A question about antenna systems.

Q I am curious as to your thoughts on the benefits/disadvantages to using repeaters with individual antennas and duplexers versus transmit combiners and receive multicouplers. 

Mostly this is in regards to single-site Capacity Plus systems. We have a few systems that started off with individual departments in a manufacturing environment getting a repeater for added coverage and then turning that into a single-site system usually with approximately 3 repeaters for all plant wide communications. We have ended up with getting the antennas near each other but there are obviously limits to that but that can create differences in coverage depending on what channel the talk group is assigned too. 

Obviously this is not a huge deal or system breaking by any stretch either way in most cases but your thoughts would be appreciated.

A A popular (though problematic) way of arranging multiple trunked repeaters is to use multiple duplexers and antennas. In this setup, every repeater has its own duplexer feedline and antenna.

The obvious benefit (and that’s why it’s seemingly popular) is that it’s cheap, easy to install and appears to work in certain cases. When the customer added the second and third, they likely just bought another repeater, duplexer and cables.

Many will argue that the insertion loss is far better than using a combiner-multicouler setup. While this is true, there is a price to pay when it comes to interference and coverage problems.

I suppose you could also make an argument about there being fewer single points of failure. The theory being that if one antenna gets hit by lightning or a duplexer drifts out of tune, you only lose that specific channel. The rest of the trunked system can keep operating at a reduced capacity.

With the above (3 repeaters, 3 duplexers, 3 antennas) configuration, in order to have sufficient isolation between each of the antennas, they would need to be spaced some distance apart. The exact spacing can be calculated using this online tool. For example, at 450MHz, to get close to the isolation a combiner would provide, the antennas would need to be 8 meters apart¹.

As a result, one antenna will be higher, one might be blocked by the others, and one might be closer to a building. As a result, the coverage offered by each of the antennas and repeaters will be slightly different. In some cases, when a radio user is in a low signal area, they could regularly lose signal when the rest channel (in the case of Capacity Plus) jumps to another repeater.

When repeater A is transmitting at 50 watts, and the antenna for repeater B (which may only be a few meters away) is trying to listen for a weak 4-watt portable radio on the factory floor, the massive signal can overload Repeater B's receiver. This is called desense and drastically shrinks the effective coverage when these conditions are met. Frustratingly, this may not always occur so the customer will sometimes get good signals and then suddenly lose signal or will hear a high bit error rate. 

The duplexer for repeater B does not provide any filtering for the signal coming from repeater A’s transmitter.

Multiple transmitters broadcasting close to one another can mix their signals in surrounding metal objects, creating new, unintended ghost frequencies. These intermodulation products raise the noise floor and further degrade system performance.

The preferred way would be to use a multicouper to distribute the received signal from the antenna to all the repeaters and a combiner to combine all the transmitters and send this to the antenna. In this architecture, TX of all three repeaters feed into a Transmit Combiner (either a hybrid or cavity variant), which outputs to a single TX output. The repeater RX inputs are fed from a Receive Multicoupler (a filtered amplifier that splits the signal) which gets its input from a single RX input. The multicoupler input and combiner output can be fed into a single antenna using a band-pass duplexer, but a two-antenna TX/RX split is sometimes used.

Because all repeaters transmit and receive from the same antenna, coverage is 100% uniform regardless of which physical channel is being used.

Because the combiners use circulators any signal from the transmitter of channel A is blocked from feeding back into the transmitter of channel B, reducing the level of intermodulation at the site.

The receive multicoupler has a pre-select filter that blocks out most out-of-band RF noise. The built-in amplifier is needed to compensate for the loss incurred by the preselector and splitter.

Combiners and multicouplers are however more costly when compared to the above multi-duplexer, multi-antenna setup described above. But the extra cost, in many cases, outweighs the problems caused by desense, intermodulation and coverage inconsistencies.

Compared to a duplexer, a TX combiner has higher insertion loss (i.e. loss of TX power between the repeater and output). A typical 3-channel cavity combiner might introduce around 4dB loss whereas the duplexer can be as low as 1,5 dB. If a repeater outputs 50W, only 20W watts might make it to the antenna. However, this is almost always offset by the fact that you can place the single antenna at a much better, higher vantage point and since there is more space on the tower, you can use a higher gain (bigger) antenna.

Those who argue in favour of multiple antennas and duplexers will use the single point of failure example. If the antenna is damaged, or the RX multicoupler amplifier dies, the entire trunked system goes down. 

A combiner is physically built for a set number of channels and tuned to specific frequencies. If they want to add a 4th channel later, the combiner must be expandable and will require a technician to physically retune the cavities.

In some cases, using multiple antennas and duplexers still makes sense. For example, when there are only two repeaters at the site. In this case, a multicoupler-combiner will not offer a significant advantage. This still needs to be weighed up against the risk of desense and intermodulation. When there are more than two repeaters on the site, then it makes sense to consider a combiner-multicoupler arrangement.

So in summary, the multi-duplexer multi-antenna method is cheap and easy but suffers from significant performance issues, including non-uniform coverage and major interference. My preferred method uses a single antenna combiner-multicoupler. While more costly and possibly introducing a single point of failure, this configuration ensures 100% uniform coverage and virtually eliminates interference.