RCG-30

Title: Comments of some issues with using ITU-R 1177 to certify radar
unwanted emissions

Written by: John Holloway[^1]

Date: Tuesday, 10 May 2005

# Introduction

This paper addresses several issues that it is contended by the author
remain unclear in the ITU document M 1177 and its application to other
ITU requirements. when used as the basis of certifying radar UE
performance.

It addresses the following topics:

-   Interpretation of Results/Confusion in Specified Limits

-   Limit Masks Issues

-   Practical Measurement Issues

# Interpretation of Results/Confusion in Specified Limits

In order to devise a method to make useful measurements and particularly
to judge the results against a pass or fail criterion of a meaningful
specification, it is necessary that published limits have units that are
clear and consistent. Currently the ITU-R documents that provide the
limits that will form the basis of testing using ITU-R Recommendation
M.1177 are inconsistent.

The confusion arises from the fact that for some services the limits are
specified at the transmitter output but for radar they are specified as
off-air measurements in line with ITU-R Recommendation M.1177.

In particular there is confusion in the specifications between Power,
Spectral Power Density and EIRP.

The peak envelope power is defined in the Radio Regulations as follows:

**\"1.157** peak envelope power (of a radio transmitter): The average
power supplied to the antenna transmission line by a transmitter during
one radio frequency cycle at the crest of the modulation envelope taken
under normal operating conditions.\"

In radar terms it is equivalent to the peak power of the radar waveform
in the time domain. PEP is expressed in units of power, dBW, dBm, W, kW
etc and is referenced in ITU-R Recommendation SM.329-9 \"Spurious
emissions\". In order to accurately measure the PEP it is necessary to
use a bandwidth wide enough to capture all the energy in the pulse. In
practice this is the impulse bandwidth of the pulse and in practice
B~imp~ ≈1/τ.

dB~pp~W is the peak power measured across the spectral envelope measured
in a specified bandwidth. This is defined in ITU-R Recommendation
SM.1541 \"Unwanted emissions in the out-of-band domain\".

EIRP is the total radiated power (can be expressed in peak or mean)
multiplied by the relative gain of the transmit antenna to an isotropic
radiator (normally the peak unless specified otherwise[^2]). EIRP is
expressed in units of power and represents the power that would be
required to generate the equivalent power density if the antenna was an
isotropic radiator i.e. of 0 dB gain. EIRP takes the units of power
sometimes written as dBwi, dBmi to indicate they are referenced to an
isotropic source. EIRP includes a term for the transmit antenna gain.

Power units are not appropriate in off-air measurements as they contain
terms related to the receive antenna gain and the range. See Annex 1 of
this paper report for a discussion on the measurement of EIRP.

## OOB Limits

The way the OOB limits are specified in M.1177 requires the use of peak
dB~pp~. The specification limits are given in ITU-R Recommendation
SM.1541 \"Unwanted emissions in the out-of-band domain\" and do not
explicitly specify the resolution bandwidth to be used. Because the
document specifies the shape of the spectrum it must be implied from
this that the resolution bandwidth should be narrow enough to accurately
measure the shape of the spectrum i.e. between approximately 0.03/τ to
0.1/τ (5% of main-lobe width). Once this is achieved, because the limits
are specified as relative dB~pp~ the relative result will not change if
the resolution bandwidth is reduced (because the spectrum shape is not
changing). However it is necessary to ensure that the measurement IF
bandwidth does not go below the 1.7 x PRR[^3] restriction or more likely
the pulse desensitisation does not limit the dynamic range of the
measurement. Because the specification envelope is expressed in relative
units, the resolution bandwidth does not become an issue. Strictly
speaking, as the limits are being measured off-air they should be
expressed as dB~pp~ EIRP, however for relative measurements, the other
factors such as antenna gain cancel out. Because the OOB region is
essentially close to the carrier any changes in receiver antenna gain
can be neglected.

## SE Limits

The SE limits are specified in ITU-R Recommendation SM.329-9 \"Spurious
emissions\". For radar systems two sets of limits apply; class A and
class B. Unlike OOB limits, SE limits do not contain a spectral mask
that has to be applied; they are specified as a simple level. ITU-R
Recommendation SM.329-9 initially specifies a 1 MHz reference bandwidth
but then confuses the situation by referring to ITU-R Recommendation
M.1177 in the notes to tables 2 and 3[^4].

SM 329-9 defines that SE limits are relative to PEP. In order to get an
accurate measurement of PEP, measurements must be made in, or corrected
for the 1/τ bandwidth if the resolution bandwidth is not wide enough.

