In the example shown, the fifteenth order (8F1-7F2)
and seventeenth order (9F1-8F2) intermodulation
products fall right on top of the receive channels. These
intermodulation products go straight through the
antenna to the receive path of the duplexer and in to
the receivers. If the amplitude of these intermodulation
products is greater than the noise floor of the receiver,
then the receiver sensitivity
is degraded leading to a loss of uplink coverage.
This is most likely to occur if the spacing between the
transmitter channels is a sub-multiple of the transmit-to-receive frequency separation. Wherever possible
WHEN AND WHERE TO USE A DUPLEXER
then, if you have to use a duplexer, combine transmit
channels where the resulting intermodulation
products do not land on a receive channel.
SEPARATE TX AND RX ANTENNAS
If we use separate transmit and receive antennas,
it takes up more space on the tower.
The big advantage is that, while passive intermodulation
still occurs in the same way between the combined
transmitted signals, there is no longer a direct path
for these products to reach the receiver. Instead, the
isolation between the transmit and receive antennas
provides additional protection.
“For single channel systems, use a duplexer.
But for multi-channel systems, separate
antennas are the more resilient option.”
If the transmitters and receivers are arranged in a
co-linear fashion (ie: one directly above the other,
generally with the receive antenna highest up the tower),
then isolations in excess of 50dB are easily achievable.
So in conclusion, for single channel systems, go ahead
and use a duplexer. But for multi-channel systems, while
separate antennas will cost you more space on each
tower, this is the more resilient option. It protects your
system better from the significant interference from
passive intermodulation as a result of those very minor
and difficult to isolate assembly or maintenance faults.