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The Other Side of RF Measurements: Out of the Lab and into the Real World. Some Selected Experiences Prepared by Ric Tell and Jim Hatfield for the Michaelson Research Conference August 11, 2001 Kalispell, Montana. Hatfield & Dawson, LLC Seattle, WA.

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the other side of rf measurements out of the lab and into the real world
The Other Side of RF Measurements: Out of the Lab and into the Real World

Some Selected Experiences

Prepared by

Ric Tell and Jim Hatfield

for the

Michaelson Research Conference

August 11, 2001

Kalispell, Montana

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

some aspects of real world rf field assessments
Some Aspects of Real World RF Field Assessments
  • Non-clean, non-lab conditions
  • Reflections everywhere
  • Often awkward exposure situations
  • Adverse environmental conditions
  • Commonly a circus environment
  • Requirement to comply with regs or standards that are not necessarily clear
  • Legal ramifications of findings!

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

factors affecting rf measurement accuracy and meaning
Factors Affecting RF Measurement Accuracy and Meaning
  • Probe calibration accuracy
  • Probe frequency response
  • Multiplicity of fields (rms response)
  • Polarization of fields
  • Spatial distribution of fields
  • Interference with field to be measured by observer (field perturbation)

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

studying the effect of field perturbation on measured rf fields
Studying the Effect of Field Perturbation on Measured RF Fields
  • Establish a “pure” test environment
  • Determine the “unperturbed” field
  • Measure influence of field perturbation caused by observer

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

measured spatially averaged rf fields at point 5 on wtc south tower walkway
Measured Spatially Averaged RF Fields at Point 5 on WTC South Tower Walkway

Based on study by Richard Tell Associates, Inc.

Mean values plus/minus one standard deviation

Normal broadcasting with FM off

Operating scenario

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

a theoretical assessment of operator interaction with fields

5

4

6

3

7

2

8

1

A Theoretical Assessment of Operator Interaction with Fields

Measurement Positions

Field propagation direction

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

montana rf a cast of characters
Montana RFA Cast of Characters

Happy Ricky

Philosophic Jimmy

Studious Davey

Smiling Andy

Late Breaking Data

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

the test site shelby montana
The Test Site – Shelby, Montana

Main antenna

Auxiliary antenna

Holaday HI-6005 probe

50 feet

Bottom pulley

Probe support isolation

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

data collection instrumentation
Data Collection Instrumentation

12 volt dc gear motor winder

Laptop & fiber optic modem

Probe was raised from a distance of 50 feet from measurement point.

Car battery

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

isotropic broadband electric field strength measurements pave paws clear afs
Isotropic Broadband Electric Field Strength MeasurementsPave Paws, Clear AFS

Holaday Industries HI-6005 fiber optic isolated, isotropic electric field probe.

Isotropic elements

Fiber optic cables

Non-conductive tripod

Sampling software to measure approximately 70 readings per second.

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

typical spatial variation of power density at 30 feet from kzin tower shelby montana
Typical Spatial Variation of Power Density at 30 Feet from KZIN Tower, Shelby, Montana

Unperturbed field

/4 at 96.3 MHz

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

polar plot of field perturbation caused by observer

1

2

8

7

3

6

Smax=85.8% MPE

5

4

Smin=61.7% MPE

Savg=71.2% MPE

Polar Plot of Field Perturbation Caused by Observer

Plotted for height of maximum unperturbed field of 86.5% MPE.

90% MPE

80%

70%

Technician faces measurement point from all directions.

60%

KZIN FM 96.3 MHz Shelby, MT 8-1-2001

Tower

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

slide13

Preliminary Spatial Average Measurement Results

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

overall average of spatial average measurements
Overall Average of Spatial Average Measurements

Based on 8 spatial averages for each scenario

Ratio max/min

Unperturbed

1.08

Ric standing

1.21

Jim standing

1.17

Ric measuring

1.38

Jim measuring

1.57

0

10

30

40

50

20

Percentage of MPE limit (%)

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

electric field strength polarization components vs distance from kzin tower
Electric Field Strength Polarization Components vs. Distance from KZIN Tower

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

polarization and sar isotropic field probes will generally overestimate resulting sar
Polarization and SARIsotropic field probes will generally overestimate resulting SAR

Relative SAR Contribution

F = 96.3 MHz

E: 100%

H: 9.3%

K: 17.6%

E

Total SAR from all field components at test point = 10% of the SAR that would be implied from a measurement of the resultant field.

K

H

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

slide17

Comparison of measurements of spatially averaged RF fields

Unperturbed field

Unperturbed field

Assessing compliance with exposure limits can be difficult.

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

comparison of two persons using the same probe at same point
Comparison of Two Persons Using the Same Probe at Same Point

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

how strong is that field
How Strong is that Field?
  • The FCC maximum permissible exposure (MPE) limits are in terms of spatially averaged values of plane wave equivalent power density over the body.
  • The limits are derived from the presumption of uniform exposure to a field having the specified MPE limit.
  • The most accurate assessment of exposure, relative to determining compliance with the FCC limits, is in the absence of any field perturbing effects introduced by either the person being exposed or the person attempting to measure the exposure.

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

tentative conclusions
Tentative Conclusions
  • Measures of spatially averaged RF fields are inherently fraught with uncertainty caused by field perturbations.
  • Operator interaction with the field can lead to significant differences in compliance measurements at antenna sites.

Hatfield & Dawson, LLC Seattle, WA

Richard Tell Associates, Inc. Las Vegas, NV

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