a Geneva drive mechanism, four high-speed cam-operated switches, S1 through S4, and an
indexing switch, S5. The motor actuates the Geneva drive mechanism which, in turn, operates the polarizing device
through 135 degrees of rotation to obtain horizontal, vertical, +45-degree or -45-degree linear polarization. In order that
the motor will stop at a precise point, the indexing or zero-seeking action consists of two parts to provide the accurate
polarization control. The first is through the Geneva drive mechanism (an 8-position-per-revolution control), which
provides a coarse control for the polarization movement, and the second is through an indexing control which
deenergizes the motor at a precise position and at the same time applies an electric braking action to the motor.
One set of contacts on the high-speed cam-operated, fine-indexing switch, S5, provides a cycling
action of a detent relay in the antenna pedestal. This cycling action operates the detent relay to interrupt the 28-volt DC
supply to the drive motor at each detent position. Therefore, when the motor-driven polarizer reaches either limit of
travel (or one of the intermediate stop positions), the voltage to the motor is cut off so that the motor stops. Then
reversed polarity of 28-volts DC can be supplied to reverse the motor and permit the polarizer to progress to the selected
The cam which operates the indexing switch turns one full revolution in progressing to each
polarization control position. There are four control positions marked H, V, RHC, or +45 , and LHC, or -45 These
positions represent horizontal, vertical, right-hand 45 degrees, and left-hand 45 degrees linear polarization, respectively.