||4 ton Radio Monitoring Van
||Reading & Co, 26 seater coach type body
||Mobile Radio Monitoring Vehicle
||18356 (907 CLB) 1961
|Date of picture
Morris FGK80, 4 ton, radio test van with specialist coach body.
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
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.
FUNCTIONS OF THE MONITORING STATION
The main functions and objectives of the monitoring station are
(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
(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
(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 MONITORING VEHICLE
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
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
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.
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.
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.
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
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
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.