BBC RD 1984/12
[DOB #
RESEARCH DEPARTMENT REPORT
The susceptibility of aeronautical
navigational aids to interference from
adjacent-band broadcast transmissions
G.H. Millard, B.Sc, F. Inst. P.
Research Department, Engineering Division
THE BRITISH BROADCASTING CORPORATION November, 1984
BBC RD 1984/12
UDC 621.396.933:621.396.97
THE SUSCEPTIBILITY OF AERONAUTICAL NAVIGATIONAL
AIDS TO INTERFERENCE FROM ADJACENT-BAND BROADCAST
TRANSMISSIONS
G.H. Millard, B.Sc, F.lnst.P.
Summary
Measurements to determine the susceptibility of some airborne navigation
receivers to interference from v.h.fjf.m. sound transmissions are described. The
measurements were designed to investigate both the response to interference radiated
in-band and to intermodulation in the receivers. The results of the measurements were
submitted to an ITU Planning Conference and the decisions of the Conference are given
together with an indication of the effects on broadcast planning.
Research Department, Engineering Division,
BRITISH BROADCASTING CORPORATION
November, 1984
(RA-204)
Issued under the Authority of
Head of Research Department
This Report may not be reproduced in any
form without the written permission of the
British Broadcasting Corporation.
It uses SI units in accordance with B,S.
document PD 5686.
THE SUSCEPTIBILITY OF AERONAUTICAL NAVIGATIONAL AIDS TO
INTERFERENCE FROM ADJACENT-BAND BROADCAST TRANSMISSIONS
Section Title Page
Summary Title page
Foreword 1
1. Introduction 1
2. Interference mechanisms 1
2.1. Adjacent-band interference 1
2.2. Radiated intermodulation 1
2.3. Receiver intermodulation 1
3. Intermodulation products 1
4. Modulation of the broadcast transmissions 2
5. The ILS system 2
5.1 . I LS system technical detai Is 2
5.2. ILS airborne equipment details 3
6. Interference criteria 3
7. Experimental arrangements 4
8. Results of measurements 5
8.1 . Effect of modulation material 5
8.2. Co-channel interference (radiated intermodulation) 6
8.3. Adjacent-band interference 6
8.4. Receiver generated intermodulation 7
9. Implications for broadcast transmissions 9
9.1. Co-channel interference (radiated intermodulation) 9
9.2. Adjacent-band interference 9
9.3. Receiver intermodulation 10
10. Response of the aircraft antenna 10
11. The decisions of the ITU Conference {First Session) 10
11.1. Radiated intermodulation 11
11.2. Adjacent-band interference 11
11.3. Receiver-generated intermodulation 11
11.4. Receiver desensitisation 11
11.5. Filtering of broadcasting transmitters 11
12. Subsequent and future developments 11
13. References 12
Appendix 1 13
(RA-204)
© BBC 2006. All rights reserved. Except as provided below, no part of this document may be
reproduced in any material form (including photocopying or storing it in any medium by electronic
means) without the prior written permission of BBC Research & Development except in accordance
with the provisions of the (UK) Copyright, Designs and Patents Act 1988.
The BBC grants permission to individuals and organisations to make copies of the entire document
(including this copyright notice) for their own internal use. No copies of this document may be
published, distributed or made available to third parties whether by paper, electronic or other means
without the BBC's prior written permission. Where necessary, third parties should be directed to the
relevant page on BBC's website at http://www.bbc.co.uk/rd/pubs/ for a copy of this document.
THE SUSCEPTIBILITY OF AERONAUTICAL NAVIGATIONAL AIDS TO
INTERFERENCE FROM ADJACENT-BAND BROADCAST TRANSMISSIONS
G.H. Millard, B.Sc, F.lnst.P.
Foreword
1. Introduction
The 1979 World Administrative Radio Con-
ference extended (with effect from 1st January
1982) the frequency band available for v.h.f./f.m.
sound broadcasting in Region 1 from 100 MHz to
108 MHz. Now the frequency band from 108 MHz
to 136 MHz is used by aeronautical navigation
systems which generally operate with low-level
signals and some of which are "safety-of-life"
systems. In Region 2 (The Americas) broadcasting
and aeronautical services have operated in adjacent
bands for some years and there have been reports
of a significant number of cases of interference to
the aeronautical services. Accordingly the ITU
Regional Administrative Conference charged with
deciding technical standards to be used when
planning the extended broadcasting band was
requested to study means of avoiding such inter-
ference. In the United Kingdom studies were co-
ordinated by a Technical Working Group (TWG)
having representatives of interested parties with a
Home Office Chairman. Under the auspices of the
TWG, measurements of the characteristics of a
number of aeronautical equipments were carried
out jointly by the BBC and the Civil Aviation
Authority (National Air Traffic Services) - CAA
(NATS). The results of this work were contained in
a report appended to the UK submission to the
Conference; this report is reproduced as Sections
1-10 inclusive of the present report.
