RCG-31

Title: Commentary on the Indirect Method Described in ITU-R
Recommendation M.1177

Written by: John Holloway[^1]

Date: Tuesday, 10 May 2005

# Introduction

ITU-R Recommendation M.1177 contains two methods for measuring spurious
and out-of-band emission for radar systems. The two methods are
described as the "Direct" and "Indirect" method.

If these two methods are to be recommended then they should ideally
produce equivalent results or at best have defined error bands that can
be used to indent the measured results to ensure compliance with
specification limits, and so as not to disadvantage the user of either
of the two methods.

Both methods must ensure that systems measured as being compliant are in
fact such.

Of the two the Direct method is well characterised the indirect method
however is much less well described and understood. This note describes
several issues that have been identified with the indirect method that
make its possible use as a method of certifying radars suspect.

# Limitations with the Indirect Method

The indirect method described in ITU-R Recommendation M.1177 has several
practical limitations that make its use limited to very few radar types.
In addition, the document does not provide a detailed budget for
measurement uncertainties, so it is not possible to verify the claims
for the accuracy made for the method as described.

However the following practical limitations do exist with the method:

-   It requires the antenna to be detached from the radar and placed on
    the near-field range to have its gain characterised. This may not be
    appropriate for large antennas or antennas where the transmitting
    and radiating function are combined, such as in active antennas.

-   It requires the use of a special absorptive filter to attenuate the
    fundamental and allow the harmonic through. Such a filter is not
    practical for use with high power radars.

-   The method also makes the tacit assumption that the antenna gain
    figure can be added to the measured transmitter power level. It
    assumes no interaction between the antenna and feed and the
    transmitter. This may be true in the OOB domain but is much more
    uncertain in SE domain.

Recommendation M.1177 documents the two sources of uncertainty in the
measurements as:

-   Near-field gain measurement uncertainty with the applied correction
    factors;

-   Measurement uncertainty in a waveguide.

The following sources of error over and above the errors associated with
the measurement uncertainties described above have been identified.
These issues do not seem to be addressed in Recommendation M.1177 or its
proposed revision.

These comments pertain to the use of the method to measure emissions in
both the OOB and SE domains including harmonic emissions:

-   In some cases it is not possible to decouple the antenna from the
    transmitter active arrays are such a type and are not appropriate
    for use with this method. However in some other cases the
    transmitter and antenna are very closely coupled, such is case for
    many "patch antenna systems". In these systems when the two are
    separated is difficult to reproduce the impedance conditions that
    occur then the to are coupled. The result is that the results
    calculated using the indirect method do not represent the results
    achieved on the combined systems.

-   The use of a "special filter" and a matched receiver will provide
    essentially a matched load at the harmonic frequencies. In practice
    the transmitter will see a mismatched antenna and feeder system at
    the harmonic frequency. The level of the harmonic power produced
    will thus vary with this match. As well as affecting the level of
    harmonic power generated, the amount coupled to the antenna will
    also vary. This poor antenna match will also affect the accuracy of
    the power measurement depending on the directivity of the
    measurement coupler. These effects need consideration when
    calculating the far-field power. Allowance needs to be made for the
    change in power and miss-match loss. It is not possible to calculate
    these changes for a general system as they depend totally on the
    layout of the microwave components and the sensitivity of the
    individual transmitter types to miss-matches. Experience has shown
    that these changes could be of the order of 10dB at harmonic
    frequencies.

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-   In the OOB domain the use of a well-matched load will not reproduce
    the impedance conditions of the transmitter antenna interactions
    which can distort the transmitted spectrum.

-   The gain of the antenna at the harmonic frequencies and the position
    of any harmonic beam formed will depend on the mix of modes excited
    in the radiators. This can vary especially in feed systems that
    comprise waveguides. The mode mix at the harmonic frequencies will
    depend on the type and position of all the feed components. ITU-R
    Recommendation M.1177 makes light of this effect speculating that
    the higher order modes will be trapped in coaxial to waveguide
    transitions. Whilst it is true that if these components are present
    within the feed they can act as mode filters, they can also generate
    higher order modes themselves depending on the plane of symmetry
    within the transition, also not all radar feed systems have such a
    transition or use coaxial cable. Thus the Equivalent Isotropically
    Radiated Power (EIRP) of the harmonic beam will depend on the exact
    arrangement of the feed. This effect is not replicated in the direct
    method and will add further errors.

