Modeling of VHF Current Conversion From Excited Antenna Mode to Differential Mode at Transmission Line Terminals

Abstract

The current induced by an incident electric field can convert from the antenna mode to differential mode at unbalanced terminals of transmission lines (TLs), causing severe interference to a number of applications such as broadband power line communication, especially in the very high frequency (VHF) band. The previous work on modeling the current conversion is not applicable to VHF. We present a new model of the VHF current conversion which includes a general formula for the antenna mode characteristic impedance and two solutions to the formulated problem: 1) a numerical solution referred to as the antenna theory numerical (ATN) approach, which gives the exact value of the characteristic impedance; 2) an analytical solution referred to as the enhanced TL approximation (ETLA) approach, which gives the mean of the characteristic impedance. This is the first reported work to obtain the antenna mode characteristic impedance by the antenna theory. Furthermore, the ETLA approach outperforms the previous frequency-independent solution and requires a reduced complexity over the ATN approach. Our model is general as it converges to the previous model at lower frequencies. Simulation results also show the relationship between the antenna mode characteristic impedance and the power of the interference caused by the current conversion.Index Terms-Antenna theory, electric field effects, electromagnetic interference (EMI), transmission line modeling, transmission line theory, VHF radio communication.