#  BPL compliance measurement procedures

## INTRODUCTION

The BPL Inquiry states that Part 15 of the Commission's rules do not
specifically provide measurement procedures that apply to BPL systems
and notes that the Commission has "...allowed measurements of radiated
emissions at three installations that the operator deems as
representative of typical installations."[^1] This approach is allowed
under Part 15 (§15.31(d)) when it is impractical to perform compliance
measurements at Open Air Test Sites (OATS). Compliance measurements must
be designed to be practical, but they should also be accurate with any
composite measurement error biased toward overestimation of actual field
strength.[^2]

In Section 5.2, it was noted that peak levels of BPL field strength may
arise from standing waves on the power lines that are generated by
reflections of signals at impedance discontinuities along the power
lines. It is essential that these standing wave conditions be addressed
during compliance measurements. These radiation conditions have little
to do with the BPL device itself; instead, they result from various
features of power lines that cannot be readily emulated in a laboratory
or at a conventional OATS.

NTIA has reviewed three proprietary reports of BPL measurements that
were performed by contractors hired by BPL proponents to test compliance
of trial BPL systems with Part 15 field strength limits. In all cases
involving outdoor overhead power lines, measurements were performed
using a one-meter high antenna on radials emanating from a power line
pole to which a BPL access device was mounted. While consistent with
§15.31(f)(5), this ad hoc measurement approach does not demonstrate
compliance with the field strength limits because as shown by NTIA's
measurements and models, peak field strength levels are not centered at
the BPL device and do not occur at a height of one-meter above the
ground.[^3] Other sources of potential BPL measurement inaccuracies
include: the measurement distance and extrapolation factor;
frequency-selective radiation effects; estimation of electric fields
using a loop antenna; and selection of representative BPL installations
for testing. Potential solutions to most of these measurement challenges
are at hand within existing Part 15 measurement procedures, as discussed
below (also see the listing of applicable Part 15 rules in Appendix A).

## MEASUREMENTS MUST ADDRESS RADIATION FROM POWER LINES TO WHICH BPL DEVICES ARE CONNECTED

Part 15 already clearly specifies that compliance measurements must
address the device under test (DUT, also referred to as equipment under
test or EUT) while it is connected to all cables, wires and companion
devices normally used with the DUT.[^4] The measurement distances are
specified to be relative to an imaginary, ground-based boundary around
the DUT and the interconnected cables, wires and companion devices.
Nonetheless, because BPL measurement contractors have applied
measurement distances with respect to only the BPL DUT, the Commission
should consider clarifications to the provisions that apply to BPL
systems.

When applying measurement distances relative to the BPL DUT, the peak
field strength may be substantially overestimated or underestimated. As
shown in NEC models of BPL radiation (see Appendix E), vertical electric
field strength varies substantially over small distances along radials
from the BPL DUT and, depending on geometric and electrical factors, the
measurement location may coincide with a local field strength peak or
trough. There is no apparent need to measure local field peaks under
power lines because radio receivers operating in the subject frequency
range inherently should not be located directly under power lines in
order to avoid degradation from ambient local power line noise.

## MEASUREMENTS SHOULD ADDRESS AGGREGATED EMISSIONS FOR THE FULLY DEPLOYED BPL NETWORK 

Part 15 specifies that the aggregate emissions from a composite system
must satisfy the field strength limits applicable for a single
device.[^5] As BPL networks are substantially deployed in a community,
the aggregated BPL emissions for the overall network are expected to
increase above the levels generated by a single BPL device. This
aggregation has already been observed by NTIA at one of the trial BPL
systems where multiple simultaneous transmissions occur.[^6]

## MEASUREMENT ANTENNA HEIGHTS SHOULD ADDRESS ALL IMPORTANT DIRECTIONS OF BPL SIGNAL RADIATION 

Part 15 measurement procedures for testing at OATS require measurement
of emissions radiated in all directions and identification of the
direction of maximum radiation intensity.[^7] This is accomplished
using: a turntable on which the DUT and interconnection cables, wires
and companion devices are rotated; a reflecting ground plane in
conjunction with predetermined normalized site attenuation; and
measurement antenna heights varied between 1 meter and 4 meters in order
to facilitate determination of the height of maximum radiation. Although
these OATS provisions are not practicable when measuring at a BPL
installation site, the underlying principles remain critical to
measurement accuracy and control of interference risks. Specifically, it
is essential that BPL compliance measurements be made in directions
where emissions may propagate to radio receivers.