-   The equation used to calculate the \"near-field\" transmit antenna
    gain is based on the FRIIS transmission equation. This equation is
    only valid for antennas spaced at a distance greater than 2D^2^/λ
    where D is the largest dimension of the two antennas. A further term
    to correct for the gain of the probe antenna is thus required. The
    lower the gain of the receiving antenna is, the less this effect
    will be as the far-field distance for an antenna increases with the
    gain of the antenna. The probe antenna will also suppress energy
    from off axis due to its gain pattern shape; any transform needs to
    include this \"probe correction\" factor.

-   The near-field to far-field gain transformation assumes uniform
    illumination. This will be in error when antennas with significant
    edge tapers are measured. The transform described is a single point
    near to far-field transform, the accuracy of which is unproven.
    Particularly for large antennas, the near-field is complex and must
    be sampled over the full extent of the azimuth aperture in order to
    get a good estimate of the far-field gain by the use of a transform.
    Errors relating to the transform should be included. It is not
    possible to calculate the error in the transform without detailed
    knowledge of the antenna under test.

-   A further issue is that of antenna-to-antenna interaction. It is
    also possible to set up a standing wave between the antenna under
    test and the probe antenna; this effect can result in fluctuation in
    the received power in the boresight as a function of range. The
    effect can be checked by moving the probe antenna towards and away
    from the antenna under test.

-   Harmonic radiation may not be radiated coincident with the
    fundamental frequency, so the effective aperture at these
    frequencies will not be the same as at the fundamental. Harmonic
    radiation can also radiate in more than one beam due to the wide
    spacing of the array elements at harmonic frequencies. If there are
    higher order modes in the waveguide feed, then harmonic radiation
    can be radiated out of the horizontal plane. This all means that at
    harmonic frequencies the effective aperture can depart significantly
    from the physical aperture used. Thus the aperture used in the
    transform from the near to far-field will be in error.

-   Harmonic radiation is likely to occur in significant amounts in a
    form cross-polarised to the fundamental frequency radiation. The
    method needs to look for the position of any cross-polar beams to
    ascertain the peak in terms of co-polarised radiation.

-   The arrangement shown in ITU-R Recommendation M.1177 gives a
    depression angle of approximately 18° to the ground plane. The
    effect of multi-path needs to be considered. The effect the
    multi-path will have depends on the elevation beamwidth of the Tx
    and Rx antennas and the reflectivity of the ground plane. Small
    antennas designed for use on un-stabilised marine platforms have
    wide elevation beamwidths (25 to 30°); methods need to be employed
    to reduce the effect of multi-path. The use of a ground plane only
    enhances the problem. The practice of adjusting the test antenna
    height prior to measurements would tend to maximise the multi-path
    error. An attempt should be made to attenuate the multi-path and
    confirm this by ensuring that the received signal is essentially
    multi-path free. This can be carried out by using the measurement
    antenna to sample the received power in the elevation plane by
    moving it up and down. The variation in this plane can then be
    reduced by the application of absorptive material to the ground
    between the antennas. The use of a high gain receive antenna will
    reduce the ripple by suppressing the reception of the multipath
    signal but will increase the errors associated with the gain
    measurement.

# Accuracy

####  {#section .unnumbered}

There has been no published error budget for the indirect method,
however two figures are quoted in Recommendation M.1177:

\"Near field gain measurement uncertainty with the applied correction
factors

The worst-case measurement uncertainty can be calculated to be ± 6 dB,
which includes, uncertainties due to a spectrum analyser, test horn
gain, cable loss and source and site imperfection. Total uncertainties
with a confidence level of not less than 95% can be calculated to be
± 4.2 dB.

The derivation of the correction factors for these distances assumes the
AUT radiating aperture to be constant at all frequencies.\"

\"Measurement uncertainty in a waveguide

The system has a measurement accuracy of ± 1.3 dB across the frequency
band 2 to 18.4 GHz for the waveguide port. Total uncertainties with a
confidence level of not less than 95% can be calculated to be ± 3.4 dB
for the waveguide port including the spectrum analyser.\"

If we take the Root of the Sum of the Squares (RSS) of the two 95%
figures we achieve ±5.4dB, however unpublished sources indicate that the
errors using this method could be as large as 20 to 30 dB.

