GPO Vehicles

Make Morris
Model FGK80
Type 4 ton Radio Monitoring Van
Body Builder Reading & Co, 26 seater coach type body
Use Mobile Radio Monitoring Vehicle
Registration Number 907 CLB
Fleet Number 18356 (907 CLB) 1961
Date of picture 1962

Morris FGK80, 4 ton, radio test van with specialist coach body.


Extracted from
The Post Office Electrical Engineers Journal
Volume 55, Part 4 (1963)

Monitoring Station for Mobile-Radio Services
L. T. ARMAN and E. J. M. PECK
U.D.C. 629.114.7:621.396.72:621.396.931

A mobile monitoring station has been provided for observing the operation of V.H.F. mobile-radio services and to obtain a knowledge of conditions in the field.  This information is needed by the Post Office for frequency planning, guarding against contravention of the terms of the licences, and also in connexion with the technical development of equipment.  A description of the mobile monitoring station is given together with its objectives and general method of operation.


The Post Office licenses mobile-radio services in the United Kingdom, assigns frequencies and exercises control over the technical standards of performance of the equipment used; it has, therefore, considerable responsibility in this matter and in order to carry it out must have knowledge of conditions in the field.  V.H.F. mobile-radio services have a comparatively short range and, as it would be out of the question to set up a number of fixed monitoring stations, a mobile station has been provided for field monitoring.  The unit is equipped at the moment mainly to deal with services operating in the V.H.F. bands, but additional provision against the development of services in the U.H.F. band has been foreseen.

It will have considerable value as a deterrent against wrongful and irregular use of mobile-radio services and will provide valuable information for
frequency planning and general development.


The main functions and objectives of the monitoring station are as follows:-

(a) Observation of Mobile-Radio Services.  Call signs are allocated to each station by the Post Office and it is a condition of the licences that these
are used so that transmissions can readily be identified. Good operational discipline whereby the messages are kept brief and confined to the business of the user is necessary for efficient use of the channels. On the technical side the licences also impose standards of accuracy in the use of the assigned carrier frequencies. These points can only be checked by radio monitoring.

(b) Information on Channel Traffic Density.  This is required generally for statistical purposes and also to resolve problems that may arise in specific frequency assignments.

(c) Investigations of Interference Problems.  As far as possible, with relatively limited specialized equipment at their disposal, cases of interference are dealt with by Post Office Regional or Area staffs. Beyond this point the monitoring unit is brought into use.

(d) Tests of Systems.  Tests are made by the Post Office on current and on new mobile-radio systems to obtain general technical information. A particular instance was trials of a 25 kc/s channel system set up specifically for the purpose of testing the practicability of 25 kc/s band-width channels for mobile-radio systems.

(e) Radio Surveys.  These are for various services operated by the Post Office, such as public correspondence services including V.H.F. maritime services. The Post Office does not make surveys for private services.


The vehicle comprises a Reading 26-sealer coach-type body, modified for the purpose, on a B.M.C. 3-ton chassis. It is fitted internally with benches, the arrangement of which can be appreciated from the pictures below.  A water-cooled petrol-engine generator-set is fitted within the framework of the vehicle on the nearside just behind the cab, access to it being obtained externally.  This provides 230 volts a.c. for the equipment and fluorescent lighting.

To minimize running noise in the coach, acoustic insulation is provided and the set is fixed to the body via rubber mountings.  Because of the need to reduce vibration and noise to the minimum, and because the vehicle would often be used in residential areas for long periods, petrol engines, rather than diesel, are used for the vehicle and the generator-set.

Frequency-Measuring Set

The rack-mounted frequency-measuring set provided in the vehicle has an accuracy of ±2 parts in 10; it has a basic frequency standard of 100 kc/s and uses frequency synthesis.  Any frequency from zero to 600 Mc/s can be obtained at the output.  For frequency measurements the signal to be measured is normally compared with the output of the measuring set by the heterodyne method on an aural basis, but the two signals can be displayed on the oscilloscope provided for this purpose.  The accuracy of the measuring set is checked quarterly to within -± 1 part in 10 against the appropriate frequency derived from a 100 kc/s Essen-ring oscillator.  A receiver is also provided in the vehicle whereby the MSF (the call sign of the transmitter used for the standard-frequency service) transmissions from Rugby on 2-5 Mc/s or 5 Mc/s can be received and be used as a ready check of the measuring set at any time should the occasion arise, e.g. in case of a dispute.  Due to propagation conditions the accuracy that can be obtained may be limited to±2 parts in 10, but this would normally be adequate.

