Everything you ever wanted to know about Combining Systems, but were afraid to ask?

Everything you ever wanted to know about Combining Systems, but were afraid to ask?

Simply put, a combining system places multiple transmit (Tx) and receive (Rx) signals onto a single common port. Within a combining system, a Tx combiner gathers all transmit signals together onto a single output port whilst a Receive Multicoupler will take incoming signals and split them to feed multiple receivers. If the Tx output and Rx input are both fed into the Tx and Rx ports on a duplexer then the resulting common (Ant) port carries both signals to/from a single antenna creating a multi-channel RF combining system.

Combiners are used throughout industry, essentially anywhere that multiple channels are required to be used simultaneously. Combiners are an essential item in any medium to large scale radio system where multiple channels are required as they facilitate the use of multiple transmitters and receivers whilst isolating these signals from each other. Without a combining system, you would have to use multiple antennas which could not only introduce radio interference but could also be quite expensive especially if antenna space has to be purchased or rented. There may also be restrictions from a local authority who will not permit a multi-antenna installation, so a single discrete antenna would need to be installed. The use of just a single antenna also permits the installation of just a single feeder cable rather than multiple cables - another significant cost saving, particularly when taking into account increasing cable costs and riggers charges.

There are generally 2 types of combiner - cavity and hybrid. Note that whilst this refers to the method of combining the Tx channels, the Rx channels would generally be handled identically regardless of Tx combining method.

Cavity Combiners
Cavity combiners use large, high Q cavities which are generally lower loss and capable of handling higher power levels. The sacrifice is often size and price as cavity combiners take up a lot of rack-space. They are also more expensive compared to their hybrid counterparts.

Hybrid Combiners
Hybrid combiners are much smaller but generally have more signal loss due to the way they process RF signals. Generally speaking, if signal loss is not an issue or can be compensated for, then a hybrid combiner is better suited for most typical applications. Another advantage of a hybrid combiner is that they can operate with little or no Tx frequency separation whereas a cavity combiner requires a minimum frequency spacing, dependant on operating band (typically 75 kHz (VHF), 250 kHz (UHF).

There are a few considerations to investigate when choosing a combiner. Frequency, losses, size and cost are all major factors. The choice of available system will very much depend upon the channel frequencies, so this could be one place to start e.g does the channel plan allow the use of a cavity system? Another starting point could be the acceptable level of Tx loss in the system, once you have factored in Tx input power level, feeder cable losses, antenna gain & permitted ERP (effective radiated power) output for the site. Available rack space may also be an important element, which could automatically rule out the much larger cavity-based solutions as could available budget.

Like many passive devices, the principles behind their operation are based on physics – these haven't changed so the associated equipment hasn’t really either. Manufacturing processes have perhaps evolved over the years so the cosmetic finish of the item may be different but the physical products still use similar technology.

Radio systems can be operating 24 hours a day/ 365 days a year, so a combining system must work as reliably as the radio system. Often combiners could be situated on a remote radio site not practical for regular engineer site visits. It is essential therefore that a combining system be a "fit & forget" solution.

Procom have built an industry wide reputation as a major supplier of high-quality, reliable RF filtering/combining products. If you have a requirement for a combining system, visit our website at http://www.procomuk.co.uk/Combiners or contact our team on +44 (0)1227 743099 or via email at sales@procomuk.co.uk.


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