Govind R. Kadambi

Polarization diversity measurements and analysis for antenna configurations at 1800 MHz

In wireless communication systems, multipath interference has a significant impact on system design and performance. Fast fading variations are caused by the coherent summation of multiple echoes from many reflection points reaching the receive antenna. Antenna diversity is one technique that can be used to overcome multipath fading. A test system used to measure the diversity performance of an antenna pair was used to experimentally determine the complex correlation coefficient between the two antenna branches. A local mean estimation algorithm based on the channel mean square error equalization was implemented. Thus, the two parameters that determine the expected diversity gain, i.e., the complex correlation coefficient and the mean level signal difference, were estimated. The test system was used to evaluate the polarization diversity performance of different antenna pairs in Rayleigh and Rician environments, both in the absence and in the presence of a human head phantom.

Electromagnetic coupling effects on the cavity measurement of antenna efficiency

Electromagnetic coupling effects on the antenna in a conducting cavity are studied theoretically and experimentally. It is observed in experiments that at the resonant frequencies of the cavity, the input resistance of the antenna attains values two or three orders of magnitude higher than that at frequencies away from resonance. It is shown via theoretical analysis that the input resistance of the antenna measured at the resonant frequencies of the cavity is not merely the loss resistance desired in computing the antenna efficiency, but is actually the sum of the loss resistance of the antenna and the coupling resistance between the antenna and cavity. This coupling effect is demonstrated quantitatively by numerical computations for dipole and monopole antennas. The computational results for the input resistance are in agreement with the measured data. A method is proposed to avoid the cavity-antenna antiresonance in the measurement.

On Wheeler's method for efficiency measurement of small antennas

The Wheeler cap method is commonly used to measure the efficiency of small antennas. This method is based on the assumption that the spherical cap does not essentially disturb the near fields surrounding the antenna. Although this method has been proved in experiments, the validity of this assumption has not been shown rigorously via field analysis. It is demonstrated, by dimensional and scaling analysis, that the power loss of a small antenna in a Wheeler cap, as well as in a certain variety of conducting cavities, is essentially the same as that of the antenna placed in free space. Consequently, one can measure the loss resistance with the cap in place.

Antenna-cavity coupling effects on measurement of antenna efficiency

The coupling between the cavity and the antenna is analyzed by placing a small antenna inside the cavity. It is seen that at the resonant frequencies of the cavity, the input resistances of the antenna attains values two or three orders of magnitude higher than that at frequencies removed from resonance. A field-circuit equivalence of this coupling system is derived using full wave analysis. We show that the input resistance of the antenna measured at resonant frequencies of the cavity is not merely the loss resistance desired in determining the antenna efficiency but is actually the sum of the loss resistance of the antenna and the coupling resistance of the cavity. The results of numerical computations are in reasonable agreement with measurements for a short wire antenna.

Equivalent Circuit for Antenna-Cavity Coupling in Wheeler's Cap Technique

An equivalent circuit is derived for an antenna coupled to a shielding cavity via field analysis. The input impedance of the antenna inside the cavity is then studied theoretically and experimentally. It is shown that the input resistance of the antenna shielded by the cavity is not merely the loss resistance, but is actually the sum of the antenna loss resistance and the coupling resistance between the antenna and cavity. This coupling effect is demonstrated quantitatively by numerical computations for dipole antennas. A method is proposed to avoid the antenna-cavity antiresonance in the measurement of loss resistance of the antenna.

Mobile station polarization diversity reception for handheld devices at 1.8 GHz

The paper describes a new approach for reliably obtaining the cross-correlation properties for two orthogonally polarized dipole antennas. The complex correlation coefficient, computing the two degrees of freedom of the signal, i.e., amplitude and phase, is the parameter that relates the cross-correlation statistics of two signals. In order to mitigate the effects of a particular environment, the amplitude and phase local means are extracted. It was found that normalizing with the local mean significantly modifies the cross-correlation function; thus a novel algorithm for amplitude local mean extraction, based on adaptive filtering, was developed.