NZPO Radio Inspector’s mobile DF equipment

By Ian Walker ZL1BFB

In the years after World War II the New Zealand Post Office took advantage of the availability of war surplus radio parts to construct mobile DF (direction-finding) equipment, utilising Catalina direction finder antennas and tuning units. There were plenty of spare parts from scrapped or decommissioned aircraft as well as components held in storage and this was one of the ways to make use of what was freely available.

A Radio Inspector's car fitted with DF equipment

A Radio Inspector’s car fitted with DF equipment

The DF Units were then fitted to the vehicles used by Post Office Radio Inspectors to locate illegal radio signals or interference to domestic reception. These vehicles were mostly located in the main centres: Auckland, Hamilton, Wellington and Christchurch.

The equipment used was the loop antenna and its associated valve tuneable loop amplifier with a feed to the main broadcast receiver in the vehicle.

In Auckland it was originally fitted to the modified Gulbransen car radio and later to the Sprague interference receiver. The Gulbransen receiver had a specially built output level meter which read in decibels. This gave the intensity of the signal which could be read whilst driving.

Some of the receivers had a BFO fitted. There was no RF indicator on the amplifier so the output meter acted as an “S” meter.

The valve pre-amplifier had two valves that operated on 12 volts DC for the filaments and HT supply. (These are no longer manufactured.)

The use of the loop antenna for locating the source of interference was very efficient, as direction could be maintained while mobile and it was especially good for locating faulty power poles and electric fence noise.

A Radio Inspector's car fitted with DF equipment

A Radio Inspector’s car fitted with DF equipment

In one of the Auckland vehicles, the pre-amp was mounted in a box in the centre of the vehicle at the rear of the front seat. The tuning knobs were visible in the rear view mirror and easily accessible from the front seat.

Just above the tuning body was a sprocket wheel (similar to a gate valve) coupled to the central pole which, when rotated, would turn the loop and the graduation marking fitted to the top of the control box gave an indication of the direction of the signal. The sense antenna, which was a standard car antenna, was switched to ascertain a North or South direction from the location of the vehicle. This antenna was normally located on the rear wing of the vehicle to reduce car ignition noise pick-up.

This unit was the better for use and accuracy of the two in Auckland. I recall vehicle No. 5693, a Holden station wagon, which I often used. The Sprague 500 receiver was mounted on the floor between the driver and passenger’s seats and was operated from the 12 volt vehicle system. This unit has been modified for 500 kHz reception and had a BFO fitted to assist with QRM and signal tracing.

Auckland later received its second unit which was mounted differently, having been modified to transistorised construction, with modern switching on the control panel which looked very neat and compact. This was mounted internally in the roof of the vehicle, and although it was (for its time) a modern design making for a clean look within the vehicle, it was not always very accurate or efficient.

Mounted within the roof space above the centre of the driver’s bench seat, the transistors frequently failed especially in the summer months due to the heat generated, so the unit was often under repair. Although the valve unit was superior and more efficient, the valves became very hard to obtain and we were forced to keep up with the modern technology of the times.

RF unit

RF unit

Returning to the description of the RF Unit, this was very compact and easy to use. It gave about 10dB gain from the beacon band to the broadcast bands and extended into the ham bands on 80 and 40 metres. There was also provision for signals above 30 MHz. The 12 volt system in the car was duplicated, as at times the antenna amplifier in the DF was left switched on and would shorten the life of the starter battery.

The unit was used for locating power line interference, electric fence noise, problems with intermods on broadcast and local transmitters. Amongst the many other uses we found we had for it, the main ones that I recall were for signals on 600 metres, and the location of Radio Hauraki and other pirate signals on the broadcast band.

The 1960s were the era of pirate radio, and as we always seemed to be the last ones to hear about them as operational, it was not surprising that we became very efficient in the use of the DF and subsequent actions.

My time with the DF was three terms of three months or whenever called upon for those duties. I then moved on to the VHF/RF which was called the “Miscellaneous Section” and although I still had to DF radio signals it was with different equipment.

The technically inclined can try duplicating the circuit of the DF Valve Set. I am unsure if the valves are still current but substitutions could be made. I have the information of the transistor amplifier used but think these types of devices are also out of service.

DF Unit

DF Unit. Click to enlarge.