Considering each of these uncertainly statements in turn:

## Measurement uncertainty in a waveguide

The errors come from several sources:

-   Receiver Errors

-   Component losses

-   Miss-match Errors

These issues do not seem to have been addressed in the published
uncertainties. This measurement as defined in ITU Recommendation M.1177
requires a measurement of absolute power of UEs. In order to ascertain
the level relative to the PEP, as required by the Radio Regulations,
either the power of the transmitter has to be measured directly or the
loss of the system has to be characterised at the fundamental frequency.

This will require a further measurement, which itself will have similar
errors. In order to avoid having to measure the PEP directly, (which
would involve the use of high power attenuators) it would be better to
characterise the loss of the system to the fundamental frequency
(including the notch filter) at low power. This measurement will have a
similar set of uncertainties to the first measurement made of the level
of the UEs. This extra level of uncertainty will need to be added to the
overall uncertainty figure.

It is also unclear in the method as presented how the systems losses at
harmonic frequency are accounted for, as it is not possible to calibrate
out the losses given the fact that the input and output waveguide are a
different size, extra tapers would be required to do this. Alternatively
each component could be measured separately and the results added, but
then their individual miss-match errors would need to be allowed for in
the overall uncertainty.

## Near-field gain measurement uncertainty with the applied correction factors

Errors associated with the characterisation of antenna result from:

-   Gain transform errors

-   Multi-path errors

-   Polarisation errors

-   Beam pointing errors

-   Power Measurement Errors

ITU Recommendation M.1177 gives an uncertainty of ± 6 dB for the gain
correction factor, and again no breakdown is given of the constituent
parts. In particular it is not clear what errors have been associated
with the near to far-field transform, which is known to be a gross
over-simplification.

The uncertainty will be dominated by the errors associated with the
characterisation of the antenna, in particular the determination of the
antenna gain.

The receiver errors associated with the \"Power Measurement\" needed to
calculate the gain prior to correction are similar to those achieved by
using the direct method, or by the UE power measurement in the first
part of this method. These are quoted at 95% value of ± 3.5 dB. If the
overall result of ± 6 dB is assumed to be an RSS of the uncertainty
obtained from the power measurement, i.e. ± 3.5 dB, plus that obtained
from the transform plus the other errors quoted, then this results in an
uncertainty value of ±5.2 dB[^2] to allow for the transform and the
other errors quoted. This figure of ± 5.2 dB seems low when all the
simplifications and limitations of the transform are considered. This
would seem to indicate that the quoted value of ± 6 dB is an
underestimate of the uncertainties.

When considering these errors, it must not be forgotten that there are
still the interaction uncertainties associated with the good match seen
by the transmitter by the use of the \"special\" filter and the effect
of the feed components at harmonic frequencies.

If the error in the gain measurements could be reduced, then the overall
uncertainty would be reduced. In order to fully characterise the test
antenna, and to overcome the limitations described, a wide-angle
characterisation of the antenna pattern over the required frequency
range is needed. Figure 3.1 shows a facility capable of making such a
measurement. The operation of this facility is described in paper
\"High-precision outdoor cylindrical near-field antenna test
facility\"[^3]. The use of such a facility should be referred to in the
upgrade of ITU-R Recommendation M.1177

This is not the only facility in the UK that could produce the type of
results required. There are several others based on cylindrical or
spherical scanners, including some indoor facilities.

Such facilities can achieve an absolute characterisation of the
transmitting antenna main beam gain of ±0.5 dB and on a -50 dB side-lobe
of ± 3 dB. Measurements of this accuracy would significantly reduce the
estimated uncertainty in the indirect method.

Whilst such facilities are automatic, considerable time would still be
required to characterise the antenna over the very wide frequency band
required.

Given that such an approach is adopted to reduce the measurement
uncertainties, there are still unknowns that are not accounted for.
These are related to the interactions between the antenna and the
transmitter, which are highly dependent on the feed arrangement,
particularly if the feed is implemented using waveguide. Coaxial feeds
are more predictable if the coaxial line can be constrained to a
Transverse ElectroMagnetic wave (TEM) mode.

