Overhead Electrical Power-Line Distribution

Methods of Feeding Overhead Electrical Power-Line Distribution Lines With BPL Signals and the Relationship of These Methods to the Radiated Emissions of the Conductors

1. Introduction 1.1 There are differences in the way that medium-voltage (MV)2 power-distribution lines conduct and radiate signals based on the way that RF power is fed to the lines. ARRL used a well-known antenna-modeling program, EZNEC PRO3 3.0 with the NEC-4.1 calculation engine4 to model a simple MV power line and two nearby amateur antennas, conservatively located 30 meters from the lines. A pictorial diagram of the model is shown in Figure 1. 1.2 Tables 1 and 2 show the results ARRL obtained by modeling three different ways of feeding the antenna: o Differential feed between two phases, at one end o One phase to earth ground, in the center o One phase fed differentially similar to the way a dipole antenna is fed, offset on the ungrounded phase

2. Description of the Model 2.1 The power-line radiator antenna model was configured with two 12.7 mm copper conductors5, 200 meters in length. They were placed 10 meters above ground. The ground was modeled with average conductivity and dielectric constant. The two conductors were parallel, spaced 1.0 meter. One of the conductors was grounded to simulate typical imbalance in the line. The ground connection consisted of four 10-meter radials, 5 cm above ground. This is a relatively poor RF ground, to simulate the typical poor RF characteristics of power-line grounds. (This also allows those that don’t have access to the NEC-4.1 software to duplicate the results using the more available NEC-2 calculation engine, which cannot handle direct ground connections the same way NEC-4.1 does.) 2.2 Differentially connected loads were placed at each end of the transmission line to properly model the signal losses from various loads present on the line (transformers, BPL modems). This power would not be radiated, so must be accounted for in the model. This also allows the software to calculate the relative efficiency of feeding the system at different points by comparing the power fed to the system and the power that reaches the load, simulating a BPL system modem or repeater. These loads are 50-j0 ohms. 2.3 Two amateur receive antennas are also included in the model. Antenna 1 is a half-wave dipole located 10 meters above ground, at the height of the power line, typical of many amateur tree-mounted antennas. This antenna is 30 meters distant from the line. Antenna 2 is a half-wave dipole located 30 meters above ground, 30 meters diagonally from the line.

The height of this antenna is representative of taller amateur tower installations. Each of these antennas has a 50-j0 ohm load in the center and EZNEC is used to calculate the power that reaches each load by radiation.

Figure 1: This is a pictorial of the model used by ARRL to calculate differences in the performance of BPL systems fed in different ways. Point 1 = Amateur half-wave dipole antenna, 10 meters high, 30 meters from line. Point 2 = Half-wave dipole antenna, 30 meters high, 30 meters diagonally from line. Point 6 = Single-phase differential “dipole” feed point. Points 7 and 8 = Two phase differential feed or load, as specified in Tables. Point 9 = Ground wire, fed where it connects to the phase. Point 10= Earth ground radials (4). 3. Results 3.1 The results of the modeling are shown in Tables 1 and 2.