Bird Technologies 21-87A-11-xx-T Series-Manual - Manual (Page 17)

TX RX Systems Inc. Manual 7-9100-5 10/17/11 Page 11
MULTICOUPLER TUNING
T-Pass transmitter combiners are pre-tuned at the
factory and usually require no adjustment. T-Pass
expansion channels are also pretuned but may
require fine tuning after being installed in an exist-
ing system. Channels that are close in frequency
(adjacent channels in the multicoupler) to the
expansion channel may also benefit from fine tun-
ing due to the slight interaction that occurs with the
new channel. The procedures that follow may be
used at any time to verify that any or all channels
are properly tuned or to correct any misalignments.
It is interesting to note that T-Pass filters, bandpass
filters and cavity filters in general can act as imped-
ance transformers as well as filters. It is for this
reason that many field service personnel claim that
they can always tune a filter better than the factory.
What isn't generally realized is that their tuning
efforts are usually producing better impedance
matching between transmitter and antenna which
can be improved by the transforming action of fil-
ters. Since the filters are usually tuned using labo-
ratory grade 50 ohm loads, the tuning adjustment
that produces this improved match will be slightly
different than the factory adjustment. While this
tuning may produce slightly greater power output
readings, it will rarely produce any discernible
change in system performance.
It is our recommendation that channel tuning only
be attempted under the previously mentioned con-
ditions or when it is suspected that the combiner
has been tampered with or subjected to extreme
shock in shipping or installation. This condition is
indicated when the channel loss is in excess of that
expected from actual measurement of power input
and output.
Tuning Specifics
Tuning the multicoupler consists of tuning the indi-
vidual T-Pass channels. T-Pass channel tuning
involves cavity filter tuning. For multicoupler mod-
els used at 800 MHz and above, isolator tuning is
never required because these isolators are fixed-
tuned at the factory for specific frequency bands
and have no user adjustments.
Transmitter
Output
Section
Termination
Two Single Section
or One Dual Isolators
T-Pass
Cavity Filter
Wattmeter
To Other
Channels
To Other
Channels
Input
50 Ohm
Termination
Fine
Tuning
Output
Coarse
Tuning
Figure 7: Using a Wattmeter for T-Pass cavity fine tuning.