||5cwt Radio Interference Vehicle
||Radio Interference Investigation
|Fleet Number Range
||201827 - 201931 (PMG 823F - 927F) 1967
|Date of picture
THE POST OFFICE ELECTRICAL ENGINEERSí
VOLUME 61, PART 1 - APRIL 1968
New Radio-Interference Equipment Vehicles
By A. S. McLACHLAN and F. W. GORMAN
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
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:-
Vehicles to carry equipment and staff.
Portable television receivers.
Portable tracing and measuring receivers.
Rotating masts for direction finding.
High masts to raise the demonstration aerials to a suitable
Benches and soldering-iron points to facilitate the carrying
out of on-the-spot suppression of small appliances.
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
every vehicle. It was decided, therefore, to have two classes of vehicle and
equipment to deal with the following two broad categories of complaint.
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.
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
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
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
5CWT 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
10CWT RADIO-INTERFERENCE 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
The vehicle (Fig. 4) is a standard Type 2 10 cwt Morris van that
has been modified in the following details:-
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.
Side windows have been provided to give the operator an
all-round view, and to provide light and ventilation.
Provision has been made for the mounting of a pneumatic mast
in the centre of the vehicle within easy reach of the operator.
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.
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.
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.
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
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
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
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.
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
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
Fig. 8 - Block schematic diagram of the panoramic receiver, Radio,
Receiver, No. 33A
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
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,
the u.h.f. receiver (No. 34A), above the panoramic receiver,
the v.h.f receiver (No. 31 A), to the right of the
television receiver, and
the medium-wave and long-wave receiver (No. 27A).
These receivers may also be carried and used with the 5 cwt
vehicle when required.
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
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.
LOCATING A SOURCE OF INTERFERENCE
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.