In the case of outdoor BPL systems, radio receivers can be located in
any direction around the BPL device and the power lines to which the BPL
device is connected. Receiving antennas on masts or buildings near the
power lines or on aircraft flying over power lines can be at high
elevation angles from the DUT and power lines, whereas land mobile
antennas will typically be at low (including negative) elevation angles.
The lowest receiver antenna heights typically will be of the order of
two (2) meters. Thus, since BPL measurements at an OATS are not
practicable (where a one (1) meter antenna height should be considered),
there is no need to measure BPL emissions at a height less than two (2)
meters. However, to adequately address emissions at high elevation
angles, it is necessary to measure BPL emissions at heights comparable
to the power line height.

Conceptually, this can be accomplished either through direct measurement
at various heights and directions or by application of a standard
two-meter or higher measurement antenna height with an adjustment factor
that accounts for other heights. The direct measurement approach may
require more measurement samples but is favored by NTIA because the
logistically simpler adjustment factor approach introduces uncertainty
and electric utilities generally have access to bucket-trucks (\"cherry
pickers\") needed to safely perform measurements at and above the
heights on MV and LV power lines. In the alternative, NTIA\'s
measurement results to date (see Appendix D, §D.5) indicate that
electric field strength generated within tens of feet of the power lines
at two (2) meters above ground level generally are 3 dB to 15 dB lower
than values generated at a height of ten (10) meters (*i.e.*, typically
one (1) meter above the height of power lines). This indicates that at
heights above a BPL energized power line, a height adjustment factor
would be needed to properly estimate the peak field strength based on
measurements made at a two (2) meter height. In light of the large range
of potentially required adjustment factors and the need for high
certainty of compliance in directions where emissions may propagate to
radio receivers, the adjustment factor approach necessarily would have
substantial bias toward overestimation of field strength.

To minimize the number of measurement samples associated with direct
measurement, it appears feasible to apply only a single, high, standard
measurement height, combined with a smaller adjustment factor, unless
the measurement height coincides with peak field strength. For example,
a 10 meter measurement height may be adequate with a small adjustment
factor that accounts for higher field strength levels than could occur
above the 10 meter height; however, further study is needed to identify
the most practical but accurate approach. Measurement at a lower height
may be superfluous since higher peak electric fields appear to
consistently occur at heights above the power lines. NTIA plans to
further address this potential measurement solution in its Phase 2
studies.

### **7.5 A SINGLE MEASUREMENT DISTANCE SHOULD BE USED FOR OVERHEAD POWER LINES AND BPL DEVICES**  {#a-single-measurement-distance-should-be-used-for-overhead-power-lines-and-bpl-devices .unnumbered}

Practical and technical considerations dictate that BPL compliance
measurements be made in the near-field. NTIA\'s BPL measurements and NEC
radiation models both manifest near-field behavior at large distances
(*e.g.*, over 300 meters) in many directions from BPL systems. In many
cases, the field strength at large distances in the near-field is at
levels too low for reliable measurement. Thus, avoidance of measurement
in the near-field is not practicable and the measurement distance must
be based on other factors, such as:

-   The possible occurrence of local peak field strength levels at
    distances beyond the measurement distance, where these local peaks
    are near or exceed the measured peak level. If this were to occur,
    the BPL emissions may cause interference over an area much larger
    than implied by the limits and conventional point-source radiation.

-   The desire to not make measurements at multiple measurement
    distances associated with different frequency ranges. Use of two
    different measurement distances above and below 30 MHz almost
    doubles the time needed to conduct the measurements.

-   The measurements should be made at distances no closer than the
    minimum typical separation between power lines and radio receiver
    antennas. Otherwise, measurement uncertainties associated with any
    extrapolation are unnecessarily incurred.

NTIA\'s measurements and radiation models, deployment of Federal
Government radio receivers, and safety considerations indicate that a
measurement distance of

10 meters from any BPL device and its connected power lines satisfies
the above conditions. Thus, NTIA recommends that a standard measurement
distance of 10 meters be used for BPL compliance measurements.

## 7.6 A MODIFIED DISTANCE EXTRAPOLATION FACTOR IS NEEDED FOR BPL {#a-modified-distance-extrapolation-factor-is-needed-for-bpl .unnumbered}

NTIA\'s measurements and radiation models indicate that at distances
within several tens of meters of the power lines, BPL field strength
does not decrease with increasing distance consistent with the existing
Part 15 distance extrapolation factors of 20 dB and 40 dB per decade
above and below 30 MHz, respectively. In several cases not deemed to be
anomalous, field strength diminishes at a lower rate and NTIA plans in
its Phase 2 studies to further investigate this extrapolation factor for
outdoor BPL systems.

