Modeling Signal Leakage Characteristics of Broadband Over Power Line (BPL) Using NEC With Experimental Verification

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Modeling Signal Leakage Characteristics of
Broadband Over Power Line (BPL) Using NEC With
Experimental Verification

• Steve Cerwin WA5FRF
• Institute Scientist
• Southwest Research Institute

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Possible Geometries for Using Power Lines As
Transmission Lines

• Single wire driven against ground not considered
  suitable as a transmission line
• G-line impractical because launchers are too big
  and power lines too discontinuous
• Balanced drive between two adjacent wires
  deemed best option to minimize radiation, and is
  the model used in the study

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Two Wire Transmission Line Models Used in the
Study
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Interpreting NEC Simulation Results
Program also calculates radiation patterns and
current distributions.
The difference between the total applied power
and the power absorbed in all loads is the amount
of power radiated from the line. This information
can be obtained from the Total Load Loss report.
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Maximum Lobe Gain and Leakage Radiation From
Matched and Balanced Straight Lines
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Radiation Patterns from Matched and Balanced Two
Wire Transmission Lines in Free Space
2MHz
5MHz
10MHz
20MHz
40MHz
80MHz
200-ft. Long Straight Line with 4-ft. Spacing, 1
Source, and 1Load
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Mismatched Source and Load Impedances Create High
SWR and Increase Line Radiation
Matched
200x4 Line _at_ 20 MHz
Mismatched
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Coupling to Nearby Resonant Antennas Shows
Normalized Frequency Response
Wavelength dependent capture area of a resonant
receive antenna compensates for frequency
dependent line leakage, normalizing coupling over
frequency.
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Position Dependence of Coupling Along A Perfectly
Matched and Balanced Line
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Scale Model Laboratory Setups Used For
Experimental Verification of NEC Models
1/60th Scale Model Used 450-ohm Ladder Line to
Represent the Power Line Under Conditions of Free
Space and Over Ground.
Full Scale 1/60th
Scale Length 500-ft.
8.33-ft. Spacing 48-in.
0.8-in. Height 30-ft.
0.5-ft. Frequency 10MHz 600MHz
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Experimental Data Agreed With Theoretical Data
Only Near Line ends Where Signal Levels Were High
Low Coupling Levels Predicted For Interior
Portion of Line Were Unachievable Because of Room
Multipath Reflections or Balun Imbalance
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Multiple Loads Create Unavoidable Impedance
Mismatches and High SWR
Multiple loads along a constant impedance line
create mismatches through cumulative loading.
Source on End
Low SWR available only on ends where a matched
termination is available.
Source in Interior
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Increased SWR From Multiple Loads Increases
Radiation from Interior by 20dB
Level in matched line
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Unequal Wire Lengths from 90-degree Turn
Imbalance Current Distribution and Rapidly
Accelerate Radiation with Frequency
Maximum lobe gain approaches 9dBi and nearly half
of the total applied power is radiated above 30MHz
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Coupling Levels to Nearby Dipole With L-line
Containing Multiple Loads Increased 10-20dB Over
Straight Line
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Unequal Wire Lengths in U-Shaped Line Cause
Severe Radiation Losses at 80 MHz
Current Distribution shows pronounced amplitude
taper and unequal wire currents.
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Bending a 200-ft. x 4-ft. Line Into a U Destroys
Transmission Line Properties Above 10Mhz
Maximum lobe gain undulates between and 6dBi
Half of the applied power is radiated above
22MHz. Less than 10 reaches the load above 30MHz.
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Current Distributions on U-line With Multiple
Loads Show Amplitude Taper, Unequal Currents in
Wires, and SWR Misalignment
40MHz
80MHz
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Power Lines As Transmission Lines at Radio
Frequencies

• Transmission lines modeled after power lines
  radiate severely because they are spaced too far
  apart for high frequencies and have too many
  characteristics that destroy balanced operation.
  Many line geometries radiate as much or more
  power than that delivered to loads placed
  directly across the line. Using these structures
  to distribute wideband data signals is
  technically flawed because of their inability to
  contain the radio frequency energy as a guided
  wave, and should be considered very poor
  engineering practice.