4.2.4
Antenna pair phasing
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Connect the antenna cables A1 and A24 to the ADU outputs labelled A1 and A24 respec-
tively.
Connect all other ADU outputs to dummy load. Use the NM3710 with portable test antenna.
On the the runway establish a point officially marked as the centre line for DDM measure-
ments. Ascertain that the measurement point is not affected by reflected signals upsetting the
direct received signals from the antenna pairs. This can be tested by moving the test antenna
a few meters back and fourth while observing the DDM reading. If the value is not changing
then the measurement position is OK.
Take a note of the DDM reading first from antenna pair A1/A24.
Connect pair A2/A23 to ADU outputs A2/A23 and disconnect pair A1/A24, dummyload A1/
A24 ADU.
Take a note of the DDM reading.
Carry out this procedure until all antenna pairs DDM readings have been noted.
If any antenna pair measures DDM different from zero, one of the antenna pair feed cable
lengths should be trimmed in order to bring the DDM reading to zero.
If the DDM reading is positive (150 Hz dominance) the antenna cable at the right hand side
(A13...A24) should be trimmed.
The sensitivity of DDM versus electrical phase error of 6° (4° CS) is given in the table below:
Summary of Contents for NM 7000 Series
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