Figure 3.1 High-precision Cylindrical Near-field Test Facility

![](media/image1.wmf){width="2.8965277777777776in" height="4.3625in"}

# Comparison of Direct and Indirect Methods

ITU-R Recommendation M.1177 does not recommend any single method
described. In order to compare the methods it is necessary to understand
the requirements of the measurement compared to the measurement accuracy
achieved.

ITU-R Recommendation M.1177 is referenced as the method of choice in
other ITU documents dealing with issues such as radar OOB or SE limits.
It is likely that ITU-R Recommendation M.1177 would be required to be
used as the basis of a type approval exercise. It is also likely to be
used for monitoring in-service performance, wideband spectrum surveys,
interference investigation and conflict resolution.

It is thus necessary to use methods that are accurate, repeatable and
practical; the direct methods seem to meet these requirements subject to
the ability to achieve the required dynamic range[^4]. Problems with
dynamic range, resulting from a lack of received signal, are however
generally clearly apparent to the operator as the measurements are being
taken, as the received signal is seen approaching the system noise and
measures can be taken to rectify the problem. Neither method described
in ITU-R Recommendation M-1177 specifies what receive polarisation is to
be used. For measurements at and around the fundamental frequency, the
polarisation should be matched to the radar polarisation. At harmonic
frequencies, it may be required to measure in two orthogonal
polarisations. Alternatively, if the source is linearly polarised, a
circular polarised receive antenna could be used, noting that this will
introduce a loss of 3 dB to any form of linear polarisation. The
indirect method as described however has severe limitations with keeping
the measurement uncertainties to an acceptable level. The method could
be improved by using a more accurate method of measuring the antenna
gain. However it is unlikely, if this approach were adopted, that the
measurement time and cost would be reduced to below that of the direct
method. This is true for direct measurements made either on a dedicated
range or in-situ.

The indirect method does however have two advantages over the direct
method:

-   The measurement of the transmitter is made in a \"closed\"
    environment and is not subject to interference.

-   If the improved near-field measurement is made, then this can
    potentially give a full hemispherical polarimetric characterisation
    of the antenna over all frequencies.

# Conclusion

The indirect method as it stands, because of both its practical
limitations and its measurement accuracy, is of limited use, given that
it can only be used on a dedicated test site and it can only be used for
certain types of radars. An improved indirect method is available with
the use of more accurate near-field systems, however the time and
complexity increase significantly.

Given the limitation of the indirect method and the inability to
quantify the measurement errors and the likely differences in the two
methods it is not considered appropriate to recommend the equivalence of
the two methods. Permitting the use of the indirect method could
seriously disadvantage users of the direct method that has been shown to
give well-quantified representative results.

It is thus recommended that in the majority of cases that the direct
method is adopted and the indirect method is no longer recommended[^5].

The only case where the indirect method should be allowed is that for
low frequency systems (HF) where the direct method is not practical.

[^1]: These are the authors' own opinions and are taken from 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]: In linear units of power
    $3\text{.}\text{98} = \sqrt{\left( x^{2} + \left( \text{10}\hat{\left( \frac{3\text{.}5}{\text{10}} \right)} \right)^{2} \right)}$

    where x is the uncertainty of the transform.

[^3]: Electronics & Communication Engineering Journal, June 1995,
    J.R.Holloway & A.H.I M^c^Cormick

[^4]: Dynamic Range is a function of the receive antenna being used and
    the distance from the radar. In service measurements will always be
    subject to the ability to find a suitable site for locating the
    receiver antenna.

[^5]: It may be that its use should be restricted only to a \"type
    approval\" method for small civil marine radars. It may have some
    applicability as a \"transfer\" method where, with the use of a well
    characterised test antenna, results can be compared with those taken
    using the direct method. This could allow variants of transmitters
    to be tested without recourse to the direct method. In this case it
    is repeatability more than its absolute accuracy that may be more
    important for comparative testing. It needs, however, to be
    established that users who elect to use the direct method for type
    approval are not disadvantaged compared to those who adopt the
    indirect method. If this cannot be established then the indirect
    method must not be used.