Bird Technologies 21-87A-11-xx-T Series-Manual - Manual (Page 15)

TX RX Systems Inc. Manual 7-9100-5 10/17/11 Page 9
a corroded tower joint, metal-roofing, transmitter
final amplifier or the receiver front-end.
Both cavity filters and ferrite isolators isolate the
transmitters connected to the combiner from one-
another thus reducing intermodulation interference.
However in all transmitter combiners, intermodula-
tion products are reduced in strength but never
completely eliminated. They have to be reduced by
an amount to meet the Federal Communications
Commission, 43 + 10 Log(Power Out) rule for spu-
rious output reduction. Because of the limitations
imposed by the tension and friction joints in con-
nectors, IM products will be down 100 to 120 dB
below carrier so they are still strong enough to
cause problems if they fall on a near-by receiver
frequency.
To avoid transmitter generated IM problems, do
not put two channels on the same combiner that
your IM software predicts will cause interference by
generating either 3rd or 5th order IM products.
Having at least two transmitter combiners allows
maximum flexibility in dealing with transmitter gen-
erated IM.
Multicoupler Checkout
Fully assembled multicouplers are factory tuned
and ready for routine operation after properly con-
necting the transmitters and antenna(s) as outlined
previously. The components used in systems that
require partial assembly have been fully interca-
bled and tuned so they will not require tuning. How-
ever, it is recommended that the performance of
the multicoupler 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 6 shows the equip-
ment hook up.
REQUIRED EQUIPMENT
If a power monitoring system is not installed along
with the multicoupler, two Bird Model 43 thruline
wattmeters 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 at the bottom of the rack and
proceed to the next higher channels. The two watt-
meters should be connected to the equipment as
shown in figure 6. Note that the use of the elbow
and/or male-male connectors allows the shortest
connections and negligible 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.
A convenient data sheet is included in Table 3 and
may be photo copied. After entering the data and
calculating the power losses, it should be retained
for future reference. A column is provided for enter-
ing the factory measured loss from the T-Pass
Thruline Data sheet that was included in the enve-
lope with this manual. The factor y data was
obtained with a laboratory network analyzer having
an accuracy ±0.05dB. The readings obtained using
the wattmeter method outlined may vary consider-
ably from the factory values and this difference is
explained in the next paragraph.
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: