Why and when a duplexer wont be sufficient?
The purpose of a regular (notch) compact duplexer, is to prevent the transmitted signal (RF energy) from entering the receiver and any received signal from entering the transmitter. It does this by notching out the transmit signal on the port of the duplexer that goes to the receiver, and by notching out the receive signal on the port that goes to the transmitter. The duplexer does not filter any other frequencies - only these two and only on the respective port.
This means that a notch duplexer is only really suitable to be used on repeater sites that have no other repeaters on it.
The duplexer shown above has three ports (connectors). The cable from the antenna goes to the middle connector. One cable will go to the transmitter and the other to the receiver - which one is determined by which frequency is the highest or lowest. Typically there needs to be more than 4MHz between the transmitter and receiver frequency to use a duplexer like this.
If there is more than one repeater (or any radio device) on this site, you have to be sure that there are no intermodulation products.
A notch duplexer will also therefore not prevent an unwanted signal from entering either the receiver or transmitter. The unwanted signal could come from the Transmitter of another repeater on the site. Any RF entering your repeaters transmitter, can generate harmonics (i.e. other repeaters transmitter frequency multiplied or divided by 2;3;5;7 and 9). These harmonics can mix with your repeaters transmitter signal - or the harmonics thereof - and produce intermodulation products. These products can in turn, mix with each other producing an inordinate number of products, in a worst case scenario.
Mixing and intermodulation will occur whenever there is a non-linear device in or near the circuit. Passive intermodulation (PIM) can occur when there is a rusty connection between a cable and connector or a rusty tower member. It can also occur when two dissimilar metals come into contact with each other (e.g. copper washer on a steel bolt).
Pass-reject duplexers are slightly better at filtering out-of-band signals but may offer sufficient filtering of signals a few hundred kilohertz away.
Having multiple repeaters and multiple duplexers means having multiple antennas. To achieve the same level of RF isolation as a multicoupler-combiner would provide, you would have to separate each antenna by, at least ten wavelengths - apart from being completely impractical, the coverage provided by the top antenna would be vastly different from that of the bottom antenna. The result will be inconsistent performance for users at the fringe of the coverage area - especially on trunked systems.
Generally, for systems where where there are two or more repeaters on the same site and in the same frequency band, I recommend to consider a multicoupler and combiner system as opposed to using 2+ duplexers and 2+ antennas.
If there are only two repeaters and the frequencies arent posing any intermodulation risk, you can sometmes get away with using two antennas and two duplexers BUT if it's three, doing this becomes a bad idea.
Since a notch duplexer does not filter anything other than a single transmitter or receiver frequency, any signal on an adjacent channel can enter the receiver of your repeater and produce RF densense. The result here would also be inconsistent performance as the interfering signal (or intermodulation product) may not always be present, making it difficult to diagnose.
The temptation to use multiple antennas and duplexers is sometimes too great because a multicoupler-combiner is comparatively expensive. However, the cost saving achieved by multiple duplexers and antennas is soon nullified by additional service calls to fix strange problems and by upset customers. (This has been proven many, many times).
Densense in analogue systems is easy to spot: you can hear the signal level drop, whenever the interfering signal is present. Intermodulation is also easy to spot, you can hear other voices on top of another transmission.
Densense in a DMR system only produces garbled audio (the interference pushes the bit error rate up). I explicitly use the word DMR because all DMR manufacturers will be affected in the same way - this is not something unique to MOTOTRBO.
A multicoupler-combiner provides the best performance and, in almost all cases, avoids (or significantly minimises) problems caused by intermodulation and desensing.
The combiner takes the RF energy and combines this into a single output. There are two methods to do this (and consequently two types of combiners). The first uses a series of cavity filters, all tuned to pass or reject a specific signal. This works if all the transmit frequencies are more than a few hundred kilohertz apart.
If the frequency separation is less, then hybrid combiners should be used. A hybrid combiner uses ferromagnetic combiners to sum the transmitter outputs. A hybrid combiner is also more compact but has more insertion loss when compared to a cavity combiner.
The multicoupler distributes the received signal, from the antenna, to the receivers of the repeaters. The multicoupler will have some filtering and amplification built-in and therefore requires power (DC or AC or both)
The input of the multicoupler and output of the combiner, would be connected to a pass-reject duplexer. This duplexer would be tuned to pass the transmitter frequencies to the antenna as well as allow received signals to pass to the multicoupler. This is much like a standard duplexer but multiple frequencies (which are within a few MHz of each other) are passed/rejected.
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