The SE limits are however required to be measured in the reference
bandwidth as defined in SM 329-9 and referred back to PEP which may or
not be defined and/or measured in the same bandwidth.

The Class A limits are specified as relative measurements of PEP (should
be PEP EIRP) for a similar argument to that for OOB limits provided the
same resolution bandwidth is used, the measurement of SE is consistent
within itself.

The following is an extract from SM.329-9:

\"Radiodetermination (7) 43 - 10 log PEP, or 60 dB, whichever is less
stringent

The following notes apply:

\(1\) In some cases of digital modulation and narrow-band high power
transmitters for all categories of services, there may be difficulties
in meeting limits close to ±250% of the necessary bandwidth.

\(2\) Use the e.i.r.p. method shown in Annex 2, § 3.3, when it is not
practical to access the transition between the transmitter and the
antenna transmission line.

\(7\) For radiodetermination systems (radar as defined by RR No. 1.100),
spurious domain emission attenuation (dB) shall be determined for
radiated emission levels, and not at the antenna transmission line. The
measurement method for determining the radiated spurious domain emission
levels from radar systems should be guided by Recommendation ITU-R
M.1177.\"

The 60 dB limit is clear. The SE signal measured in the reference
bandwidth must be at a level 60 dB below the PEP signal measured in 1/τ
bandwidth if different from B~ref~.

There is however some confusion in the *43 - 10 log PEP* limit. For
other services the PEP is as defined in the Radio Regulations as
follows:

\"1.157 peak envelope power (of a radio transmitter): The average power
supplied to the antenna transmission line by a transmitter during one
radio frequency cycle at the crest of the modulation envelope taken
under normal operating conditions.\"

That is, it is the power supplied to the antenna transmission line. For
radio determination, when the measurements are made off air, (referred
to in note (2) above, as the EIRP method), the question is then, is the
PEP referred to in \"43 - 10 log PEP\" the transmitter power, or the
EIRP? It is not clear which should be used.

Table 10 of ITU Recommendation SM.329-9 \"Absolute levels of spurious
domain emissions -- Category A\" leads to a further confusion. The
levels are specified in PEP Watts and dBm, (PEP for the transmitted
level, dBm for the SE), but the units for both should be in EIRP watts.
If not, the level specified for PEP depends on the Tx and Rx antenna
gains and the measurement range.

\"Radiodetermination(7) --13 dBm if PEP ≤ 50 W

10 log PEP -- 30 if PEP \> 50 W

For radiodetermination systems (radar as defined by RR No. 1.100),
spurious domain emission attenuation (dB) shall be determined for
radiated emission levels, and not at the antenna transmission line. The
measurement method for determining the radiated spurious domain emission
levels from radar systems should be guided by Recommendation ITU‑R
M.1177.

A gain refers to power not EIRP, \"Maximum permitted spurious domain
emission power in the relevant reference bandwidth (see further
recommends 4.1) (dBm) with P, PEP or X (W)\"

There is now however an inconsistency at the boundary between OOB and SE
and also for any OOB emissions that fall in the SE domain. In order to
make the two limits consistent, the same resolution bandwidth would have
to be used. This would imply using a measurement bandwidth (equal to the
SE reference bandwidth) that may well be too wide to demonstrate the
performance in the OOB domain, an alternative is to measure the SE
region using a bandwidth narrower than the reference bandwidth and
correcting the data to allow for this.

For Class B limits are defined as :

\"Radar systems in the radiodetermination service:

Fixed radiodetermination stations (5), (6), (7), (8) (wind profiler,
multi-frequency and active array radars are excluded) --30 dBm or 100 dB
attenuation below the PEP, whichever is less stringent

\(5\) For radiodetermination systems (radar as defined by RR No. 1.100),
spurious domain emission attenuation (dB) shall be determined for
radiated emission levels, and not at the antenna transmission line. The
measurement method for determining the radiated spurious domain emission
levels from radar systems should be guided by Recommendation ITU-R
M.1177.

\(6\) European and some other countries have determined that insofar as
they are concerned, Category B spurious limits for radar systems should
apply to transmitters used in those countries and installed after
1 January 2006.

On a site-by-site basis, administrations may permit the use of maritime
mobile radar equipment in fixed installations (e.g. vessel traffic
services radar), using the appropriate limits for mobile radars.

Further study is to be undertaken by the relevant regional body, any
interference will be handled on a case-by-case basis.\"

There are further complications in the class B limits. These are
specified as -30 dBm or 100 dB below PEP, whichever is less stringent.
As for the above Class A, there is not an issue for the relative
measurement, however -30dBm is specified as PEP. This has the units of
power, it needs to be specified explicitly as an EIRP to be meaningful,
and a resolution bandwidth must be assigned. There will also be a
problem of the SE/OOB boundary, however the shape of the OOB mask and
the class B SE OOB boundary have yet to be defined within the ITU.

