Himatangi aerial feedlines

Rex JohnsonBy 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.

Himatangi Radio aerial farm

Himatangi Radio aerial farm showing the complex maze of feeders, aerials, staywires and aerial switches, c1970s. Photo: Chief Technician Collection, provided by Jon Asmus

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.

Philips transmitter aerial feeders, spark gaps and fluorescent tubes

Philips transmitter aerial feeders, spark gaps and fluorescent tubes, c1980s. Photo: Jon Asmus

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.

Aerial switching console, which remotely controlled switches in the antenna field

Aerial switching console, which remotely controlled switches in the antenna field.
Photo: Jon Asmus. Click to enlarge.

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.

Neville Carwell Cooke at the aerial console

Neville Carwell Cooke at the aerial console, c1953. The open cabinet doors show some of the bus wiring and control relays. The 10 lights in the top right corner of the panel are the Coomans Array indicators.
Chief Technician’s Collection, Rex Johnson

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.

Interior of Aerial Control Panel

Interior of Aerial Control Panel showing multi-selector bank switches and associated power inter-locking relays, c1953. Chief Technicians Collection, Rex Johnson. Click to enlarge

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.

Aerial switch panel at Himatangi Radio in the 1980s

Aerial switching console at Himatangi in the 1980s, designed for the new coaxial cable feedlines. This shows the coax switch matrixes as two rectangular groups of light bulbs to the left and right sides which indicate (with lamps) the plunger selection positions and RF feed paths. Photo: Jon Asmus

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.

Feeder poles carrying RF energy out into the aerial farm at Himatangi Radio

Feeder poles carrying RF energy out into the aerial farm at Himatangi Radio, c1970s. Photo: Rex Johnson

Once outside via the feed-through bolts, the feeders were carried on a set of ‘feeder poles’ out to aerial switches and aerials.

Redifon transmitters for Chatham Islands and Raoul Island at Himatangi Radio, c1970

Redifon transmitters for Chatham Islands and Raoul Island at Himatangi Radio used coaxial feedline, c1980s. Photo: Jon Asmus

The alternative to open-wire feed transmitters were coaxial feed transmitters. Two coax-feeder Redifon transmitters were installed at Himatangi pre-1970.

Harris and Collins coax-feed transmitters at Himatangi Radio

Harris and Collins coax-feed transmitters at Himatangi Radio, c1980s. Photo: Jon Asmus

From about 1973, two Collins and two Harris transmitters were installed, and these too had coaxial outputs.

Feeder ‘Baluns’

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.

STC DS12 transmitter with blue balun on top

STC DS12 transmitter with blue balun on top, c1970s. Photo: Rex Johnson

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.

STC DS12 transmitter with black balun mounted on adjacent wall

STC DS12 transmitter with black balun mounted on adjacent wall, c1980s. Photo: Jon Asmus