Specifications

MDS 05-3438A01, Rev. E MDS 4790/9790 Series I/O Guide 87
MDS 4790/9790 Simplex Radios—With Antenna Switch Mod-
ules
Simplex radios do not have a duplexers. In its place, an Antenna Switch
Module is placed to switch the antenna system between the radios trans-
mitter and receiver. The switch does not have any frequency-sensitive
elements and covers the radio’s entire operating band. Radios equipped
with this module can be programmed to any frequency within their oper-
ating range. However, two things need to be considered.
1. The radio’s antenna system’s performance may not be satisfactory
on the new frequency. This can quickly be checked by measuring the
antenna system’s standing-wave-ratio (SWR) for a ratio of 1.5:1 or
less at the antenna.
2. The receiver’s front-end helical coils will need retuning if the
change in receive frequency is more than 5 MHz.
Figure 43. Antenna Switch Module
MDS 4790/970—Front End Helical Coil Alignment
Changing the radio’s receive frequency by more than 5 MHz will
require re-alignment of the receiver’s front-end helical coil set. Signifi-
cant degradation in receive bit-error rate (BER) is likely if the coils are
not aligned properly. For assistance, please consult MDS Technical
Support for additional details about tuning procedure.
8.5 Testing and Removing an Internal Duplexer
Testing
If you suspect that the internal duplexer is not functioning properly, per-
form the following steps to determine if it needs to be replaced.
1. Measure the RF power out of the antenna jack.
If the power registers approximately +37dBm (5 watts), the internal
duplexer is probably functioning correctly. (See Table 20 on Page 100
for dBm-volts-watts conversion chart.)
If the power registers significantly less than +37dBm, proceed with
Step 2.
2. Open the radio chassis, locate the TX cable, and disconnect the
cable from vertical interface board (Figure 44).