By Rex Johnson
From installation in 1953, Himatangi Radio was designed for the majority of transmitters to use open-wire 600 ohm balanced feeders for transferring (feeding) RF energy to antennas.
The RF output from 600 ohm transmitters appeared on top of each transmitter on a set of insulators. From there the open-wire feeders led up to roof level where they connected to a pair of ‘feed-through’ bolts mounted on insulating pyrex bowls.
Below the bowls were mounted lightning arrestor spark gaps and fluorescent tubes. The tubes were an indicator device for technicians as they would be excited by the RF energy in the feeder and consequently would flash in time to CW (Morse code) keying, or would flicker brightly for FSK or sideband transmissions.
The very neat and tidy aerial console pictured above was designed and constructed by the NZPO Radio Section staff in Wellington East Post Office. The console hid a mass of wiring and relays. Over time the outside aerials changed, requiring the control panel layout and switch operation to be changed. A significant panel change was to remove from the upper right corner the ‘Coomans’ phased curtain array used on early long distance circuits to the USA and UK, and to use that area to show more modern coaxial feed antennas.
Aerial selection was made using a ‘multi-selector’ switch associated with a particular transmitter. Each switch was ‘power-interlocked’ to a transmitter power supply so that the switch could not be rotated while the transmitter was active. With transmitter power removed, it became possible to depress an adjacent stud which then allowed that switch to be turned by hand to the selected position. In the new position a series of relays within the cabinet would operate, these switching control wiring out to further relays in all the affected remote aerial switches. Mains power at the remote switches would control 230-volt motor and gear systems to rotate all the aerial switch contacts, rotation being halted by sensor contacts.
Once each aerial switch had finished rotation, further contacts would close. Those closures would then be sensed back in the aerial console and panel lighting would come on, showing the effective route of the transmitter to antenna feeder. It would take 15-20 seconds for all the switches to rotate and lock in their new positions.
For the coaxial feeds, a vertical/horizontal matrix of ‘plunger’ switches was mounted on the wall inside the transmitter hall. Movement of a plunger in or out switched RF energy flow into horizontal or vertical rows/columns. This matrix allowed a versatile selection of transmitters access to coax-fed aerials.
Once outside via the feed-through bolts, the feeders were carried on a set of ‘feeder poles’ out to aerial switches and aerials.
The alternative to open-wire feed transmitters were coaxial feed transmitters. Two coax-feeder Redifon transmitters were installed at Himatangi pre-1970.
From about 1973, two Collins and two Harris transmitters were installed, and these too had coaxial outputs.
Arrangements were made to ensure the mix of open-wire and coaxial feeds did not limit the ability of transmitters to access the most appropriate aerials.
The Redifon coaxial feeds went to ‘baluns’ (balanced to unbalanced transformers) which converted the transmitter’s unbalanced coaxial feed to a balanced 600 ohm output. This meant these transmitters could use the radio station’s existing open-wire feeder services the same as other balanced-output transmitters.
A coax ‘unbalanced’ feed had the outer screen of the cable at earth potential. The balanced open wire feeders were not earthed but carried equal energy (were balanced with respect to earth) on both feed wires.
The two Collins and two Harris transmitters used baluns to allow them to access open-wire feeder services when required.
By the 1980s there were enough coaxial feeder services installed that baluns were being used the other way around. This meant the balanced output of an open-wire transmitter was fed into a balun whose output was coaxial and was connected to the rest of the coaxial feed services.