The First Session of the Regional Administra-
tive Conference for FM Sound Broadcasting in the
VHF Band was held in Geneva in August-
September 1982, being the first part of a two-part
planning conference. The decisions of this Confer-
ence departed in a number of respects from the
results given in the UK report and these are
outlined in Section 1 1 .
On the advice of CAA/NATS the measure-
ments carried out by BBC and CAA/NATS were
confined to Instrument Landing System (ILS)
receivers. The Conference, however, decided to
extend consideration to VHF Omnidirectional
Range (VOR) and to VHF communications.
Interference to the two latter systems is not
considered in this report.
The World Administrative Radio Conference
in 1979 gave Region 1 an allocation of the band
87.5 MHz to 108 MHz for f.m. sound broadcasting;
thus those transmissions would become adjacent in
frequency to aeronautical navigational services
operating in the band 108-1 18 MHz. Of the two
services operating in this band, i.e. VOR and ILS,
only ILS seems likely to be affected in any critical
way. This paper considers possible interference
mechanisms and describes measurements of protec-
tion ratios on five ILS localiser receivers.
2. Interference Mechanisms
2.1 . Adjacent band interference
The broadcast transmission if sufficiently
strong will break through into the ILS channel or
cause the ILS receiver to lose sensitivity. This
mechanism has been called "brute force"
interference.
2.2. Radiated intermodulation
When two or more broadcast transmissions
are radiated from a single antenna, or from
adjacent antennas, intermodulation products will
be generated, albeit at low level. An intermodu-
lation product (i.p.) of sufficient strength falling in
or near an ILS channel will cause interference.
2.3. Internal receiver intermodulation
When an ILS receiver is subject to strong
signals from two or more broadcast transmissions,
intermodulation products will be generated in the
receiver. If the frequency relationships are such
that a product falls on the wanted channel,
interference will result.
3. Intermodulation Products
When signals are applied to a non-linear
resistance, various harmonic and sum-and-
difference terms are generated. The non-linear
(RA-204)
resistance may be asymmetrical (rectifying) giving
rise to even harmonics and certain sum and
difference terms as well as a d.c. component, or it
may be symmetrical giving rise to odd harmonics
and certain sum and difference terms. The latter are
particularly important in that some of these fall on
frequencies adjacent to the original signals and
may be radiated efficiently by the antenna.
Consider signals acos27itf a , bcos2n:tf,
b>
ccos2?itf c applied to a symmetrical non-linear
resistance where fc> fb > fa. The following third-
order products will fall in the upper adjacent
frequency band:
c 2 acos2jrt(2f c — f a )
c 2 bcos2jrt(2f c -f b )
b 2 acos27it(2f b -f a )
a bccos2rct(f b + f c — f a )
(where f b - f a > f e - f b )
Fifth order terms may also be produced of the
types:
c 3 b 2 cos2ret(3f c -2f b )
c 3 bacos27it(3f c - f b - fj
c 2 ba 2 cos27it(2f c + f b -2fJ
In the above expressions the amplitude multi-
pliers indicate the dependence of the amplitude of
the intermodulation product on the amplitude of
the generating components. Thus for the inter-
modulation product
ab 2 cos2;rt(2f b -f a )
an increase of 1 dB in level of the signal of cos 2:rt f a
will result in a 1 dB increase in the level of the i.p.
whereas a 1 dB increase in the signal bcos 27ttf b will
result in a 2dB increase.
4. Modulation of the broadcast
transmissions
Band II broadcast transmissions in
Region 1 are frequency-modulated with a maxi-
mum deviation of 75 kHz. In a stereophonic
transmission the pilot-tone deviates the carrier by
between 8% and 10% of the maximum so that the
deviation will remain finite even in quiet passages.
Other subcarriers will be present to carry data or
for control purposes so that the carrier will not be
undeviated in a monophonic transmission either. It
may also be noted that the maximum deviation of a
third order intermodulation product will be three
times as great as that of the constituents. It is
therefore necessary to consider deviations of the
broadcast transmission lying in the range
5 kHz-225 kHz.
5. The ILS system
5.1 . ILS system technical details
A channel spacing of 50 kHz is employed in
the band 108-1 12 MHz and the use of the band
employs an interleaving pattern between ILS and a
further ICAO navigational system (VOR) viz.
108.10 MHz ILS
108.15 MHz ILS
108.20 MHz VOR
108.25 MHz VOR
11 1.80 MHz VOR
11 1.85 MHz VOR
11 1.90 MHz ILS
1 1 1 .95 MHz ILS
Guidance in the horizontal plane is provided
by two overlapping beams with separate amplitude
modulations (90 and 150 Hz) with the indication of
deviation from the centre line obtained in the
aircraft receiver by a measurement of the difference
in the depth of modulation (DDM) of the signals
from the two beams. The system specifications
include all necessary system tolerances to ensure
that a standardised indication (specified in /iA) is
provided on the cockpit indicators irrespective of
the type and manufacture of the indicator itself.
