888-2406-002 5-7
WARNING: Disconnect primary power prior to servicing.
Maintenance and Alignment
5.7 Directional Coupler Removal and Replacement
First, note that 2 of the couplers are labeled Forward sampling and one is Reflected.
They cannot be interchanged as the coupling ratio is different. The difference is how
far the coupler is inserted into the base. The Forward couplers are not inserted as far
as the reflected and have a small gap between the coupler and the base.
Before removal, mark each coupler for the following:
a. Proper position - the couplers should go back into the same hole from which
they came.
b. Orientation - mark the couplers with an arrow pointing up to be sure and
replace them correctly. Reversing a forward coupler makes it read reflected
and vise-versa.
c. Cable Number - write on the coupler, the number of the BNC cable which
attaches to it.
NOTE:
The Directional Couplers Are Held In Place On The Harmonic Filter With 2
Hose Clamps. Be Sure To Place The Clamps As Far Toward The Edge Of The
Coupler Body As Possible And Do Not Over-tighten.
d. A calibration of the Forward or Reflected power should be done depending on
which coupler was replaced
NOTE:
If both couplers are being replaced at the same time, Reflected Power should
be calibrated first, then the Forward Power. An External meter is required to
calibrate Reflected Power in this case.
5.7.1 Typical Coupling Ratios
Typical Coupling Ratios for the directional couplers is as follows:
• Forward Couplers: 48.8dB@ 88MHz to 47dB@ 108MHz
• Reflected Couplers: 43.8dB@ 88MHz to 42dB@ 108MHz
Tolerance is +/- 0.1dB with directivity greater than 30dB. This is not guaranteed
since any minor variations can be factored out by using the calibration procedures.
For frequencies between 88 and 108MHz the actual coupling ratio can be
extrapolated linearly. For example: At 98 MHz the typical Forward Coupling ratio
is 47.9dB.
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