GPO Vehicles

Make Ford
Model Anglia 309E
Type 5cwt Radio Interference Vehicle
Body Builder  
Use Radio Interference Investigation
Registration Number PMG 924E
Fleet Number Range 201827 - 201931 (PMG 823F - 927F) 1967
Date of picture March 1967

VOLUME 61, PART 1 - APRIL 1968

New Radio-Interference Equipment Vehicles

At the present time the British Post Office maintains a force of 350 radio-interference investigation officers throughout the country. These officers deal annually with some 65,000 complaints concerning the radio and television broadcast services. To aid them in this task they are supplied with special vehicles equipped with measuring and tracing apparatus. Two types of vehicles are used in this work: for the simpler work a small 5 cwt vehicle is used; for the more difficult interference cases , and for dealing with interference with reception in Bands IV and V, a more fully equipped 10 cwt vehicle has been provided.


The number of complaints of interference with radio and television broadcast services, received annually by the British Post Office, has steadily decreased from a peak of nearly 170,000 in 1955 to just over 65,000 in 1966. Although the annual number of cases has been reduced the problems of tracing sources of interference remain.

When it became clear that the existing vehicles and equipment used in radio-interference work were reaching the ends of their useful lives a study was made to determine the requirement for replacements to deal with the present-day problems. In the course of the survey, Telecommunications Regions and Telephone Areas were consulted, and many of the ideas and suggestions received were incorporated in the design of the new vehicles and equipment. At the time when the preliminary design and development was taking place the new ultra-high-frequency (u.h.f.) television service was being introduced, and facilities for dealing with this were incorporated from the beginning.


For some years the numbers of complaints relating to medium-wave and long-wave reception have been decreasing, while the number of complaints relating to television reception has been increasing. Special attention had, therefore, to be given to the problems of tracing sources of interference with
television reception, especially sources of continuous-wave or modulated continuous-wave interference. This type of interference is generated, for example, by the frequency-change oscillators of radio or television receivers and by industrial and medical radio-frequency (r.f.) apparatus.

Interference caused by Barkhausen oscillations in the line output valves of television receivers can be traced satisfactorily only by the use of a portable television receiver. This type of interference is usually referred to as ďwindscreen wiperĒ interference because, when the source is a television receiver tuned to a B.B.C. program in Band I interfering with a receiver tuned to I.T.A. in Band III, it takes the form of a vertical band moving slowly back and forth across the television screen. The same type of interference is experienced in Bands IV and V from receivers tuned to either Band I or Band III, but in this instance, because the line standards are different, the band is broken up and appears as dots or lines in a pattern that moves diagonally across the screen.

For tracing a source of continuous-wave or modulated continuous-wave interference which may be close in frequency to the carrier of the wanted signal the only suitable method is to use a panoramic receiver coupled to a direction-finding aerial. A television receiver, a high mast and appropriate aerials are also necessary for demonstrating to a complainant the effect of an efficient receiving system.

For economic reasons it is desirable that as many of these different functions as possible should be combined in the one piece of equipment, and that, as far as possible, standard commercial equipment should be used.

To deal with all the types of interference encountered in the field it is, therefore, necessary to provide the following equipment:-

  1. Vehicles to carry equipment and staff.

  2. Portable television receivers.

  3. Portable tracing and measuring receivers.

  4. Panoramic receivers.

  5. Direction-finding aerials.

  6. Demonstration aerials.

  7. Rotating masts for direction finding.

  8. High masts to raise the demonstration aerials to a suitable height.

  9. Benches and soldering-iron points to facilitate the carrying out of on-the-spot suppression of small appliances.

  10. Power supplies to run the equipment.

Not all this equipment is necessary for every case of interference, and most can be dealt with using only a portion of this apparatus. As the present force of 350 investigation officers requires some 300 vehicles to carry out its tasks it would have been uneconomic to provide the full facilities in
every vehicle. It was decided, therefore, to have two classes of vehicle and equipment to deal with the following two broad categories of complaint.

  1. Those where the source of interference is known or may be relatively easily traced by simple means, and where suppression may be effected by standard methods. These cases form the bulk of the work.

  2. Those in which the source of interference is difficult to trace and may be a long way from the affected receiving installations, and where measurement and demonstrations are necessary. At present, all complaints of interference with u.h.f. reception are included in this category.