Additional and tighter system tolerances are app-
licable to the higher Category systems (CAT III)
relating to the uniformity of the course beam
structure to cater for those approaches in which
automatic flight control coupled from the ILS
signals is used. Large aircraft tend to universally
employ coupled approaches, as do all aircraft when
landing under reduced or zero visibility conditions.
Interference to the system can affect either the
audio identification or the actual guidance signals,
or both. Disruptive interference to the identi-
fication or any undue significant effect on the
guidance signals must be considered as potentially
highly dangerous.
The ILS service area, i.e. the area over which
the signal is provided to a given value and where it
is protected from other ILS signals, is arranged to
provide approach guidance in a sector extended
from the touchdown point outwards such that
aircraft can be guided by other means, e.g. radar
control, into the approach "funnel" from which
point ILS provides the guidance signal right down
to touchdown point. The dimensions of this service
area are shown in the diagram at Figure 1 where
the point of origin is the localizer aerial, which is
situated at that end of the runway furthest from the
<RA-204)
— 2-
course line
46-3km(25NM)
31-5 km (17 NM)
-185km
(10NM)
Fig. I - ILS Service Area.
approach path. Before an ILS is approved for use
there is also an associated operational assessment
of the approach path, its physical layout (i.e.
obstructions) from which is derived a mandatory
procedure for landing aircraft including obstacle
clearance limits to ensure a safe approach even
when aircraft are at the lower limits of the glide
path beam.
The minimum field strength for the localizer
signal is specified to be 40/jV/m for Category I
systems (approach guidance down to a height of
200 feet) and 100/iV/m at 10 n miles, increasing to
200/iV/m near touch down for Category III
systems (approach guidance down to and along the
runway). This minimum signal strength is required
over the horizontal areas specified in Fig. J,* and in
the vertical plane up to 7 degrees from the
horizontal and at the touchdown point to 2000
feet, or to 1000 feet above the elevation of the
highest point in the approach path whichever is the
highest. In regard to the protection of the ILS
signal, in the general case the figure of 40^V/m
* The backward service is not always specified.
would need to be assumed for the level of the
wanted signal although in specific instances the
higher figure of 100/vV/m might be appropriate.
5.2. ILS airborne equipment details
Different versions of ILS receiver were chosen
for test in this investigation: 3 receiver types which
might typically be found on large passenger
carrying aircraft and 2 equipments from the
General Aviation or Light Aircraft market. They
each have one of the following approval
classifications:
1 Fully approved
2 Limited approval (LA) Class 1
The fully approved equipments are for use on
any aircraft without restriction but are, generally,
used on larger passenger-carrying aircraft such as
found in airline service. LA Class 1 receivers are
restricted to aircraft weighing 5700 Kg or less i.e.
General Aviation/Light Aircraft.
For commercial reasons the receivers tested
will not be identified here under manufacturers
type number but will be identified in numerical
order from I to 5;
Receiver 1 is a fully approved ILS receiver
Receiver 2 is a fully approved ILS receiver
modified specially for flight cali-
bration purposes
Receiver 3 is approved under LA class 1
Receiver 4 is a fully approved equipment but
would be found more often on
general aviation aircraft such as
executive aircraft
Receiver 5 is approved under LA class 1
6. Interference criteria
The presence of interference may be observed
in different ways;
(i) change of localizer (course guidance)
current
(ii) change of flag current or appearance of
flag
(iii) change of a.g.c. current/voltage
(iv) impairment of audio channel
The localizer display is a centre-zero micro-
ammeter with a calibrated scale over the range
±150/iA (or 5 dots on the pilot's instrument).
When the difference in the depth of modulation
(DDM) of the input signal was adjusted to give a
(RA-204)
— 3^
deflection at 1 50 /^A, or slightly below, the effect of
an interfering signal was to reduce the deflection.
There is no generally agreed "permissible" reduc-
tion but a value of 5% (i.e. 7.5/* A) of the full
calibrated scale is used for the purposes of the tests.
With the lowest input levels, some of the receivers
tested showed a tendency for a random variation of
deflection so that small changes, in some instances,
are difficult to measure.
The "flag" is an electromechanical device
incorporated into the course guidance indicator
which indicates the quality of the course inform-
ation to the observer. There are two types of "flag"
operation, but both types sense the sum of the
"left" and "right" guidance currents in some way.
In the first type the flag is in effect a meter needle
following the flag current. In the second type the
flag is operated by a discrete signal when the sum
of the guidance outputs falls below a predeter-
mined level. The normal level of flag current (in the
first type) is 350 /(A, and the effect of interference is
to reduce this.
Measurements of a.g.c. current were taken
during the tests and showed reactions that differed
from receiver to receiver. However this proved to
be an insensitive method of assessing interference
and is not used here.
