GPO Vehicles

Note the Chrysler badge on the lower nearside front panel

The equipment is installed in a modified 2500 Series, Commer general-service vehicle. Eleven of this type were made and this bought the compliment of detection vehicles to two in each Telecom Region.

A dummy roof houses the aerial-movement mechanism. To prevent water entering the dummy roof and being driven into the mechanism by the forward movement of the vehicle, the slot necessary to allow fore-and-aft movement of the front aerial is closed by a heavy-duty zip-fastened neoprene cover. Two sliders are used: one is mounted on the front of the aerial support, the other on the rear. The cover is thus opened in front of the aerial and closed behind it as the aerial moves. A purpose built console in the body of the vehicle provides the operating position. Extra seats are fitted for use of passengers during demonstrations.
The inside and roof of the vehicles are lined with washable plastic to give a good appearance and reduce condensation. Hot air from an additional vehicle heater is ducted to the side windows to prevent misting and to the dummy roof to prevent the seizing which would occur if the moisture formed by condensation were allowed to freeze in low air temperatures. The exterior of the vehicle is pale blue in colour with red sign-writing, and the interior is 2-tone grey. The vehicle is fitted with automatic transmission in order to give smooth take off and slow speed during detection runs.

AERIAL MOVEMENT, CONTROL CIRCUITS, AND POWER
Aerial Movements
Two separate movements are required. When the vehicle is not on a detection run the aerials are parked in line along the centre of the roof. Prior to operation it is therefore necessary to rotate them to face either the near side or offside of the vehicle. Normally, detection is arranged to take place on the nearside of the vehicle but, in a one-way street, it may be necessary to work on the offside. The rotation of the aerials is carried out by two cam-and-microswitch-controlled d.c. shunt-wound motors via worm-and-wheel gears to ensure that the aerials are locked in position when the power is disconnected from the motors.
The second movement is the linear movement of the front aerial which is necessary to obtain the constant spacing of 6u between the aerials. The front aerial is mounted on a light trolley, which moves on wheels on two Z-section rails. The trolley is fastened to an endless toothed-belt driven by a third d.c. shunt-wound motor.

Control Circuits
The positioning of the front aerial is controlled by a d.c. servo system. The u.h.f. tuner shaft is coupled directly to one potentiometer and via a shaped cam to a second potentiometer, which provides a correction voltage to convert from the straight-line frequency law of the tuner capacitor to the straight-line wavelength law required for the linear movement of the aerial. A third potentiometer is coupled to the main motor gear-box to provide an error voltage. The three voltages are combined in a differential amplifier with a reference voltage from a Zener diode. The differential output is fed via two Schmitt-trigger circuits, which are included to prevent hunting, to switch a high-power transistor bridge. This controls the power fed to the d.c. motor, to drive the aerial in the required direction.

Operator's Controls and Display
The operator's controls are mounted on a display panel on a console. To connect power to the detector, the vehicle ignition must be switched on, and the aerial position switch must be at PARK. In this condition the main on/off switch in the ON position connects power to the equipment. When the on/off switch is set to OFF, power is not disconnected from the servo system until the aerials return to the parked position. In case of an emergency such as fire, an EMERGENCY OFF position on the switch is used to disconnect all power from the equipment. Pilot lamps and an aerial-position indicator are provided to let the operator see at a glance the position of the aerials.

The panoramic receiver has three controls: frequency setting with associated digital-logging display, gain, and sweep width. Brilliance, focus and beam-shift controls are provided for the cathode-ray tubes, and an aerial-control switch is fitted to select the main or either of the auxiliary sensing aerials.

Power
Power is provided by two heavy-duty 6-volt batteries in series. The batteries also provide for the normal electrical system of the vehicle and are charged by the vehicle alternator. A 230-volt 50 Hz supply obtained from a solid-state invertor is used to power the panoramic receiver and the power unit for the cathode-ray tube displays. Low-voltage supplies for the transistor circuits are obtained from the display power pack.

CONCLUSION
A new system of television detection has been designed, and a working prototype produced by the Post Office Radio Regulatory Division. The production models are being manufactured by a contractor. The equipment is of modular construction to facilitate fault finding and reduce out-of-service time by the replacement of faulty modules by spares. It is expected that the new transistor receivers will have a lower fault liability than the old thermionic-valve receivers.
The new detection equipment enables an accurate location of v.h.f. and u.h.f. television receivers to be made in the presence of reflections, and the sensitivity allows for the low levels of radiation from modern transistor tuners.

ACKNOWLEDGEMENT
Acknowledgement is made to Messrs. Vosper Ltd., Portsmouth, for the details of their improvements to the mechanical design of the aerial-movement mechanism and for their assistance in the design of the servo-mechanism.

References
BURLING, K. G. and FANNING, J. C.
A New Detection System for Television and V.H.F. Radio Receivers. P.O.E.E.J. Vol. 55, p. 219, Jan. 1963.

Parts extracted from the POEEJ, Vol 62, Page 143, October 1969.

Last revised: February 04, 2021