A new method is proposed in this paper for reducing the MF broadcast wave induction field on overhead power transmission lines during maintenance and inspection work of the line. Power transmission usually has to be stopped in the circuit being worked on, and the conductors are grounded to the steel towers at both ends of the worked section of the line to prevent electric shocks that may be caused by the commercial frequency induction field induced by the current running through the transmission circuit. In these situations, a very strong RF induction field is sometimes observed in the circuit undergoing maintenance work when a high power MF broadcast antenna is located near the transmission line. It has been found that this strong RF induction is caused by the resonance of one or two wavelengths in the closed loop circuit consisting of the conductors and the steel towers (including the ground), and that the strong induction due to the MF field can be avoided by inserting induction coils of appropriate values between the conductors and the steel towers. In this paper, a simple alternative method for reducing the MF induction field by carefully selecting appropriate towers for the grounding is proposed. In this method, the two towers to be grounded are chosen from among the four towers adjacent to the towers that are being worked on. By selecting the correct two towers to be grounded we can ensure that the resonance frequency does not correspond with the frequency of the broadcast wave, and we demonstrate that the RF induction field can be considerably reduced.

For emission from a printed circuit board (PCB) by the common-mode current, the suppression method based on the image theory by placing a conducting plate under the PCB is presented. In order to evaluate the suppression effect by this method the radiation power from the PCB is calculated by using FDTD method. The numerical results show that placing the conducting plate suppresses the emission by the common-mode current. Especially, using the conducting plate bent the sides, it is possible to suppress the emission by the small conducting plate. Further, the experimental results of a radiation power and a maximum electric field intensity show the validity of the numerical results.

In this paper, we report the detailed investigation of novel printed disc monopole antennas for ultra-wideband (UWB) applications focusing on miniaturization of the disc radiator. First, the basic property was examined for the case of a circular disc with diameter of 50 mm, and it was found that the VSWR is less than 2 in the UWB band of 3.1-10.6 GHz when the feed gap length is between about -0.1 and 0.2 mm. Next, in order to reduce the size of the disc radiator, various dimensions of elliptical discs were investigated. It is shown that if the dimensions of the elliptical disc are chosen appropriately, a smaller disc size antenna can be achieved. To decrease the antenna size further, a triangular notch and an exponentially curved notch on the ground plane of the antenna were examined. It is observed that the use of the notched ground is very effective and that the diameter of the circular radiator can be reduced to 17 mm. The proposed antenna has an omnidirectional pattern in the x-y plane. The influence of the notch on the radiation pattern is very small. Details of the simulation results using the FDTD method and experimental results for the proposed antenna are presented and analyzed. These features are very attractive for UWB applications.

This paper focuses on the S-parameter method that is a basic method for measuring the input impedance of balanced-fed antennas. The basic concept of the method is summarized using the two-port network, and it is shown that the method can be enhanced to the unbalanced antennas using a formulation based on incident and reflected waves. The compensation method that eliminates the influence of a measurement jig and the application of the S-parameter method for the measurement of a radiation pattern with reduced unbalanced currents are explained. Further, application of the method for measuring the reflection and coupling coefficients of multiple antennas is introduced. The measured results of the input impedance of a dipole antenna, radiation patterns of a helical antenna on a small housing, and S-parameters of multiple antennas on a small housing are examined, and the measured results obtained with the S-parameter method are verified.