The localizer receiver is provided with an
audio frequency output which normally carries the
identification of the particular ILS ground install-
ation. This output was either monitored aurally or
measured during the course of the tests but without
a wanted signal being present. For some of the tests
the modulation of the interfering signal was evident
at unwanted signal levels lower than those needed
to produce flag or guidance current effects: for
Receiver 1 however, particularly where the interfer-
ing signal frequency had certain values of frequ-
ency offset, there was no audible effect.
7. Experimental arrangements
Reference was made in Section 5 to ILS
localizer field strengths of 40/iV/m, 100^V/m and
200/xV/m being specified for different ranges or
system categories; the corresponding values in
logarithmic measure are +32dB(/iV/m),
+ 40dB(/iV/m) and + 46dB(/iV/m). If the air-
craft antenna were isotropic and the feeder from it
to the receiver were loss free, the signal levels at
the receiver would be 11. 4 n\, 28.5 /iV and 57 /*V
( — 86, —78, — 72dBm) measured across 50 ohm
at a frequency of 108 MHz. The wanted signal
levels used in the tests were 8/iV, 20 /iV and 40 (iW
altenualor
0-60dB
variable
source f
atlenualor
0-90dB
variable
in
ILS
simulator
attenuator
60dB
fixed
w
in
terminate
OdB coupler
out
ILS receiver
AGC current
d) 6 6
flag i
currenl I
deviation
current
Fig. 2 - Arrangement for measurement of co- and
adjacent-channel interference.
source
filter f 2
reject
"L
_r
r~\
in
in
— c terminate
OdB coupler
out
-< 20dB coupler
out
■20
dB
filter f A f 2
reject
attenuator
0-90dB
variable
+ 26dB
amplifier
spectrum
analyser
in
ILS
simulator
attenuator
60dB
fixed
— <OdB coupler
P£t_
m
AGC current
ILS receiver
d> cb <b
t
flag
currenl
deviation
current
Fig. 3. - Arrangement for measurement of receiver
inter modulation.
(RA-204)
source f^
filter / 2
reject
in
I — in
J \J
— <? terminate
OdB coupler
-i out
in
hi
out
20 dB coupler
20
dB
filler A, f 2
reject
attenuator
0-90dB
variable
+ 26dB
amplifier
non-linear R
spectrum
analyser
filter f A f z
reject
ILS
simulator
attenuator
60dB
fixed
! n —< OdB coupler
out
ILS receiver
W ^— r—
GC current Q (t)
t
flag
current
deviation
current
Fig. 4 - Arrangement for measurement of co-channel
interference with wide deviation.
(-89, -81, -75dBm) so that in effect the
antenna gain for the wanted signal was assumed to
be 3dB less than isotropic.
Two basic experimental arrangements were
150r
<
Fig. 5 - The effect of modu-
lation on interference levels for
Receiver J.
o
o
100
50
50
used. That shown in Fig. 2 was used for the
measurements of co-channel, adjacent -channel and
adjacent-band interference. A signal from an
interfering source f, was combined with a simulated
ILS signal and applied to the receiver. The
modulation on the ILS signal was set to give a
localizer current near full scale and input levels at
the receiver were set at the levels given above.
The arrangements for the
receiver intermodulation shown
great care to ensure that the
product was being generated
receiver and not to any significant
The level of directly-generated
was less than — 95 dB relative
sources f L and f 2 , which were mai
measurement of
in Fig. 3 needed
intermodulation
in the localizer
extent elsewhere.
intermodulation
to each of the
ntained equal.
In order to assess the effects of deviation most
of the measurements were made with just two
values, 5 kHz and 75 kHz; the former gives most
interference when co-channel and the latter when
adjacent-channel. Measurements of the effect of an
intermodulation product radiated from a trans-
mitter required a deviation of 225 kHz (see section
4). As the available signal generators were not
able to produce deviations greater than 100 kHz
measurements using a deviation of 225 kHz were
made by generating an intermodulation product in
a non-Jinear resistance using the arrangement of
Fig. 4.
8. Results of measurement
8.1 . Effect of modulation material
An initial experiment was made with one
receiver (Receiver 1) to determine whether the
receiver was sensitive to the type of modulation on
two broadcast transmissions generating in the
receiver an i.p. of the same centre frequency as the
wanted signal. Fig. 5 shows results obtained with
Receiver 2 using tone, music and band-limited
wanted.
unwanted:
1081MHz,
-89dBm
1037MHz,
1059MHz
deviation 5kHz 75kHz
'» 1kHz tone • •
music k x
noise * *
_i_
J_
_L
■40 -30 -20
level of interfering signals.dBm
-10
(RA-204)
noise with maximum deviations of 5 kHz and
75 kHz. It was concluded that the use of 1 kHz tone
modulation would give representative results, and
this was used in all the tests. Where it was
necessary to generate an i.p. from two sources,
modulation of 1 kHz was used on one and 5 kHz
on the other to avoid a slow beat of the modulation
frequencies.