Because the radio-interference staff may have to spend long hours in their vehicles in the course of tracing interference, monitoring transmissions or suppressing apparatus, special attention was paid, in the vehicle designs, to the layout and seating. The vehicles have been fitted with interior lining to reduce condensation and to provide a good internal appearance.

Vehicles in Post Office garages must be left unlocked because of fire risk; therefore, lockable reinforced-plastic security screens have been provided between the driverís compartment and the rear compartment in all vehicles. These screens can be removed for normal working.

Fig. 1 - Five cwt radio-interference vehicle


A commercial 5 cwt van has been used as the basis for the vehicle designed to deal with the complaints described earlier as category (i). This vehicle is illustrated in Fig. 1.

It is often inconvenient for an investigation officer to carry out suppression work on small appliances such as vacuum cleaners, sewing-machine motors, etc., in peopleís homes.

Owners are naturally reluctant to lose the use of their appliances for any length of time, especially where interference affects a broadcast receiver other than their own. From the Post Officeís point of view, it is uneconomic to have to make a special journey back to a central workshop with each item that has to be suppressed. The most efficient method of working from every point of view is, therefore, to provide in the vehicle facilities for fitting suppression components on the spot. For this reason the 5 cwt vehicle has been provided with a small working top, underneath which a nest of six drawers is provided for storing tools and suppression components.

Light for the working area is provided by a green-tinted roof light which can be opened, if required, for ventilation. For soldering, a 12-volt d.c. supply is provided from an enlarged vehicle battery. Anchorage points are fitted at the rear and front edges of the bench so that apparatus can be secured for travelling.

The usual passengerís seat is dispensed with and its place is taken by a working position set back into the body of the vehicle. This seat, because of its position and the consequent difficulty in fitting safety belts, is not suitable for carrying a passenger, and the 5 cwt vehicle is satisfactory only for one-man working. Fig. 2 shows a rear view of the 5 cwt radio interference vehicle with the equipment which may be normally carried.

When the issue of these vehicles is completed there will be approximately 200 of them in service.

Fig. 2 - Interior view of the 5 cwt vehicle


For dealing with the complaints referred to earlier under category (ii) a limited provision is being made of 75 vehicles fully equipped to investigate all type of interference. This provision is on the basis of one vehicle for each of the 59 Telephone Areas plus an additional vehicle for Areas which are large territorially or have a large number of difficult cases.

To meet all the requirements as economically as possible each separate piece of equipment has been made to serve as many individual functions as possible. Fig. 3 shows in block schematic form the connexions of the equipment in the vehicle, and the functions of each part are described in the following text.


Fig. 3 - Arrangement of equipment and power supplies in the 10cwt vehicle

The vehicle (Fig. 4) is a standard Type 2 10 cwt Morris van that has been modified in the following details:-

  1. Part of the rear floor has been lowered to accommodate an operatorís seat and to provide the necessary leg room. This seat has a movable back which can be positioned either for a passenger facing forward in the usual manner or for an operator sitting directly in front of the panoramic receiver and facing the off-side of the vehicle.

  2. Side windows have been provided to give the operator an all-round view, and to provide light and ventilation.

  3. Provision has been made for the mounting of a pneumatic mast in the centre of the vehicle within easy reach of the operator.

  4. Two box compartments have been mounted on the roof to carry normal commercial-television and Band II aerials, which are used for direction finding, field-strength measurements and demonstrations.

  5. A large hatch has been provided in the roof above the operatorís seat to allow easy access to the head of the mast, for fitting the aerials as required, and to gain access to the aerial boxes without climbing on to the roof of the vehicle. The hatch also enables the operator to take a direct bearing on a source of interference by looking directly along the path of the main beam of the aerial.

  6. A heavy-duty battery and a large a.c. generator have been substituted for the normal vehicle battery and generator to provide power to operate the television receiver and the panoramic equipment.

  7. An apparatus bench with universal fixing points and safety rails has been fitted. The shape of the bench has been designed so that the face of the television receiver can be seen by the operator when seated in the operatorís seat, but cannot be seen from the driverís seat, or from directly in front or from the off-side of the vehicle. A writing flap is positioned in slides under the television receiver so that it can be extended into the writing position when required. The bench has three drawers and two open lockers easily accessible from the operatorís position, and a closed locker accessible from the rear of the vehicle.


