8
Explanation: Fig. 5 shows different TX power levels arriving the transceiver.
Because of different cable lengths, the cable loss for antenna 1 is higher than for antenna 2. The modem is driving currently
-10dBm on the TX line and antenna 1 is online. The arriving TX power at the BUC is now -20dBm, which is below the 1dB compres-
sion point of -15dBm. Now the VSAT DivKit switches to antenna 2. The cable to antenna 2 has only a loss of 5dB which means that
with a TX power of -10dBm the arriving TX power at the BUC input is about -15dBm.
This is exactly the 1dB suppression point of the antenna. If the Uplink Power Control (ULPC) sends a command to the modem to
increase the TX power (because of bad weather for example), the antenna works beyond the compression point and the signal
is distorted. That reduces the C/N value and ULPC sends a command to increase the TX power once more which reduces the C/N
value again.
To avoid this loop, you can turn the power regulator for TX2 to 6dB attenuation.
Now the arriving power at both antennas is equally at -20dBm.
Measuring the arriving TX power
Normally, the output power of the modem is regulated by the UPLC (Up Link Power Control) of your satellite service. It monitors
the arriving power level at the HUB and sends the modem the command to increase or decrease the output power.
Two antennas usually means different cable lengths and hence different arriving TX power levels at the input of the transceiver of
the antenna. The transceiver itself is not able to output the arriving TX power to the system, and so you have to request the UPLC
for the incoming power level at the HUB. When you know the corresponding values for both antennas, it is highly recommended
to match these power levels to avoid missing TX power margins, or even worse, saturation of one transmitter.
Fig. 5: Example for mismatching power levels of TX at the output of the modem.
ACU 2
EPAK DIVERSITY KIT
Modem
ACU 1
Arriving TX power: -20dBm
Cable loss: 10dB
TX power: -10dBm
Antenna 1
Antenna 2
Arriving TX power: -15dBm
Cable loss: 5dB
Compression Point:
-15dBm
Compression Point:
-15dBm
TX1
RX1
TX2
RX2
TX1
RX1
TX2
RX2
TX
RX
