By Rex Johnson
This page discusses the Himatangi Radio Station carrier room and its role in interfacing HF traffic links with New Zealand Post Office (NZPO) land-based communications networks.
The carrier room was so named because its main purpose was for processing multi-channel telephone and telegraph circuits arriving at Himatangi Radio over ‘carrier’ circuits.
A standard carrier circuit was a ‘group’ of 12 audio channels which were multiplexed together. The multiplexer output was a 4-wire send and receive circuit which could be passed over two pairs of wires or a radio bearer circuit. At the receive end of a circuit they were ‘demultiplexed’ back to individual voice circuits, plus any signalling associated with each circuit. Carrier groups (of 12 channels) were a basic building block and many such blocks could be multiplexed together where bearers had bandwidth to support them.
The carrier room was part of the original station building commissioned in 1953. Because of the high levels of radio-frequency energy to be generated from nearby transmitters, there was potential for interference with low-level signals handled by the carrier equipment. Such interference was minimised by fitting copper sheets over the walls, roof and floor of the carrier room and shielding the rear of the door. Mounting screws were also of copper to avoid possible rectification caused by dissimilar metals.
The external circuits terminating in the Himatangi Radio carrier room were (in 1958) a 12-channel group from the South VHF system and five individual channels to the North VHF system. An additional 12 channel group from the South VHF system passed through to Palmerston North where it provided 12 toll circuits between Palmerston North and Wellington. The audio circuits from Wellington were from Wellington Radiotelephone Terminal (Radphones) for international circuits, Pacific, Chathams, Raoul and Campbell Islands, Scott Base and shipping radiotelephone circuits. The circuits could also have carried facsimile weather picture data from the Met Office in Wellington.
In addition to multiplexed audio circuits, it was possible to multiplex telegraph signals so that many telegraph circuits could be carried within single audio circuits. A common Voice Frequency Telegraph (VFT) standard was for 24 telegraph channels carried over one voice circuit. For operating Himatangi Radio transmitters these channels may have carried Mose code (from remote operators – such as at Wellington or Awarua Radio) or 50 Baud teleprinter traffic (from Wellington or Auckland Telegraph offices). Consequently, the carrier room had many racks of equipment associated with demultiplexing audio and telegraph circuits.
COMPAC Cable Backup
A patch panel in the carrier room had rows of phone sockets prewired to connect traffic to radio transmitting services to Australia. These jacks were part of the Himatangi Radio backup service for the COMPAC cable.
Some traffic bound from New Zealand for the COMPAC Cable was routed through Himatangi (and probably the received portions of those channels were received at Makara Receiving Station). In normal use, a series of jackplugs were in place connecting the transmit side of some COMPAC Cable traffic straight back out of the station to the cable terminal in Auckland. If the COMPAC Cable failed, Himatangi Radio would have been instructed to activate the radio services backup, which meant pulling out the jackplugs from the linkfield so the cable circuits would be diverted to Himatangi Radio’s always-active COMPAC-standby transmitters. A similar activity would probably have occurred on circuits received by Makara.
In later years more modern equipment was purchased to improve voice frequency telegraph (VFT) circuits transmitted over radio paths. By the early 1980s Quindar multi-frequency telegraph equipment was in use to both Scott Base and the Chatham Islands. This system used a mix of redundant code bits to provide forward error correction (FEC) and dual-keyed tone pairs to provide audio frequency diversity.