There are however potential drawbacks in specifying the absolute level
in PEP EIRP. With the same value as currently given due allowance needs
to be taken for the antenna, otherwise it will apply to transmitters of
much lower powers than those stated in Table 10 of ITU Recommendation
SM.329-9.

# Limit Masks Issues

Given that the OOB mask is defined in dB~pp~ and the SE limits in PEP,
if an unwanted emission mask is to be applied, then it will have a
discontinuity at the OOB/SE boundary. A way to resolve the inconsistency
is to make the resolution bandwidths equal. This means that the
bandwidth has to be wide enough to measure the peak power, which will
result in a distortion of the spectral shape.

A further problem in applying the mask occurs when it is not possible to
use a receiver with a resolution bandwidth wide enough to measure the
peak power without desensitisation. This occurs for pulses less than
200/300 ns and the most common receivers have maximum bandwidths between
3 & 5 MHz. The question then arises as to how to set the reference level
for the emission mask? If the mask is set to the peak of the main
spectral lobe then it implies that all the SE signals have the same
desensitisation factor.

In the measurement campaign the OOB masks were referenced to the peak of
the measured pattern. The SE region however was not corrected for PEP or
resolution bandwidth to reference bandwidth.

Units are rationalised when the resolution bandwidth equals the
reference bandwidth.

The following is the proposed method for applying the limit masks in the
future.

-   The maximum level of the fundamental is corrected for the PEP level
    using 20log(B~res~/B~imp~)

-   The level of signal in the SE domain is corrected using between 0
    and 20log(Bw~m~/B~ref~) depending on the signal type.

-   In the OOB domain, no correction is applied and the OOB mask is
    positioned on the peak of the fundamental.

-   The OOB/SE boundary is taken as 10 x B~-40dB.~

#  Practical Measurement Issues

## Interpretation of ITU-R Recommendation M.1177

When making SE and OOB measurements, it is necessary to understand
several practical issues if the results are to be meaningful and
consistent. The ITU recommendations are only specified limits in terms
of measured relative power or EIRP levels. They do not specify how these
limits should be applied in practice. Take for example SE limits. They
require the SE\'s to be a certain number of dB relative to the
fundamental.

ITU-R Recommendation M.1177 does not specify whether measurements are to
be made on the elevation bore-sight of the radar antenna or whether
measurements are required at different heights or elevation angles off
of the bore sight to determine off-axis unwanted emissions, nor is the
measurement distance defined. It does not say if the limits apply over
all space, (i.e. relative to fundamental peak gain wherever it is), or
at all points in space, (i.e. relative to the local fundamental
frequency power levels) or any other interpretation such as a mean
level.

The nature of the harmonic performance of antennas is such that harmonic
beams are generated at different angles in both the azimuth and
elevation planes relative to the position of the fundamental frequency
beam. For electronically scanning systems, the position of the harmonic
beams vary with both scan angle and frequencies.

Consider the antenna performance at SE region frequencies. Generally SE
measurements have been made in the azimuth plane with the antenna
rotating. This removes any beam modulation in azimuth that occurs in
antennas with travelling wave feeds or frequency sensitive phase
shifting components. However in the SE region, even antennas that do not
have travelling wave feeds can squint in both azimuth and elevation and
produce multiple \"grating lobe\" beams.

Consider the simplest of antennas, the horizontal half wave dipole
spaced λ/4 above a ground plane. At the design frequency the current
distribution is a half sinusoid with a peak normal to the arms. This
produces a beam normal to the arms of the dipole. The reflection from
the ground plane is such that this reinforces the broadside pattern. At
the frequency of the 2^nd^ harmonic the current distribution is still
such as to form a beam on broadside but the reflection from the group
plane is in anti-phase. This results in a beam with a null normal to the
arms of the dipole and peaks either side of normal. The radiation
pattern is said to have bifurcated. At the third harmonic, the peak
returns to the normal but extra nulls form in the plane of the dipole
arms.

The same effect can occur in reflector antennas if the higher order
asymmetric modes are formed in the feed horn. These can be excited by
discontinuities of asymmetric feed excitation. In waveguide fed horns,
these modes can be generated by the feed run and the exact mix depends
on the design and spacing of the feed components.