8.2. Co-Channel interference (radiated
intermodulation)
(Type A in CCIR Report 929)
This test simulates the effects of an inter-
modulation product radiated from a broadcast
transmitting station having two or more frequency
assignments in use. It is not necessarily applicable
to any other type of spurious emission that may
arise. The centre frequency of the intermodulation
product is assumed to be sufficiently near to that of
the ILS localizer for some of the energy to fall
within the ILS channel. The ILS frequency alloc-
ations are in pairs spaced by 50 kHz (108.1, 108.15,
108.3, 108.35 MHz etc.) and the broadcast frequ-
ency allocations are likely to be in multiples of
1 00 kHz so that the effects of interference are
required to be known in steps of 50 kHz.
Fig. 6 shows the effect on the receivers tested
using the 7.5 /iA harmful interference criteria
discussed in Section 6; the wanted signal level was
set at — 89dBm. It may be noted that the receivers
are most sensitive to the interference when the
deviation is lowest (5 kHz). This is to be expected
since the energy of the interfering signal is then
concentrated within the ILS channel. As the devi-
ation is increased the sensitivity to co-channel
interference falls but interference centred on adja-
cent channels becomes more significant. With the
maximum deviation of 225 kHz intermodulation
products having centre frequencies within
±250 kHz of the ILS centre frequency will give
discernible interference.
Measurements made with different levels of
the wanted ILS signal demonstrated the linear
nature of this interference mode (as far as the
receiver is concerned) so that it is justifiable to give
results in terms of the protection ratio, i.e. the
required level of the wanted signal divided by the
level of the unwanted signal. Thus when ratios are
expressed in decibels, a positive value of protection
ratio indicates that the wanted signal must be
stronger than the unwanted signal and a negative
value vice versa. It may also be noted that the
absolute frequency scale of Fig. 6 may be changed
to a scale of frequency difference relative to the ILS
channel (a few measurements were made with ILS
receivers tuned to channels higher than 108.1 MHz
to verify that this was valid). These changes have
been incorporated in Figs. 7 and 8 to give results
that are applicable to any channel in the ILS
frequency band.
For planning purposes it is necessary to take
into account the performance of the poorest
receiver and all the possible values of deviation of
the interfering broadcast signal. This is done in Fig.
9 which is an envelope of the curves in Figs. 7 and
8.
8.3. Adjacent-band interference
Fig. 10 shows the maximum permitted levels
of a single broadcast transmission into each
receiver when the level of the wanted signal is 8 ^V
(-89dBm) and its frequency is 108.1 MHz. It may
be noted that for frequencies below 108 MHz the
sensitivity of the receivers to interference falls off
by 5-20 dB per MHz. It is also evident that
Receiver 1 has the greatest sensitivity to inter-
ference of all the receivers, the margin being about
E
CD
T3
Or
<U —
3
<J
Q>
O
C
D
■o
g
v
o
XL
o
<t.
IO
h-
O)
=1
O
o
1)
k_
0>
k_
"o
c
-100
Hi
- ILS signal 8/iV(-89dBm
■ receiver 1
— — receiver 2
receiver 4
-receiver 5
J
107 8
1081
107-9 108
« frequency of interfering signal, MHz
Fig. 6 - Interference close in frequency to the wanted
signal.
(RA-204)
protection ratio, dB
20 r
-0-4-0-3 -0-2 V T
T-10 ->
spacing from
ILS frequency, MHz
I I I
0-2 0-3 0-4
protection ralio.dB
20 r
-80
• Rx 1, 5kHz x Rx 2,75kHz
■ Rx 1,75kHz * Rx 3, 5kHz
Fig. 7 - Responses of Receivers 1 , 2 and 3 to a
radiated inier-modulaiion product.
protection
20
ralio dB
1
I
10/
I S\
spacing from
S. ILS frequency, MHz
i\ 1 1 1
0-4
-0-3
^-01
-10
-20
-30
-40
-50
-60
01 N. 0-3 0-4
-04 03
1 1
spacing from
ILS frequency, MHz
1 02 0-3 0-4
Fig. 9 - Protection ratio required for a radiated
inlermodulation product.
-80 L
receiver 4 receiver 5
• • 5kHz dev.
x x 75kHz dev.
* * 225kHz dev.
Fig. 8 - Responses of Receivers 4 and 5 to a radiated
inlermodulation product.
15dB at frequencies above 107 MHz. Some mea-
surements were made with Receiver 4 and Receiver
5 tuned to 110.1 MHz; they show that the permi-
ssible level of interference may be expressed in
terms of a frequency difference from the ILS
frequency. Additional measurements with the
wanted signal at levels of 20 /iV ( — 81dBm) and
40 n\ ( — 75dBm) show that the interference mech-
anism is linear so that the results may be expressed
in terms of a protection ratio as in Fig. 1 1 .
8.4. Receiver generated intermodulation
Fig. 12 shows the effect of equal signals
intermodulating in the receiver to give a third-
order product at the frequency of the wanted signal
for three levels of the wanted signal. The result of
fifth-order intermodulation is also shown. It may
be noted that a particular increase in wanted signal
permits the interfering signals to rise by a lesser
amount, i.e. the interference mechanism is there-
fore non-linear and the concept of protection ratio
may not be used in this case.
