Bird Technologies 28-88-04B - Manual (Page 15)

TXRX Systems Inc. Manual 7-9176-4 06/23/04 Page 11
2. Check the unit label. If needed, the duplexer may be field tuned. Consult the instructions and/or the factory if the duplexer is still under
warranty or beyond field tuning capability.
3. Check cable, by substitution, using a termaline wattmeter, or a thruline wattmeter into a known good load. Check the antenna line input
for reflected power.
4. To eliminate high input VSWR reduce the number of between series adapters by making up proper interconnect cables. UHF connec-
tors are non-constant impedance, and certain combinations can transform a 1.1:1 VSWR into a 2.0:1, or vice versa.
5. Consult the instruction manual for field tuning procedures, or the factory, if the unit is still under warranty or beyond field tuning capabil-
ity. (We trust that our products will not be prone to this problem).
6. Consult the factory. The affected antenna cables may be field replaceable, or a “baking out” process may be possible.
7. To prove this condition, place a bandpass filter between the Tx and duplexer to clean up the spurious, and put the wattmeter between
the bandpass filter and the duplexer to measure reflected power from the duplexer. The bandpass filter selectivity should be equal to or
better than that of the duplexer at about the 3.0 dB points.
8. Operate the duplex system into a dummy load. If no desensitization occurs, check out all line, antennas, and look for potential bad joints
close to the radiation antenna where re-radiation of noise may be possible back into the antenna system receiver. Loose metal-to-
metal contacts on tower guying systems have also been known to create system noise. Note the effect of vibrating tower guys on sys-
tem noise.
9. Change the length of cable between the transmitter and duplexer, traversing through half wave in increments of between 1 and 2 inches
until the desensitization ceases or is minimal. A ferrite isolator will also cure this condition when it is installed between the transmitter
and duplexer. However, this is a much more expensive remedy.
10. If the IM is in the duplex transmitter, a ferrite isolator in the duplex transmitter line (NOT antenna line) will show this by either reducing
or eliminating it. More isolation can be obtained by cascading isolators if needed. However, IM of this magnitude indicates the system
should be studied for possible revision to reduce the production of the IM.
11. Cables such as RG-8a/u and RG-213/u should be kept at least 3 - 4” apart over 5”-6” runs. Use of double shielded cable will reduce the
susceptibility to this problem.
12. Consult the radio manufacturer. This condition can be verified by operating the transmitter into a dummy load while injecting a minimum
quieting signal into the receiver. Some radios require special modifications before they are suitable for repeater operation.
13. If this problem is suspected, contact the radio manufacturer for recommended duplex isolation for Tx noise suppression and carrier
suppression. Duplexer isolation should be measured first per instruction manual to verify rated specifications are present. If more
duplex isolation is required, contact TX RX SYSTEMS for recommended filtering.
14. Consult the factory. Bandpass filter tests can be made to confirm this. In extreme cases, adjustments to the transmitter may be
required.
15. Either de-ice the antenna, or use an antenna less sensitive to ice. A ferrite isolator can also be put at the transmitter output to improve
the impedance match. Ferrite isolators cannot be put in antenna lines, as they will attenuate Rx signals.
16. A mismatch may possibly be reduced by lengthening the cable which runs between the power amplifier output and the duplexer input
until the receiver desensitization disappears, as follows:
30 MHz to 512 MHz RANGE; BNC or N type adapters may be inserted in the original cable, one at a time and not to exceed a total of
1/2 wavelength, until desensitization disappears.
800 MHz to 1.3 GHz RANGE; Prepare a cable length 3/4” longer than the original cable and insert. If desensitization does not disap-
pear, repeat with cables each 3/4” longer than the previous length, not to exceed 1/2 wavelength.
17. We find that this cause most commonly relates to shifting impedance of the transmitter or power amplifier with temperature. The
duplexer appears detuned, since a “conjugate match” (canceling reactance, and matching resistance component) is approached by
shifting the duplexer passband above or below the 50 ohm point, as determined by an increase in output power on the wattmeter. In
this case, temperature control of the room is the only answer, other than upgrading the transmitter.