It is thus possible that in the SE region, particularly at harmonic
frequencies, the peak of the antenna gain can deviate from the azimuth
plane. Thus measuring in the azimuth plane may not measure the peak of
the SE beam.

The problem is further exacerbated in array antennas where changes in
the phase shift in the beam-forming network combine with element pattern
changes to scan the beam at SE region frequencies. If the antenna
contains phase-shifting components to steer the beam, then the position
of the beam formed at these frequencies is a function of the position of
the beam at the fundamental frequency.

As well as the squinting of the main beam, if the antenna is an array,
then at SE frequencies \"grating\" beams can also occur. These are
further representations of the main beam caused because the element
spacing exceeds λ/2 at frequencies in the SE region.

Thus in order to be sure that the maximum SE is measured, it is
necessary to \"look\" over the full hemisphere enclosing the radar.
Using the direct method this approach would be impractical both in
physical terms (i.e. the positioning of the measuring antenna), and in
time. This is made less practical by the need to do the measurement at
all beam positions.

It is thus necessary to decide a pragmatic approach that has to be
taken.

## The Pragmatic Approach

Initially measure the system in the azimuth plane. Harmonic beams are
rarely unique or well formed, so it is highly likely that any SE will be
detected in the azimuth plane. If any emissions are detected at
frequencies that are likely to cause interference with receivers
situated in the elevation plane (i.e. airborne), then further
investigation is required.

For rotating antennas the interpretation would seem to be that the
comparison should be made of the maximum levels seen in the measurement
plane independent of azimuth position. This would seem a good
engineering solution as it makes the measurement in the plane close to
the plane that the maximum interference potential exists. However, this
does not address any SE in the elevation plane.

This approach needs to be generalised for antennas that do not rotate
but electronically scan in azimuth. Because the harmonic radiation is
also a function of the scan angle, measurements may need to be made at
several fixed azimuth positions, one on-boresight and one each at either
end of the scan.

Making direct measurements in the far-field is virtually impossible
above the principle azimuth planes, since generally radars are
positioned such that this area of the radiation pattern is not
accessible. For radars in an elevated position, it is generally possible
to gain access below the principle plane if this is assessed as the
worst possible plane for potential interference.

The best approach to addressing SE in the elevation plane would seem to
be based on assessing the likely interference potential from
measurements in the azimuth plane. Interference caused in the elevation
direction is likely only to effect airborne and space systems. If the
measurements in the azimuth plane shows no radiation in these bands,
then it may be safe to assume that there will be no significant
radiation into the elevation direction. If however significant SE are
found in the candidate bands, then further investigation may be
required. The extent of this further investigation requires further
study. The investigation could take the form of scanning the antenna to
high elevation angles and measuring in the azimuth plane to ascertain if
spurious emissions are being produced. This investigation however would
need to be tailored to the specific radar under test with detailed
knowledge of the design and the likely performance in the spurious
emission region.

As well as considering where relative to the radar head the measurements
should be made, it needs to be considered in what operational modes the
radar should be in when measured.

If possible the radar needs to be measured with a fixed pulse pattern.
If OOB emissions are being measured then this can be considered against
the appropriate OOB mask. If the radar cannot be operated in a single
mode then it needs to be operated in its \"normal\" mode with comparison
being made against the widest appropriate transmission mask based on the
modulation being used. When interpreting the results however, the level
of pulse desensitisation must be considered.

For SE measurements, it is most appropriate to operate the radar in its
normal mode of operation.

Once the measurements have been made and the levels have been
determined, it is necessary to decide whether the measurements pass or
fail the specified criteria.

The subject of measurement tolerances needs to be addressed. Generally
two options are available; make the decision based on indicated values
or require the limits to be exceeded by a margin equal to the estimated
measurement errors (so called indented limits). When using indicated
values the method of measurement needs to be defined so as to limit the
measurement errors involved. Given the difficulty in making these
measurements, the likely level of error and the possible insensitivity
to errors, it would seem that the appropriate approach to be adopted
would be to use indicated values based on a well described method of
direct measurements.

## Interpreting ITU Limits Pass & Fail Criterion

As has been previously discussed, the measurement bandwidth required in
the SE domain does not only depend on the fundamental signal alone but
on how the specific SE emissions are related in the time domain to the
fundamental signal.

It could be argued that there is no one appropriate bandwidth for
measuring the whole SE region. ITU-R Recommendation M.1177 in Appendix 1
to Annex 1 step 7 discusses the need to use differing resolution
bandwidths to identify any of these issues. In the case of quantifying
the level of the Π~-1~ mode this is particularly important. As there is
no appropriate resolution bandwidth, so it is not appropriate to apply a
continuous mask as described in the ITU recommendations over the full SE
region. Parts of the resolution mask may have to be offset by the
appropriate desensitisation levels. These offset levels would have to be
determined by the use of different resolution bandwidths.