Fig. 13 shows how the frequency separation
(RA-204)
lOr
£
m
>
0)
.o
0)
a
■1055 1060 106-5 1070
broadcast frequency, MHz
107-5
108-0 f
ILS frequency
Fig. 10 - Maximum permissible level in ILS receiver of a single adjacent-band broadcast transmission.
protection ralio.dB
n20
broadcast frequency, MHz
I l
-10
ILS frequency
l_t
105
• — ■
* —
106
receiver 1
receiver 2
receiver 3
receiver 5
receiver 4
107
108
-10
--20
-30
-40
-50
-60
-70
-80
-90
-1-100
Fig. 11 - Protection ratio for a single adjacent-band
broadcast transmission.
between the two interfering signals affects the
permissible level; in each case the frequencies of the
interfering signals were chosen so that the third-
order product coincided with the wanted ILS
frequency. Receiver I and Receiver 4 show con-
siderably more tolerance to widely-spaced trans-
missions whereas Receiver 5 shows little difference
with spacing. This may reflect the slope of the r.f.
rejection characteristics for each receiver although
other mechanisms may be responsible. The re-
ceivers are less sensitive to deviations of 75 kHz
than those of 5 kHz, as would be expected from
Figs. 7 and 8.
It is concluded that when the above conditions
apply, i.e. when the interfering carriers have the
worst frequency relationship with the ILS, and the
latter is at its lower field strength limit,
+ 32dB(jtV/m), the level reaching the receiver
should not exceed — 32dBm for broadcast signals
spaced 2.2 MHz or more. This level corresponds to
a field strength at the aircraft antenna of
+ 89 dB (/iV/m). If the ILS field strengths are at the
higher levels of + 40 dB (/iV/m) and +46dB
(/iV/m) the corresponding permissible interfering
field strengths are + 93 dB (/iV/m) and
+ 96dB(/iV/m).
When the frequency relationships are such
that the i.p. falls near, but not on, the ILS
frequency, the curve of Fig. 9 may be used as a
derating curve until the point at which the curves
of Fig. 11 become the dominant requirement.
(RA-204)
— 8 —
I50 r
<
1 100
o
o
en
a;
2 50
wanted signal level = -89dBm -81dBm -75dBm -89dBm
' Y '
third order fifth order
wanted signal frequency: 108'1MHz 108 1MHz
unwanted signal frequencies: 1037MHZ/I05-9MHZ -1015MHz, -103 7MHz
Fig. 12 - Inter-
modulation
generated in
Receiver 1.
_L
■50
-40 -30 -20
level of interfering signals, dBm
--10
9. Implications for broadcast
transmissions
Broadcast transmitting aerials commonly have
significant directivity in the vertical plane so that
an aircraft flying directly over a transmitter may
experience lower field strength than one several
kilometres away passing through the main beam.
To take this into account it would be necessary to
look at specific flight paths and transmitter loca-
tions in detail. For an initial assessment it will be
assumed that the broadcast transmitter is omni-
directional in horizontal and vertical planes, that
only the horizontally-polarised component of the
transmission gives rise to interference and that the
field strength at the aircraft may be calculated
assuming free-space conditions. Fig. 14 shows the
range at which interference will occur as a function
ex 5
415
0) Q) ^
£°3
o o>
Q. —
c ° A
^0
deviation kHz 5 75 5 5 5 75
>Rx1 ORx2 DRx3 «Rx4 ARx 5
-40 -30 -20 -10
permissible level of two equal interfering
signals producing an i p on wanted frequency , dBm
Fig. 13 - Intermodulation in ILS receivers from two
equal broadcast signals.
of protection ratio for a range of effective radiated
powers (e.r.p.) from the broadcast transmitter.
Each interference mechanism may be considered in
turn.
9.1 . Co-channel interference (radiated
intermodulation)
From Fig. 1 1 it may be seen. that a protection
ratio of 14dB is sufficient for the worst of the JLS
receivers measured. ITU regulations []] for spuri-
ous emissions of transmitters having a mean power
above 25 W require that the spurious components
in the transmission line to the broadcast antenna
shall not exceed a power of 60 dB below the
fundamental or 1 mW, whichever is the lower.
Thus a high-power transmitter just meeting this
requirement* would have an interference range of
75 km. In practice broadcast transmitters in the
United Kingdom meet the ITU requirements with
a margin in hand and an e.r.p. of 1 mW would be
typical for the i.p. from 125kW transmissions
spaced 2.2MHz (i.e. a relative level of — 81 dB). A
typical interference range is then 30 km. This is still
quite Jarge and the broadcaster should aim for a
relative i.p. level of — 95 dB or less when specifying
transmitter combining units. However, there is no
certainty that such levels will be obtained with a
common aerial installation.
