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BBC RD 1984/12 

[DOB # 


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 


BBC RD 1984/12 

UDC 621.396.933:621.396.97 


G.H. Millard, B.Sc, F.lnst.P. 


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, 

November, 1984 

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. 


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 


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G.H. Millard, B.Sc, F.lnst.P. 


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" 

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 


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, 


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 

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 

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 


— 2- 

course line 

31-5 km (17 NM) 


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 

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) 

(ii) change of flag current or appearance of 

(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 


— 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 



source f 









OdB coupler 

ILS receiver 

AGC current 

d) 6 6 

flag i 
currenl I 

Fig. 2 - Arrangement for measurement of co- and 
adjacent-channel interference. 


filter f 2 






— c terminate 

OdB coupler 


-< 20dB coupler 




filter f A f 2 


+ 26dB 








— <OdB coupler 


AGC current 

ILS receiver 

d> cb <b 



Fig. 3. - Arrangement for measurement of receiver 
inter modulation. 


source f^ 

filter / 2 


I — in 
J \J 

— <? terminate 

OdB coupler 

-i out 




20 dB coupler 


filler A, f 2 


+ 26dB 

non-linear R 


filter f A f z 




! n —< OdB coupler 

ILS receiver 

W ^— r— 

GC current Q (t) 



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 



Fig. 5 - The effect of modu- 
lation on interference levels for 
Receiver J. 





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 


in the localizer 

extent elsewhere. 


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 





deviation 5kHz 75kHz 

'» 1kHz tone • • 

music k x 

noise * * 




■40 -30 -20 

level of interfering signals.dBm 



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 

8.2. Co-Channel interference (radiated 
(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 

8.3. 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 





<U — 






















- ILS signal 8/iV(-89dBm 

■ receiver 1 

— — receiver 2 

receiver 4 

-receiver 5 


107 8 


107-9 108 

« frequency of interfering signal, MHz 

Fig. 6 - Interference close in frequency to the wanted 


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 


• 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. 


ralio dB 



I S\ 

spacing from 

S. ILS frequency, MHz 
i\ 1 1 1 







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 








■1055 1060 106-5 1070 

broadcast frequency, MHz 


108-0 f 
ILS frequency 

Fig. 10 - Maximum permissible level in ILS receiver of a single adjacent-band broadcast transmission. 

protection ralio.dB 

broadcast frequency, MHz 
I l 


ILS frequency 


• — ■ 

* — 


receiver 1 

receiver 2 

receiver 3 

receiver 5 

receiver 4 







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. 


— 8 — 

I50 r 


1 100 





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- 
generated in 
Receiver 1. 



-40 -30 -20 

level of interfering signals, dBm 


9. Implications for broadcast 

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 


0) Q) ^ 

o o> 

Q. — 

c ° A 


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 

9.1 . Co-channel interference (radiated 

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. 


40 r 





I" -20 





e.r p. 
0-1mW ^1mW 



10km 100km 

range for perceptible interference 


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 

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 


— 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 

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- 

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 


— 11 — 


Mildenhall\ Honing ton 

Fig. 15 - 1LS service areas in East 

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 



Appendix I 

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 


— 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 — 

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