This issue is highlighted in the case where the receiver does not have a
sufficient bandwidth to measure the fundamental signal without
desensitisation. This is the case for very narrow pulses, where if the
emissions in the SE region are time related to the fundamental then it
is appropriate to set the reference equal to the fundamental peak, or if
they are not related then it may be appropriate to offset the mask by
the desensitisation factor.

# Annex 1 Measurement of EIRP

The aim of the direct measurement is to measure the EIRP of spurious
emissions and to relate them to a peak envelope power level.

Consider the power received by the measuring antenna under far-field
conditions. The power received is the power density at the receiver
multiplied by the collecting area of the receive antenna.

The equation, which describes the coupling between two antennas in the
far-field and free space, well matched in bandwidth and polarisation, is
the FRIIS transmission equation.

$P_{r} = \frac{G_{t} \times P_{T}}{4 \times \pi \times R^{2}} \times A_{\text{eff}}$

where:

P~r~ =Power Received (watts)

G~t~ =Transmit Antenna Gain

P~T~ =Power Transmitted (watts)

R = Range (m)

A~eff~ = Effective aperture area of receiving antenna (m^2^)

This can be turned into the FRIIS equation by using the relationship,

$A_{\text{eff}} = G_{r} \times \frac{\lambda^{2}}{4 \times \pi}$

and substituting into the above equation,

$$P_{r}\  = \ \frac{G_{t} \times P_{T} \times G_{r} \times \lambda^{2}}{(4 \times \pi \times R)^{2}}\  = \ (G_{t} \times P_{T}) \times \frac{1}{4 \times \pi \times R^{2}} \times \frac{G_{r} \times \lambda^{2}}{4 \times \pi}$$

The equation can be rearranged, using the following terms:

EIRP = $G_{t} \times P_{T}$ a factor relating to the transmitter power
and antenna gain.

Range Factor = $\text{Rf} = \frac{1}{4 \times \pi \times R^{2}}$ , a
factor solely related to the range at which the measurement is made.

Antenna factor
=$\text{Af} = \frac{G_{r} \times \lambda^{2}}{4 \times \pi}$, a factor
that converts incident power density into received power which is a
function of receiver antenna gain and frequency only.

In dB form:

$P_{r}(\text{dBW}) = \text{EIRP}(\text{dBW}) + \text{Rf}(\frac{\text{dB}}{m^{2}}) + \text{Af}(\text{dBm}^{2})$

It can be seen that if the gain of the receiving antenna increases as a
function of frequency squared, then the antenna factor is independent of
frequency.

In practice this would mean that the antenna effective area would have
to be constant with frequency, and the only antenna that exhibits this
behaviour is an ideal aperture.

The EIRP can thus be calculated:

$\text{EIRP}(\text{dBW}) = P_{r}(\text{dBW}) - \text{Rf}(\frac{\text{dB}}{m^{2}}) - \text{Af}(\text{dBm}^{2})$

Thus to measure the level of the spurious emission EIRP~s~ relative to
the fundamental frequency EIRP~f~ ∆~eirp~ at a given range (R), then
assuming an ideal aperture receiving antenna[^5].

$$\Delta_{\text{eirp}} = \text{EIRP}_{f} - \text{EIRP}_{s} = P_{R_{f}} - \text{Rf} - \text{Af} - (P_{R_{s}} - \text{Rf} - \text{Af}) = P_{R_{f}} - P_{R_{s}}$$

This shows that in this relative measurement it is not necessary to know
the gain of the receiving antenna or the range.

If the absolute EIRP is required to be measured then this can be
obtained from knowledge of the measurement range and the receive antenna
factors.

[^1]: These are the authors own opinions and are taken form the authors
    input to JRC-19 2003

    "The Report of an investigation into the characteristics operation
    and protection requirements of civil aeronautical and civil marine
    radar systems."

[^2]: Sometimes side-lobe power is expressed as EIRP

[^3]: This is where the pulse spectrum method is no longer valid and the
    Fourier transform method is applicable.

[^4]: There are moves afoot within ITU working party 8B to resolve these
    inconsistencies. The proposal is that for radar the reference
    bandwidth should be 1 MHz or 1/τ whichever is the smaller.

[^5]: If an ideal aperture is not available, then a suitable correction
    factor needs to be applied. This is specific to a given antenna and
    can be derived from the gain of the antenna at the spurious emission
    frequency