NORMARC NM 7000 Series, Installation & Commissioning Handbook

Page: 79 / 189

For CS = 4°:
For other CS values use the formula:
EXAMPLE
A5 DDM for CS = 5°:
PROCEDURE
Insert the 90°-stub into the SBO path.
Connect a directional coupler with 50
Ω
load to antenna O/P A3.
Connect the NM3710 (use 20 dB attenuator at NM3710 I/P) to the directional coupler's for-
ward port.
Adjust the SBO attenuator (for the Tx to air) until the DDM value equals the previously calcu-
lated value for the CS.
Move the directional coupler to A4, A5, A6 and compare readings with calculated values. If
measured DDM is lower than the calculated values for A4, A5, A6 then increase the SBO
power, or vice versa, until a fairly good matching set of values are obtained.
Adjust the SBO attenuator for the other Tx to the same setting.
Remove the 90°-stub.
2.3.2 DDM field check
Set the localizer to normal radiation. From the centreline of the opposite threshold determine
points perpendicular to the CL 105 meter to both sides of the runway. Use the NM3710 with
portable antenna and measure DDM at these points. The readings should be close to -15.5%
at the 90-side and 15.5% at the 150-side. However, some unsymmetry may be expected due
to reflections and tolerances in the antenna system.
Check the CL DDM at two or three positions in order to estimate an average value.
Expected value: 0.0%
±
0.2% DDM.
% DDM µA %SDM
A2 -34.8 -337 40.0
A3 -32.7 -316 40.0
A5 -28.1 -272 40.0
A6 -30.4 -294 40.0
DDM
A n
( )
DDM
4
° ( )
4
° ⋅
CS
---------------------------------
=
DM
A3
28.1 4
⋅
–
5
---------------------
22.5% – = =