Fig. 4 - Ten cwt radio-interference vehicle

Fig. 5 - Interior layout from the operatorís seat

Pneumatic Mast
The pneumatic mast has three main functions. Firstly, it is fitted in the centre of the vehicle, in rotating bearings, and can be used in the un-extended position as a mast for direction finding. Secondly, while fitted in the vehicle, it can be extended for field-strength measurements to a maximum height of 24 ft by a small electrically-driven pneumatic pump, which is mounted under the bench. So that the mast may be freely rotated when in the extended position a non-return valve is fitted in the air inlet and connexion is made via a flexible tube and a bayonet connector. Water is precipitated inside the mast by the action of compressing and releasing the air, and by condensation. To prevent this water from entering the vehicle the air is exhausted outside the vehicle when the mast is lowered; this is done by means of a second flexible tube and bayonet connector.

Fig. 6 - Operator erecting the pneumatic mast for use remote from the vehicle

The third function is its use to demonstrate to a complainant the effect of an efficient outdoor aerial. For this purpose the mast may be easily removed from the vehicle by lifting it through the split bearing on the roof after undoing one hand-tightened turn-screw at the bottom. A tripod, accommodated in the rear cupboards, is provided to support the mast when it is used remote from the vehicle. Fig 6 shows the erected mast on its tripod. This function has proved especially useful in dealing with complaints of interference with u.h.f. reception: over 70 per cent of complaints referring to this service have been shown to be due to the use of inefficient aerials or otherwise faulty receiving installations.

The mast consists of six high-tensile light-alloy sections. It is not keyed and will remain erect under air pressure for about half an hour. To ensure rotation of an aerial at the head of the mast, and to keep the mast erect for long periods, clamping collars are provided. When required, these must be tightened by hand, section by section, as the mast is erected.

Television Receivers
Two types of television receiver have been provided: one is a general-purpose hand-held tracing receiver, and the other is a modified version for use in the 10cwt vehicle. The television receivers are commercial transistor-type dual-standard models, and capable of receiving all television channels in the v.h.f. and u.h.f. bands. A block schematic diagram of the receiver used in the 10 cwt vehicle is shown in Fig. 7.

For direction-finding work the receiver used in the 10 cwt vehicle has been specially modified so that its i.f. outputs can be connected to the panoramic receiver. In this way the operator can observe on the cathode-ray-tube display of the panoramic receiver all signals present in the i.f. pass band of the television receiver. A switch for disabling the automatic gain control (a.g.c.) and replacing it with a manual gain is provided. This is necessary because the effects of the a.g.c. would make direction finding difficult by tending to oppose the variations in received-signal amplitude on which direction finding depends. The input stages of the receivers have been screened to increase the immunity to direct break-in of signal or interference by paths other than the aerial.

The minimum amplitude of an interfering signal which is easily visible on the panoramic display tube in the presence of the vision carrier is about 1 uV, giving the direction finding equipment an overall sensitivity of approximately 6 dB (uV/m) at v.h.f. and 13 dB (uV/m) at u.h.f. This order of sensitivity enables the equipment to be used in investigating the more difficult interference cases where the interfering field strength affecting television reception may be as low as 6 dB (uV/m) at v.h.f. and 13 dB (uV/m) at u.h.f. However, if an interfering signal as low as this exists at a complainantís premises it is usually possible to move the vehicle and orientate the aerial so that the received interfering signal is effectively increased by approximately 10 dB, thus facilitating direction finding.

In addition to its role in the vehicle, the television receiver may be used remote from the vehicle as a hand-held tracing receiver, and is particularly valuable in tracing the windscreen wiper type of interference. The receiver is also used when the investigation officer wishes to demonstrate an efficient aerial and receiving installation to a complainant. When used remote from the vehicle the receiver is powered by its own inbuilt rechargeable batteries.

