Principal Engineer Ian Graham explores a common question in radio system design: whether to deploy duplexers or to use separate transmit and receive antennas.
On the face of it, using a duplexer to reduce the number of antennas required by the system sounds like a great idea. But, as usual with RF, it is never quite that simple… Let’s examine the technicalities of both approaches.
What is a duplexer?
A duplexer is a three port filtering device which allows transmitters and receivers operating at different frequencies to share the same antenna. A duplexer typically consists of two band pass filters connected in parallel. One filter provides a path between the transmitter and the antenna, the other provides a path between the antenna and the receiver. No direct path between the transmitter and receiver exists.
Common antenna with duplexer
The obvious advantage of using a duplexer is that we can transmit and receive with only one antenna. With space on towers at base station sites at a premium, this is a real advantage.
In single channel systems, where there is only one transmitter and one receiver, use of a duplexer so they can share a common antenna is a straightforward choice. However, when multi-channel systems with several combined transmit and receive channels are considered, the situation becomes more complex.
The main disadvantage of using duplexers in multichannel systems can be seen when we consider transmitter intermodulation. This is the mixing of the multiple transmit signals on the antenna.
Consider this UHF example: Imagine we have two combined transmit channels at 400MHz and 400.5MHz, and two related receive channels at 396MHz and 396.5MHz (4MHz transmit to receive frequency separation is typical at UHF).
Once the transmitters are combined, any non-linearities or imperfections in the duplexer, the cabling, or the antenna itself will cause the transmit signals to mix and produce intermodulation products on the antenna side of the duplexer. Closest to the transmitted signals are the third order products, 2F1-F2 and 2F2-F1, followed by the fifth order, seventh order products etc. Corroded connectors or damaged cables are classic causes of this effect, known as Passive Intermodulation or PIM.
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 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. 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.
This article is taken from Connection Magazine, Edition 4. Connection is a collection of educational and thought-leading articles focusing on critical communications, wireless and radio technology.
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