Modeling Signal Leakage Characteristics of Broadband Over Power Line (BPL) Using
NEC With Experimental Verification
Steve Cerwin WA5FRF
Institute Scientist
Southwest Research Institute
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
Two Wire Transmission Line Models Used in the Study
Interpreting NEC Simulation Results
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.
Program also calculates radiation
patterns and current distributions.
Maximum Lobe Gain and Leakage Radiation From Matched and
Balanced Straight Lines
Radiation Patterns from Matched and Balanced Two Wire Transmission Lines in Free Space
2MHz 5MHz 10MHz
20MHz 40MHz 80MHz200-ft. Long Straight Line with 4-ft. Spacing, 1 Source, and 1Load
Mismatched Source and Load Impedances Create High SWR and Increase Line Radiation
Matched
Mismatched 200’x4’ Line@ 20 MHz
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.
Position Dependence of Coupling Along A Perfectly Matched and Balanced Line
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 ScaleLength: 500-ft. 8.33-ft.Spacing: 48-in. 0.8-in.Height: 30-ft. 0.5-ft.Frequency: 10MHz 600MHz
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
Multiple Loads Create Unavoidable Impedance Mismatches and High SWR
Source on End
Source in Interior
Low SWR available only on ends where a matched termination is available.
Multiple loads along a constant impedance line create mismatches through cumulative loading.
Increased SWR From Multiple Loads Increases Radiation from Interior by 20dB
Level in matched line
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
Coupling Levels to Nearby Dipole With L-line Containing Multiple Loads Increased
10-20dB Over Straight Line
Unequal Wire Lengths in U-Shaped Line Cause Severe Radiation Losses at 80 MHz
Current Distribution shows pronounced amplitude taper and unequal wire currents.
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.
Current Distributions on U-line With Multiple Loads Show Amplitude Taper, Unequal
Currents in Wires, and SWR Misalignment
40MHz
80MHz
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.