Bird Technologies 73-67-25 Series - Manual (Page 13)

TX RX Systems Inc. Manual 7-9120-5 07/26/10 Page 7
3) Verify proper operation of each channel by
measuring the power output for each individual
channel.
TRANSMITTER COMBINER CHECKOUT
It is recommended that the performance of the
transmitter combiner be checked initially and data
recorded for future reference. This is done by mea-
suring the input and output power of each channel
and recording the data. Figure 7 shows the equip-
ment hook up.
Required Equipment
If a power monitoring system is not installed along
with the combiner, two Bird Model 43 thruline watt-
meters or equivalent can be used. They should be
equipped with elements for the frequency band of
interest and rated for the expected transmitter
power output. The use of two wattmeters elimi-
nates errors that can occur from changing cable
lengths. The measurements should only be done
one channel at a time because most wattmeters
cannot accurately measure the total power of two
or more transmitters simultaneously. A pocket cal-
culator with Log functions makes for easy calcula-
tion of power loss in dB using this measured data.
PROCEDURE
Start with channel 1 and proceed to the next higher
channels. The two wattmeters should be con-
nected to the equipment as shown in figure 7. Note
that the use of the elbow and/or male-male con-
nectors allows the shortest connections and negli-
gible hook up loss. Longer cable lengths will tend
to increase measurement error.
It is important that the same wattmeters and watt-
meter elements be used in the same position
throughout the tests. The serial numbers of the
wattmeters should be recorded for future refer-
ence. Wattmeter elements may not have serial
numbers so they need to be labeled or otherwise
keyed to a specific wattmeter to assure repeatabil-
ity of the measurements.
MEASUREMENT ACCURACY
The Bird thruline wattmeter has a measurement
accuracy of +/- 5% of full scale. When using a 100
watt element in this meter, the measurement error
can be as great as + or - 5 watts.
As an example of the actual dB loss readings that
might be produced using the wattmeter method,
consider a T-Pass channel that has a factory mea-
sured loss of 3.0 dB. We would expect that a 100
watt transmitter would produce 50 watts out of this
channel but the actual wattmeter reading for the
input power could measure as low as 95 watts to
as high as 105 watts. The measured output power
could run from 45 to 55 watts. It is possible that the
output reading may be 5 watts low while the input
reading is 5 watts high or just the opposite. These
two extremes would yield the following dB loss val-
ues:
For a Power Out (PO) of 45 watts and a Power
Input (PI) of 105 watts:
Loss (dB) = 10 Log10 (45/105)
Loss (dB) = -3.7
For a PO of 55 watts and PI of 95 watts:
Loss (dB) = 10 Log10 (55/95)
Loss (dB) = -2.4
So the calculated loss for this channel can run from
-2.4 to -3.7dB and be acceptable considering the
measurement error factor. The actual error could
be much gre at er if a 25 0 wa tt elem e nt was
used; the measured values could vary by as much
as +/- 12.5 watts so using a wattmeter element with
the smallest possible rating is important for accu-
racy. Use of between series adapters or UHF type
connectors for making connections to the wattme-
ters, device under test or loads could make this
error even worse due to the additional impedance
mismatch that these connectors can cause.
GENERAL TUNING INFORMATION
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 or duplexer 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 transform-