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.
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 which used coaxial feeders.
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.