Theoretical analyses are carried out on the height dependence of the antenna factor of an EMI antenna to develop an antenna calibration method that can provide the free-space value of the antenna factor. It is found that the antenna factor in general varies with the antenna height in a quasi-periodic way with a period of about λ/2. Thus, the present paper proposes to take an average of the antenna factors over a height range of about λ/2 to obtain an accurate estimate of the free-space antenna factor. Effective antenna arrangements are also proposed for the antenna calibration. Deviations in the estimate from the free-space antenna factor are less than 0.1 dB for tuned dipoles in the frequency range above 50 MHz. But the errors increase up to 0.3 dB at about 35 MHz. For broadband antennas, the free-space antenna factor can be accurately estimated by taking the average of the antenna factors. Errors are estimated to be less than 0.3 dB in the frequency range from 30 MHz to 1000 MHz.

Electromagnetic noise radiated from microwave ovens may cause serious interference problems in wireless systems using the 2.4-GHz band. Since oven noise waveforms show strong dependence on the frequency selectivity of the receiver filters, the effect of band limitation on the interfering oven noise is an important issue for evaluating or comparing the performance degradation of wireless systems subject to interference. To understand these effects, theoretical and experimental investigations of the waveform, peak amplitude, and pulse width of band-limited oven noise are carried out. It is found that the peak amplitude of the received noise changes with the bandwidth in a way very similar to the case of a simple RF tone-burst input. The pulse width of the received noise also changes with the receiver bandwidth but takes a minimum value at a certain receiver bandwidth, which is an essential feature of band-limited microwave oven noise. In addition, an appropriate resolution bandwidth is determined for using a spectrum analyzer to obtain accurate oven noise parameters.

This paper reviews recent developments in small-sized broadband antennas for EMI measurements, especially in the microwave frequency region. Transient EMI measurements are also discussed by introducing complex antenna factors and conversion of frequency-domain data into time-domain data. This paper also focuses on considerable improvements achieved in calibration techniques for conventional EMI antennas in VHF/UHF bands.

Concerned is a spectral profile of electromagnetic (EM) emission from a signal line on a high-speed digital circuit. The authors have proposed and examined an a priori method to predict the peak frequencies on spectral profile of EM emission from printed circuit boards (PCBs). Profile of an EM spectrum is determined by the resonance of digital circuits. It is the purpose of this paper to investigate the parameters that determine the spectral profile of EM emission from a signal line on a PCS. In this paper, measurements and calculations of EM spectra were carried out for different load capacitances. EM emissions were measured with a small loop antenna at a 50mm from the surface of the PCB. Measured EM spectra had two peaks. Calculated EM spectra, which was based on transient current given by the analog simulator SPICE, had two peaks too. Results of calculations of EM spectra for different internal capacitances of an IC tell that lower peak frequency is determined by the resonance frequency of the resonant loop which is composed of an IC package and a decoupling capacitor. Comparison with measured EM spectra and calculated EM spectra for different load resistances tell that sharpness of the other peak depends on Q factor of a resonant loop which includes a signal line. Therefore the peak frequencies of EM emission spectrum can be predicted as two resonance frequencies of two resonant circuits.

The antenna factor measurement of the dipole antennas for electromagnetic interference (EMI) measurements is studied theoretically and experimentally. The 3-antenna method is applied to near-field. Near-field transmission characteristics between the transmitting and receiving dipole antennas is obtained by using the electromotive force (EMF) method, where sinusoidal current distributions are assumed. It is shown that the antenna factors can be measured from transmission values between two antennas and near-field correction factors at any height of each antenna.

Theoretical and actual experimental investigations are made on a Loop Antenna System (LAS) consisting of N-gap loop antennas. A general formula for the LAS response to an external EM field is derived by using both the method of moments and techniques for transmission-line analysis. Numerical evaluation verifies that the LAS has favorable characteristics, such as invariable response within a wide frequency range. In addition, differences between the LAS method and the present open-field method in the results of measurement are evaluated. It should be noted that the LAS gives a much lower output for an electrical source than for a magnetic source, even if both sources produce the same value with the open-field method. It is concluded that conversion factors for the results from the LAS method to the open-field method strongly depend on the orientation of the source as well as its characteristics. Consequently, new conversion factors are proposed to obtain practical EMI measurements.

It is important to develop methods of measuring radiated electromagnetic interference level that will produce identical results at all measuring locations. We have considered a number of problems which prevent the achievement of identical results, and proposed some solutions. However, agreement of measurement values adequate for practical purposes has not been achieved. After our successive studies, we finally became aware that there is a causal relationship with changes in the line-to-ground impedance of the power supply. It is presumed that power cables of AC-powered devices operate as antenna elements that produce emission. Thus changes in the power line-to-ground impedance cause variations in the radiation efficiency to produce a different EMI level. We therefore made plans to measure the values of line-to-ground impedance at the AC power outlet for the frequency range of 100kHz to 500MHz at various locations where measurements are made of EMI from EUT (Equipment Under Test). The impedance varies greatly between 6ohms and 2 k-ohm, not only according to the frequency, but also according to the measurement location. In such cases, the EMI level shows a different value even with the same EUT, and it usually increases-especially for vertical polarization. We have developed a new type of LISN (Line Impedance Stabilization Network or Artificial Mains Network) to stabilize the power line-to-ground impedance to get consistent measurement conditions. The LISN consists of feed-through capacitors and an disk type RF resistor. The measurements confirm the consistency in the impedance value which is maintained at 50 ohms in the frequency range from 1MHz to 500MHz. Thus the newly developed LISN improves consistency of measurement values at all locations, while it was difficult to obtain good correlation before employing the LISN. We feel confident that incorporation of the method discussed here in the pertinent technical standards of EMI measurements, such as CISPR, would lead to a major improvement in getting consistent measurements values.