©1999 Navia Aviation AS 21833-3.4 Maintenance Procedures
OPERATING MANUALNORMARC 7013
INSTRUMENT LANDING SYSTEM
C-9
7. Determine if the results are within tolerance, and record the actual modulation percentage
on FAA Form 6750-32. If the results are out of tolerance, circle the entry and take correc-
tive action.
5-16 SYSTEM PHASING, FAR-FIELD AND IN-LINE.
a. Object.
These procedures provide methods to adjust for optimum phase relationships between the
carrier and sideband-only radiated signals.
b. Discussion.
(1) RF Misphasing.
The effectiveness of the sideband only (SBO) radiated energy in producing off-course guid-
ance (clearance) is compromised by rf misphasing. A few degrees of sideband-to-carrier mis-
phasing from optimum is not significant. As misphasing increases toward 90°, however, the
effectiveness of the sideband energy decreases at a cosine rate to zero. Rf misphasing indi-
cations are increased course width and decreased clearances with normal sideband-to-carrier
power ratios.
(2) Phasing Methods.
There are two methods of phasing a localizer, far-field quadrature and in-line.
(a) Far-field quadrature.
1. The far-field quadrature method is the most reliable and can be used at any localizer once
a far-field phasing point is established. (Refer to paragraphs 5-173 and 5-174) It negates
the need for airborne phasing and must be used to initially confirm the in-line method.
2.
The far-field quadrature phasing procedure is used to check the composite carrier-to-
sideband phase relationship in the far-field. It is applicable at all localizers and will reflect
phase changes corresponding to changes in the antenna or transmission line system.
The reliability and accuracy of this procedure depends on the establishment of a proper
far-field check point (refer to paragraphs 5-173 and 5-174) and the proper length of the
90° phasing section (refer to paragraph 5-151). Once established, far-field quadrature
phasing can be used as standard to verify proper phasing of a localizer antenna system. It
must be used to initially
establish or subsequently reestablish the validity of other phasing procedures.
(b) In-line.
1. The in-line procedure is easier to accomplish and offers more repeatable readings.
2. The limitations of in-line phasing must be recognized for this method to remain valid. Ini-
tially, the validity of in-line phasing must be established by comparing the results with the
far-field phasing method. Results of the two methods should be within ±10°. Also, in-line
phasing monitors only one feedline. Changes can occur elsewhere in the system and will
not be reflected in the readings from the in-line detector. Unexplained changes in width or
clearance require a far-field phasing check to verify proper system phasing.
c. Test Equipment Required.
1. Far-field phasing.
(a) A portable ILS receiver (PIR).
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