Radio Transmitters
A transmitter unit which can be tuned to any frequency : n the 80 Mc/s and 160 Mc/s bands is provided, the radio-frequency power output being about 7 watts.  It includes a common modulator unit, but the frequency-multiplying, driving and power-amplifier stages are separate for each of the two bands.  Oscillator stages are not incorporated ; instead, either the output of the frequency-measuring set, or a signal generator, is connected to the transmitter at the appropriate frequency.  Only amplitude modulation is catered for at the moment, but the provision of frequency-modulation transmitters is being considered.

Monitoring Receivers
Receivers of a typical communication type are provided, tunable over the 80 Mc/s and 160 Mc/s bands and suitable for amplitude-modulation and frequency-modulation.  However, the increasing use of 25 kc/s channel services necessitates a higher degree of selectivity and more precise adjustment than is generally available on normal communication receivers; accordingly consideration is being given to the use of conventional 25 kc/s receivers, as employed on mobile-radio services, together with the frequency-measuring set, or a signal generator, in a manner complementary to the radio transmitters described earlier.  Another asset of this type of receiver is its greater sensitivity compared with those at present provided.  A tape recorder is also available for connexion to the receivers as required.

Facilities are provided for mounting three aerials, one on a retractable telescopic mast, which provides a maximum aerial height of about 35 ft above ground, and the other two on the roof of the vehicle.  The telescopic mast is raised through the roof via the block-shaped mounting near the front of the vehicle.

Test Set
Equipment for testing the main characteristics of mobile-radio sets and systems is provided.  This comprises a variable-frequency generator and a test set.  The range of the former incorporates the frequency ranges used by mobile-radio services in the V.H.F. and U.H.F. bands; it can be used on amplitude-modulation and frequency-modulation equipment and has facilities for modulating the carrier wave with an audio tone of 1 kc/s.  The test set includes a deviation meter, an R.F. power meter, an audio power meter and also d.c. meters for current and voltage measurements.

Field-Strength Measuring Set
The field-strength measuring set is the Post Office Interference Measuring Set R12 used for general radio-interference work.  It covers the range approximately of 30-200 Mc/s and is normally used in conjunction with a portable telescopic dipole aerial the length of which can readily be adjusted according to the frequency at which measurements are to be made.

Auxiliary Power Supply
As stated earlier there is a 3-7 kW generator driven by a petrol engine to supply power, additional to that obtained from the vehicle and generator-set engines.  A 24-volt battery is necessary for the starter winding of the generator set, but only 12 volts is needed for the ignition system, which comprises the starter solenoid, the engine fuel pump and water temperature and oil pressure alarms.  The generator is set to supply 230 volts at 50 c/s; there is a manual regulator to control this voltage, and if need be the frequency can be adjusted or set to another value, e.g. 60 c/s, on the machine.  A standard electric running-hour meter is connected to the 230-volt output to record the total number of hours run by the petrol generator.  The 230-volt supply is taken via a change-over switch to a distribution fuse box and from there fed to the electronic equipment, fluorescent lighting and, exceptionally, a heater for the coach.  The load is maintained at a prescribed figure by the connexion of artificial loads according to the actual load at the time.

It should be noted that the potential output of 3-75kW from the generator set allows a considerable margin  for any additional demand that might arise in the future; a contributory factor to having the large reserve power was that there was no model available between 1-5 kW and 3-75 kW.  A public supply of 230 volts a.c. may be used instead of the generator.

The 24-volt battery is also used to heat the crystal oven in the frequency measuring set and provision is made for refresher charging of this battery when a mains supply is used.

The 12-volt battery for the vehicle itself is used to supply power when necessary for equipment, such as mobile-radio sets, which may be brought into service for special tests.


BACK Home page BT/GPO Telephones Search the Site Vehicles Home Page Quick Find All Telephone Systems

Last revised: February 06, 2021