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168 AUSTRALIAN RADIO — THE TECHNICAL STORY

The first system was installed on the Sydney-Maitland route in
New South Wales and others were subsequently placed in service in
other parts of Australia. Later on, an additional group of seventeen
channels was added to some of the existing systems, thus providing a
total of 34 telephone channels on each pair of a multi-pair cable.

Although it might be regarded as a non-standard system when
related to current CCITT recommendations, the installation of the
Sydney-Maitland link was an ambitious undertaking which was very
successful and broke new ground in communication engineering.

During the latter part of the 1950s expansion beyond 24 channels
became a matter of urgent necessity. To meet this requirement, it
became general practice to take five of the basic twelve-channel
groups and modulate them with the appropriate group carriers to
translate them upwards in frequency to form a super-group of 60
channels occupying the spectrum between 312 and 552 kHz.

When more than 60 channels are required, the appropriate super-
group carriers can be used to translate up to fifteen super-groups to
produce 960 channels occupying the spectrum between 60 and 4028
kHz. For very large channel requirements to be handled by wide-
band coaxial cables or broad-band radio bearers, three sets of 960
channels can be assembled to provide up to 2700 channels requiring a
bandwidth extending slightly beyond 12.5 MHz.

The above description is intended to indicate only in broad terms
the means by which large numbers of channels are obtained for
transmission over a single medium. Nevertheless, while the
underlying principles are relatively simple, the assembly of
equipment required for very large systems is quite complex. Its
performance is based upon the use of precision oscillators for
generating the channel, group and super-group carrier supplies and
high performance filters of exceptional stability. Crystal filters have
been used extensively in some areas of the equipment for several
decades but in recent years inductance/ capacitance (LC) filters using
high stability ferrite cores and high quality capacitors are being
widely used.

The medium over which the multiplexed channels are transmitted,
whether cable or broad-band FM radio bearer, must provide a very
high order of performance with negligible variation in frequency
response or phase shift over the entire bandwidth required by the
channelling equipment.

7.5 Broad-Band Radio Bearers

During the early stages of World War 2, provision ofa radio link asa
back-up for the submarine cable between Tasmania and the
Australian mainland became a matter of some urgency. To this end, a
contract was awarded to STC in Sydney to design and manufacture
equipment suitable for the purpose.

MULTIPLEXED COMMUNICATION SYSTEMS 169

This equipment operated in the 39-42 MHz band with a carrier
power of 250 watts capable of 100% amplitude modulation over the
frequency range 30 Hz to 35 kHz. The system was thus capable of
interfacing with a three-channel open wire carrier telephone terminal
and accommodating a programme channel at the same time.

The absence of suitable test instruments from commercial sources
added considerably to the problems of developing such equipment
during the early 1940s. For example, to evaluate the performance of
the receiver adequately, it was necessary to construct a low-powered
version of the transmitter for use as a signal generator. The detailed
engineering design of the transmitters and receivers was carried out
by R. R. (Bob) Long and W. E. (Bill) Joiner respectively. This VHF
system, the first multi-channel radio bearer designed and
manufactured in Australia, was placed in service in 1943. To ensure
continuous operation during maintenance periods, the equipment
was fully duplicated in all respects.

Following commissioning of the Bass Strait radio link, little more
took place in the field of radio bearers until the middle to late 1950s
when local development of frequency modulated equipments began.
These comprised “low capacity” systems designed for operation in
the VHF (nominally 150 MHz) and UHF (nominally 450 MHz)
bands and initially for 12 and 24 channels respectively. They
employed valves throughout with type 4CX250B triodes in the high
power stages to provide transmitter powers of 20 to 30 watts at 150
MHz and 10 watts at 450 MHz.

Being designed to interface directly with 12 or 24 channel
frequency division multiplexing equipment as already in use on cable
systems, the new frequency modulated radio bearers were
immediately in demand. Large numbers of these systems were
manufactured by STC and placed in operation during 1961-62.

By 1962, local development had progressed to the point where
valves had been eliminated and all solid-state equipments were
available with 10 watt transmitters. Channel capacity had been
increased to 24 at 150 MHz and up to 60 at 450 MHz. For special
long-haul circuits in some overseas applications valve amplifiers
providing up to 200 watts at 150 MHz were also produced.

In the meantime, local development of equipment for use at much
higher frequencies was receiving close attention, so much so that by
1964 a series of all solid-state broad-band bearers for the 2000 MHz
(2 GHz) band were being manufactured in Sydney by STC. These
microwave equipments featured transmitter powers of | watt with
capacities of 120 channels in the 1.7 to 1.9 GHz band and 300
channels (or a television programme) in the 1.9 to 2.3 GHz band.
Production of the lastnamed equipment is understood to be the first
time such performance with all solid-state equipment had been
achieved anywhere in the Western world. The engineering team
responsible for this achievement was led by G. E. (George) Gissing.
As in the case of the VHF and UHF systems, much of the microwave