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Capacity Plus at the National Theatre

Capacity Plus at the National Theatre

For some years now the National Theatre had run a Linked Capacity Plus (LCP) system, with 3 repeaters, each repeater being in one of the theatres. This gave good but limited communications throughout the National, only 2 simultaneous conversations were possible.

The quantity of radios and users groups had grown over the years and more capacity was needed on the system. One approach was to simply add another LCP repeater at each location, but this approach would have been expensive in hardware terms; it would of required either a small antenna combining system in each theatre, or at least 2 antennas with circulators to comply with best engineering practice. Space is limited in the theatres without much room for second antennas.

The other idea was to bring all the repeaters together in a Capacity Plus system, giving 6 simultaneous calls at the outset (instead of duplicating the repeaters on LCP which would only give 4 simultaneous conversations), combine it all, and then distribute the signals through the venue using Heliax cable. The disadvantage of this method was the length of the cable runs involved. From a central location in an equipment room where we could house the repeaters and antenna combining we had cable runs of the following lengths; 50m, 90m and 110m. This meant installation of cable trays in some locations, and diamond drilling of the thick concrete in parts of the National Theatre so that we could get cables through walls!

However, this approach was a similar cost to the addition of 3 LCP enabled repeaters and immediately gave 50% more talk-paths, and with the addition of one more repeater at relatively low cost would enable 8 talk-paths, as we specified 4-way transmitter combining to allow for future expansion

The length of the cables and the potential RF losses due to distances meant using some very large diameter and expensive Heliax cables. Large diameter Heliax doesn’t bend very well and is easy to damage on installation, never mind fitting the connectors, which is more like plumbing work than electronic engineering! In some areas, I think the Heliax is supporting the cable trays rather than the other way around.

Because of noise and interfering with the running of the National, we have only been able to undertake this work on Sundays. So, for the past month we have been persuading thick and difficult to manage Heliax to fit into cable trays with many twists and turns en-route, we managed to fit one cable per Sunday, and have since spent another 4 days terminating cables, fitting antennas, and testing, culminating in a day spent relocating and reprogramming the repeaters (and radios) fitting them to a rack with antenna combining, and battery backup.

We intend to also implement Over the Air Programming (OTAP) of the radios using Remote Management 2. However, because of the age of some of the radios (they had early version firmware which is not supported via RM2), many units required a firmware upgrade before reprogramming, making the reprogram for Capacity Plus far from quick. OTAP will simply future radio changes, as the radios are kept in many different areas of the building, and collection, and re distribution after reprogramming is currently very time consuming for the National’s technical staff.

The Wilkinson Splitter after the combining feeds 4 cable runs (the 4th which is for the car park area is not installed yet, so is terminated with a dummy load) – the picture also shows the size of the Heliax and connectors compared to standard RG213.

Heliax cable in the National Theatre




















A big thanks goes to Mike himself for taking pictures and writing this article, all whilst working on the job!


Interested in Capacity Plus? Read more about the system and how you can hire it here.

Created On  28 Feb 2017 15:21  -  Permalink

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