## 7.7 BPL FREQUENCY AGILITY AND POWER LINE FREQUENCY SELECTIVE EFFECTS MUST BE ADDRESSED IN THE MEASUREMENT PROCEDURES {#bpl-frequency-agility-and-power-line-frequency-selective-effects-must-be-addressed-in-the-measurement-procedures .unnumbered}

Many BPL devices feature frequency agility**,** where the band of
frequencies used by each device can be remotely adjusted via network
control software.[^8] Because the standing waves generated in any given
power line depend on the BPL device frequency, it is necessary to
perform compliance measurements with the BPL device sequentially tuned
across the entire frequency range that it is capable of using. For
example, a BPL device that occupies a 3 MHz bandwidth located anywhere
in the 4 MHz to 22 MHz frequency range would have to be tuned to five
(5) different center frequencies during successive measurements (*e.g.*,
5.5 MHz, 8.5 MHz). The uncertainty in estimating the peak field strength
stemming from measurement with the BPL device operating at only one of
many possibly frequency settings could exceed tens of decibels.

## 7.8 NEAR FIELD MEASUREMENT ERRORS MUST BE MITIGATED {#near-field-measurement-errors-must-be-mitigated .unnumbered}

At frequencies below 30 MHz, Part 15 measurement procedures dictate the
use of loop antennas to estimate electric field strength.[^9] Loop
antennas inherently respond to magnetic fields and are relatively
insensitive to electric fields; yet Part 15 applies to limits on
electric field strength. Hence, as noted in several comments in response
to the BPL Inquiry, the magnetic field strength measured with a loop
antenna must be converted to an estimated electric field strength
assuming a certain ratio of electric-to-magnetic field strength. This
ratio, which is related to wave impedance, is assumed to be 377. This is
a reasonable assumption if not exact in the far field of the radiating
structure. However, as noted above, BPL compliance measurements must be
made in the near field where the impedance is highly variable and will
be substantially higher than 377 in many locations.

Loop antennas are used below 30 MHz at OATS in order to avoid effects of
reflections that are more vagarious for electric than magnetic fields.
In other words, loop antennas yield better repeatability of
measurements, but such a goal is readily achievable only in a laboratory
or at an OATS (rather than at a BPL installation site). Rather than
derive impedance values for various BPL measurement heights, NTIA
recommends consideration of BPL compliance testing below 30 MHz using a
calibrated rod antenna.

## 7.9 APPROPRIATE CHOICE OF POWER LINES USED FOR BPL MEASUREMENTS WILL REDUCE STATISTICAL SAMPLING UNCERTAINTIES {#appropriate-choice-of-power-lines-used-for-bpl-measurements-will-reduce-statistical-sampling-uncertainties .unnumbered}

One reason Part 15 requires measurement at three or more representative
installation sites in cases where OATS are impractical is that there is
a significant chance that one such installation site will not manifest
the highest field strength levels that will occur in practice. This
possibility exists even with three or more measurement sites unless the
sites are selected (or established) to yield the highest field strength
levels. Measurement venue notwithstanding, CISPR 22 requires use of an
adjustment factor accounting for statistical sampling uncertainty.[^10]
Rather than deal with these adjustment factors, which may lead to
significant overestimation of BPL field strength, NTIA recommends that
BPL installations be selected (or established) in a manner ensuring that
the highest possible levels of BPL field strength will be generated.

Conceptually, testing should be conducted using various lengths of power
lines that include substantial impedance discontinuities at various
distances from the BPL device that may result in the generation of
standing waves that are associated with the highest possible levels of
field strength. This is because the distance between the BPL device and
the impedance discontinuity affects the distribution of standing waves
(and spatial distribution of field strength) at a given frequency. Based
on NTIA\'s measurements and modeling to date, and noting that further
study is needed of the effects of power line branches and turns, the
following types of test site selection criteria (or standard test
facility design factors) are suggested for further consideration and
refinement for the case of outdoor, overhead power lines:

-   The BPL device should be located near the center of a straight
    section of power lines at least 600 meters in length that is devoid
    of significant impedance discontinuities. This ensures that at the
    lower frequencies (longer wavelengths), at least four standing wave
    crests can be generated in the straight section of power lines in
    order to establish a minimally sufficient number of radiating power
    line sections. Because BPL devices themselves may establish
    impedance discontinuities, the other BPL devices operating with the
    BPL DUT should be located beyond the nearest impedance
    discontinuity.

-   If a standard test facility is established and the power lines used
    for testing are not a segment of operational power lines that extend
    well beyond the test facility, the lines should be terminated in the
    characteristic impedance of the power lines as tested. This avoids
    inadvertent, unrealistic radiation caused by non-typical termination
    of the power lines.

-   A variety of representative MV power line configurations should be
    present in the test site (or standard BPL test facility). For
    example, the site should include single and three-phase power line
    segments, sharp turns in the power line, and risers that connect
    overhead lines to underground lines.