9.2. Adjacent band interference
The highest frequency that is likely to be used
for broadcast transmissions, assuming a channel
* A typical antenna gain, and free-space propagation are assumed
in this calculation.
(RA-204)
40 r
20
0-
m
■o
I" -20
o
--40
-60
-80
-100
e.r p.
0-1mW ^1mW
10mW
_L
J
10km 100km
range for perceptible interference
1000km
Fig. 14 - The relation between protection ratio and interference range.
spacing of 100 kHz is adopted, is 107.9 MHz. All
the ILS receivers listed showed a slow increase in
discrimination between this and lower frequency
channels. The protection ratio of the poorest
receiver (Receiver 1) at 107.9 MHz is — 55 dB for
which the corresponding interference range for a
125 kW station is 110km. However, the other
receivers would require ranges of 20 km or less. For
a frequency of 107 MHz, the interference ranges
become 50 km for Receiver 1 and less than 7 km for
the other receivers. No measurements have been
made with two or more carriers present but it is
suggested for this mechanism that a power summ-
ation be made at the input to the ILS receiver with
adjustment for different protection ratios if
appropriate.
9.3. Receiver intermodulation
It is shown in Section 8.4 that for the ILS
signal at its lowest specified limit and interfering
broadcast transmissions on frequencies 2.2 MHz
and 4.4 MHz below the ILS frequency (i.e. such
that they can intermodulate to produce a term
centred on the ILS frequency), the maximum field
strength of each at the aircraft antenna is
+ 89 dB(//V/m) for Receiver 1 and Receiver 5. If
the broadcast transmissions have an e.r.p. of
125 kW it follows that the interference range is
90 km. For Receivers 2, 3 and 4 the interference
ranges are respectively 44 km, 35 km and 27 km.
1 0. Response of the aircraft antenna
The measurements described above take no
account of the frequency response of the aircraft
antenna, which could reduce the levels of the
broadcast signals reaching the receiver. It would
not help in respect of radiated intermodulation.
CCIR Report 929 [2] suggests that the
response for a navigation antenna to broadcast
signals should be 3 dB plus 1 dB/MHz below
108 MHz, in which the first term was intended to
represent the losses between antenna and receiver.
Such a response would be of benefit in reducing
receiver-generated intermodulation especially for
the lower broadcast frequencies.
(This concludes the Report attached to the
UK submission to the ITU Conference.)
11. The decisions of the ITU
Conference (First Session)
The decisions of the Regional Administrative
Conference for FM Sound Broadcasting in the
VHF Band (Region 1 and certain countries con-
cerned in Region 3), First Session, Geneva 1982 are
contained in the Report to the Second Session of
the Conference [3], which is to be held in 1984.
Those relating to compatibility between the broad-
casting and aeronautical services are contained in
(RA-204)
— 10—
Chapter 5 of the report and extend to fifteen pages.
The main differences between the Conference
decision and the UK submission will be highlighted
here.
11.1. Radiated intermodulation
This was designated type Al interference. The
protection ratios given in the UK submission were
accepted with the addition of a 3dB margin for
multiple interference entries resulting from FM
broadcast emissions. Accordingly the curve of Fig.
9 was raised by 3 dB.
11.2. Adjacent-band interference
Where interference resulted from power in the
aeronautical band radiated from a broadcast
transmitter at a closely spaced frequency (i.e. from
the far-out sidebands), it was designated type A2
interference. The Conference considered that there
were insufficient data available to define the typical
levels of power of FM broadcast transmissions
between 200 kHz and 500 kHz from the carrier and
called for further studies.
11.3. Receiver -generated intermodulation
An effect resulting from intermodulation pro-
ducts generated within the aircraft receiving install-
ation was designated type Bl interference. Differ-
ent criteria were defined for the cases of two and
three unwanted signals. Where the intermodulation
involves two unwanted signals, taking the form 2f L
— f 2 = f a it was decided that unacceptable degrad-
ation of receiver performance may occur if
1.71 N, +N 2 + 60^0
where N x and N 2 are the levels, in dBm, of the two
broadcasting signals at the frequencies f : and f 2
respectively at the receiver input and f a is the
receiving frequency.
For equal interfering signals:
N, = N 2 = -22dBm
This is lOdB less stringent than was found
necessary for the worst receiver in the UK tests.
Theoretically, the coefficient of N t would be
expected to be 2; the coefficient 1.71 arose from
fitting a straight line to some experimental results
submitted by France.
Where the intermodulation involves three
unwanted signals, taking the form f 1 + f 2 — f 3
= f a , it was decided that unacceptable degradation
of receiver performance may occur if
Nj + N 2 + N 3 + 73 >0
For equal interfering signals
N x =N 2 = N 3 = -24.3 dBm
This was a theoretical extension of the two-
frequency case, allowing for the 6dB higher level of
a three-frequency term for a given degree of non-
linearity.
