By Rex Johnson
This page discusses the Drive Room at the New Zealand Post Office’s Himatangi Radio transmitting station, and its role in interfacing audio and telegraph traffic with radio transmitters.
Drive Room Background
The Drive Room was so named because its main purpose was for converting external audio and voltage-switched telegraph circuits into a form suitable to drive radio transmitters.
While Continuous Wave (CW) Morse might just require a transmitter to be keyed on and off by applying a keyed voltage, other modulation forms include modulated continuous wave (MCW), amplitude modulation (AM), single sideband (SSB), independent sideband (ISB) double sideband reduced carrier (DSB-RC) and telegraph circuits modulated for radio-teletype (RTTY) or facsimile (FAX) pictures.
Drive units handled the ‘pre-processing’ of traffic into the required modulation form. A discussion of various sideband modulation types may be found on Wikipedia.
The NZPO (and its international counterparts) used a radiotelephone mode termed amplitude-companded single sideband modulation (ACSSB). The companding (compression/expanding) of speech was performed at the Wellington Radiotelephone Terminal (Radphones) where the Himatangi and Makara send and receive circuits were managed for connection to the telephone network.
The drive room was not a part of the original station building. Initially a small copper-shielded ‘instrument room’ was built as an annex off the main transmitting hall.
When the station was commissioned in 1953, drive equipment was an integral part of the system design and that drive equipment was located in the main transmitting hall.
The conflict of low powered, sensitive equipment operating in a high-powered RF environment obviously caused difficulties.
In 1966 the original instrument room was considerably extended, (as an addition to the transmitter building) to provide a separate ‘Drive Room’ to house the transmitters’ low-power frequency generating and drive unit equipment and provide more instrument space for maintenance staff.
– George King, NZVRS Bulletin, Feb 2007
Due to the increasing cost of copper screening, the extension was lined with 22 swg securely earthed tinned steel sheeting which proved an effective barrier to any radio frequency interference from the adjacent high power transmitters.
George King, NZVRS Bulletin, Feb 2007
Audio and telegraph traffic circuits were cabled to the Drive Room from the Carrier Room.
The drive units could be thought of as mini transmitters. They were fed with audio or telegraph traffic and their output was a modulated signal on 3.1 MHz at a nominal 1 watts RF (sideband drive unit) or 4 watts (frequency shift keying drive unit, also termed FSK Exciter).
Driver signals were patched to a transmitter by cabling which left the Drive Room through a screened cable panel in the wall and under-floor cabling beneath the transmitting hall.
At a transmitter the drive signal was mixed with an RF oscillator frequency offset by 3.1 MHz. The required modulated output RF signal was then boosted in power by the transmitter’s linear RF power amplifiers, the output of that process being fed to antennas for radiation.
The RF oscillator frequency (offset by 3.1 MHz) may have been derived either from crystal oscillators at each transmitter, or in later years (c1973) from frequency synthesizer sources in the Drive Room.
Collins Drive Unit
The Collins transmitters were modern auto-tune models used for SSB radio-telephone shipping circuits. The drive unit (exciter) for Collins transmitters merely required selecting an operating frequency and pushing a button to tune and enable the transmitter.
Marconi frequency synthesizers were installed c1973 to provide more frequency selection options and frequency accuracy and stability than was available using crystals.
A Racal RA17 receiver was used to monitor transmitter output signals as required. Before the construction of the drive room it has been seen in pictures located beside the control console. In the Drive Room the receiver was connected to a basic longwire antenna run to the building from a nearby pole.
FSK Exciter Units
Approximately nine frequency shift keying drive units (FSK Exciters) were provided to support transmission of radio-teletype (RTTY) traffic.
Sideband Drive Units
There were about twenty drive units which were normally allocated to a transmitter each, with all incoming and outgoing circuits being permanently connected. They provided SSB and ISB modulation of audio, voice frequency telegraph (VFT) and facsimile traffic. Most of the drive units were manufactured by STC but the matching drive units for the Marconi HS51 were made by Marconi.
Work benches and shelves of test equipment were located in the Drive Room, continuing the purpose of the original Instrument Room.
Published: September 2016