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.