11.4. Receiver desensitisation
Adjacent-band interference caused by the
inability of the aircraft receiver to withstand a
broadcast transmission at a closely-spaced fre-
quency was designated Type B-2 interference. The
Conference decided that an unacceptable
degradation of 1LS localiser receiver performance
may be caused, due to desensitisation, if the level of
a broadcasting signal exceeds —20 dBm at the
receiver input on a frequency near the band edge
(108 MHz). For broadcasting signal frequencies
from 108 MHz to 106 MHz, it was decided that the
threshold level increases linearly from —20 dBm to
-5 dBm.
11 .5. Filtering of broadcasting transmitters
The Conference decided that by fitting impro-
ved combining filters, and paying careful engineer-
ing attention to possible sources of non-linearity
following the output stages of the transmitters, it is
technically feasible to reduce the radiated power of
the third-order intermodulation products to
— 85 dB relative to the effective radiated power.
Fig. 1 5 shows 1LS service areas in East Anglia in
relation to the high-power v.h.f./f.m. transmitting
station at Tacolneston. The broken line shows the
range up to which interference to ILS could occur
on the basis of the Conference criteria. Accordingly
the broadcast frequencies need to be co-ordinated
with the frequencies of all the ILS installations
whose service areas are intersected by the circle, i.e.
seven in the case of Tacolneston. This is a severe
restriction on frequency planning.
12. Subsequent and future developments
The Conference decided that the criteria
adopted would appear to constrain the full exploit-
ation of the broadcast band from 100 to 108 MHz
and recommended that the CCIR initiate studies
both of the way that the immunity of airborne
equipment could be improved and of the maximum
(RA-204)
— 11 —
Lakenheath
Mildenhall\ Honing ton
Fig. 15 - 1LS service areas in East
Anglia.
suppression of spurious emissions that could be
achieved at broadcast transmitting stations.
In response, the CCIR formed two Interim
Working Parties to carry out the studies, namely
IWP 8/12 and IWP 10/8. IWP 8/12 was charged to
consider by how much the value of immunity to
FM sound broadcasting interference of the air-
borne equipment can be improved and issued a
report after one meeting. This report, however,
proposed criteria for existing equipment that were
more stringent in some respects than those adopted
by the Conference. A significantly better perfor-
mance was promised for new equipment but the
implementation time was so long that it would
have little relevance to the 1984 Planning Confer-
ence. IWP 10/8 was charged to determine the
maximum suppression of spurious emissions from
broadcast transmitting stations, lying in the
108-137 MHz range, which can be maintained
continuously. It met twice and produced a report
supporting the Conference assumptions on achiev-
able levels of spurious emissions, but held out some
hope of improvements in specific cases. Material
submitted by the UK to IWP 10/8 has been
published in Reference 4.
As the above studies have not improved the
chances of producing a broadcast frequency plan
compatible with aeronautical usage in the adjacent-
band, a Joint Interim Working Party 8 10/1 has
been set up by the CCIR to give further study to
the problem before the Second Session of the
Conference. The terms of reference are given in
Appendix I.
13. References
1. International Telecommunication Union,
1982, Radio Regulations, Appendix 8.
Table of Maximum Permitted Spurious
Emission Power Levels, Geneva 1982.
2. CCIR, 1982, Compatibility between the
broadcasting service in the band of about
87-108 MHz and the aeronautical services in
the band 108-1 36 MHz. Report 929, Vol.
XIII, Geneva 1982.
3. International Telecommunication Union,
1982, Regional Administrative Conference
for FM Sound Broadcasting in the VHF
Band (Region 1 and certain countries
concerned in Region 3). First Session
Geneva 1982: Report to the Second Session
of the Conference.
4. MILLARD, G.H. Intermodulation between
v.h.f./f.m. broadcast transmitters and the
protection of adjacent-band aeronautical
services. BBC Research Dept. Report
1984/2.
(RA-204)
12-
Appendix I
JOINT INTERIM WORKING PARTY 8-10/1
Terms of Reference
Consider the reports of Interim Working Parties 1 0/8 and 8/12 as well as new contributions.
Determine whether further improvements in compatibility can be made, particularly as regards:
— the possibility of improving the immunity of receivers in the aeronautical radionavigation service to
interference caused by FM broadcasting emissions (Recommendation CC, formerly terms of reference of IWP
8/12)
— the maximum obtainable suppression of spurious emissions in the band 108-1 37 MHz from broadcasting
stations operating in the band 87.5-108 MHz (Recommendation DD, formerly terms of reference of IWP 10/8)
Prepare a consolidated draft report intended for submission by CCIR to the Second Session of the Regional
Administrative Conference for FM Sound Broadcasting in the VHF Band (Region 1 and certain countries concerned
in Region 3). The Joint Working Party shall report to the Interim Meeting of Study Group 8 (May-June 1984).
(RA-204) — 13 —
Printed by BBC RESEARCH DEPARTMENT, Kingswood Warren, Tadworth, Surrey, KT20 6NP