Fig. 7 - Block schematic diagram of the Television Receiver No. 4A

Fig. 8 -  Block schematic diagram of the panoramic receiver, Radio, Receiver, No. 33A

Panoramic Receiver
The panoramic receiver is a commercial panoramic display unit, tunable over the frequency range 5.2-60 MHz. A block schematic diagram of the receiver is shown in Fig. 8. Its frequency coverage enables it to be used with any receiver having an i.f. output in the range 5.2-60 MHz. The panoramic receiver is a double superheterodyne having i.f.s of 5.2 MHz and 720 kHz. A maximum sweep width of 1 MHz can be displayed on the cathode-ray tube; this sweep width may be narrowed to less than 30 kHz, allowing signals close to each other to be separated.

In its normal role in the vehicle the receiver is connected to either the 625-line or 405-line i.f. output socket of the television receiver. Since only 1 MHz of the television receiver i.f. can be displayed at any one time it is necessary for the operator when searching for the interfering signal to tune the panoramic receiver through the i.f. pass band of the television receiver.

It is possible with this receiver to distinguish a low-amplitude interfering signal when it is only 10 kHz away from the wanted carrier. For tracing interference with reception of a v.h.f. frequency-modulated signal the panoramic receiver may be used in conjunction with the v.h.f. tracing receiver, and it may also be used with communication receivers if the need arises.

Portable Tracing and Measuring Receivers
When the provision is completed it is intended to have transistor-type portable tracing and measuring receivers for each of the three frequency bands 0.15-1.6 MHz, 30-220 MHz and 470-875 MHz. The medium and long wavelengths are already catered for by Radio Receiver No. 27A, which is a portable transistor-type tracing receiver with measuring facilities; it covers the frequency range 0.15-1-6 MHz.

A new transistor-type receiver (Receiver, Radio, No. 31A) for the v.h.f. bands is being developed to replace the existing obsolescent valve receivers. For the u.h.f. bands a transistor-type tracing receiver with measuring facilities (Receiver, Radio, No. 34A) has been developed and is now in service.

The three receivers are shown in Fig. 5 and are, from left to right,

  1. the u.h.f. receiver (No. 34A), above the panoramic receiver,

  2. the v.h.f receiver (No. 31 A), to the right of the television receiver, and

  3. the medium-wave and long-wave receiver (No. 27A).

These receivers may also be carried and used with the 5 cwt vehicle when required.

Audio Amplifier
The vehicle is equipped with a small portable transistor-type audio amplifier that is primarily for use with the radio measuring and tracing receivers, which are not provided with their own loudspeakers. When extra audio amplification is required the amplifier is connected to the audio phone jacks of the radio equipment.

Coaxial Patching Panel
The coaxial patching panel is conveniently mounted on the right of the operating position. Its purpose is to terminate, at a central point in the vehicle, all the inputs and outputs of the radio equipment. This greatly simplifies the operation of the equipment, and the operator needs only short coaxial patching cords for interconnecting the equipment. A 40 dB general-purpose attenuator and the whip aerial are also brought out on coaxial sockets on the patching panel.


When tracing a source of television interference the operator moves his vehicle into a position where the interference is visible on the screen of the television receiver, and, by observing the nature of the degradation of the picture, he can decide on the method to be used for tracing the source. If the interference is continuous he can, by noting the number of interference lines on the picture and referring to the curves shown in Fig. 9, estimate the frequency separation between the wanted and unwanted signals. Then, by suitable adjustment of the sweep-width and tuning controls of the panoramic receiver, he can display the interference on the cathode-ray tube of the panoramic receiver. Once the interference is displayed in this way the aerial can be used for direction finding on an intensity basis, and the vehicle can be moved to take check bearings. Great care and a certain amount of skill are necessary to use this equipment in the u.h.f. bands because of the presence of large reflections which can be very misleading.

Note: If the interference pattern is diagonal, the frequency separation between the vision carrier and the interference is obtained by counting the number of diagonal black bars intersecting the top of the screen.

Fig. 9 - Relationship between the number of black bars produced by continuous-wave interference on a television screen and the frequency separation between the vision carrier and the interfering signal


Provision is being made of vehicles and equipment to meet the needs of the radio-interference services for the next decade. For economic and operational reasons two types of vehicles have been used, and most of the equipment is portable and capable of being used in a number of different roles. This provides for flexibility and gives scope for Telephone Areas and Telecommunications Regions to deploy the equipment to the best advantage in whatever circumstances obtain locally.




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