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.
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.