## 7.10 BPL DEVICE OUTPUT POWER SHOULD BE REDUCED AS NEEDED FOR COMPLIANCE WITH RADIATED EMISSION LIMITS {#bpl-device-output-power-should-be-reduced-as-needed-for-compliance-with-radiated-emission-limits .unnumbered}

The measurements should be initially conducted while the BPL device is
operating with maximum power output as required by §15.31(g). This may
yield field strength values that exceed the limits, in which case the
BPL device output power should be reduced to the extent necessary to
obtain compliance with the limits. Because different limits are applied
above and below 30 MHz, and because all possible power line
configurations are not being measured, at most two different BPL output
power levels may be determined for compliance with the limits (*i.e*.,
one above and one below 30 MHz).

In the event that an output power reduction is needed to achieve
compliance, all measurements must be made at the reduced output level
including any measurements preceding discovery of field strength in
excess of the limiting value.

**7.11 THE RESULTS OF RADIATED EMISSION MEASUREMENTS**

**SHOULD BE PROPERLY RECORDED IN MEASUREMENT**

**REPORTS AND APPLIED IN BPL OPERATIONS**

The measurement report should record all measurements including those
preceding any BPL output power reductions needed for compliance with the
field strength limits. If a power reduction is needed for compliance,
the amount of necessary reduction and the means by which it was achieved
during testing should be recorded in the measurement report. As a
condition for authorization, where BPL output power can be adjusted, the
BPL power control software, firmware and hardware should be modified to
prevent operation at output power levels higher than those yielding
compliance with the field strength limits. In other cases where BPL
device output power is not adjustable, inclusion of a fixed attenuator
or suitably lower-power output stage should be mandated in the
authorization. In no case should BPL operators be equipped to exceed
output power levels at which compliance is obtained.

**7.12 CONCLUSION**

The Phase 1 analyses assumed that for outdoor overhead power lines,
compliance measurements were performed using a one-meter high
measurement antenna. This ad hoc measurement approach does not
demonstrate compliance with the field strength limits because, as shown
by NTIA's measurements and models (Section 5), peak field strength
levels are not necessarily centered at the BPL device and do not occur
at a height of one-meter above the ground. Moreover, all of the
receiving antennas assumed in the Phase 1 analyses were located at least
two meters above the ground. Other potential sources of measurement
underestimation of BPL field strength include: the measurement distance
and extrapolation factor; frequency-selective radiation effects;
estimation of electric fields using a loop antenna; and selection of
representative BPL installations for testing. Solutions to most of these
measurement challenges are at hand within existing Part 15 measurement
guidelines.

In light of the above considerations and the high perceived interference
risks, NTIA recommends that field strength limits for BPL systems not be
relaxed and that measurement procedures be refined and clarified as
described in this section to better ensure compliance. These risk
reductions should be effected as quickly as possible in order to better
protect radio communications.

[^1]: BPL Notice of Inquiry, at ¶2 and 21-23.

[^2]: *See e.g.*, *Information technology equipment -- Radio disturbance
    characteristics -- Limits and methods of measurement*, CISPR
    22:2003, ("CISPR 22"), Section 7.1.2. "The significance of the
    limits for equipment shall be that, on a statistical basis, at least
    80% of the mass-produced equipment complies with the limits with at
    least 80% confidence."

[^3]: *See* *Potential Interference from Broadband over Power Line (BPL)
    Systems to Federal Government Radiocommunications at 1.7 -- 80 MHz*,
    NTIA Report 04-413, ("NTIA BPL Report"),Volume I, Section 5.2.

[^4]: *See* 47 C.F.R. §15.31(g)-(k).

[^5]: *See* 47 C.F.R. §15.31(h)-(k)

[^6]: *See* NTIA BPL Report, Volume II, Appendix D, § D.1

[^7]: *See* *e.g*., 47 C.F.R. §15.31(f)(5) and ANSI C63.4-2001, clause
    8.3.1.2. When measurements are made at an installation site, ANSI
    C63.4-2001, clause 8.3.2 requires identification of the radial of
    maximum emissions.

[^8]: Reply Comments of UPLC, BPL Inquiry, August 20, 2003, ("UPLC Reply
    Comments"), at 7; Ambient Comments at 8; Ameren Comments at 9-10.

[^9]: *See* 47 C.F.R. §15.31(a)(6) and ANSI C63.4-2001, clauses 4.1.5.1
    and 4.1.5.2.

[^10]: *See* CISPR 22, Section 7.2.2. Here, the error from statistical
    sampling uncertainties is biased in a manner "...which assures with
    80% confidence that 80% of the \[equipment\] type is below the
    limit."