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Lam, P. T.

Publications and source records attributed to Lam, P. T..

7 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Compensation of relector antenna surface distortion using an array feed

The dimensional stability of the surface of a large reflector antenna is important when high gain or low sidelobe performance is desired. If the surface is distorted due to thermal or structural reasons, antenna performance can be improved through the use of an array feed. The design of the array feed and its relation to the surface distortion are examined. The sensitivity of antenna performance to changing surface parameters for fixed feed array geometries is also studied. This allows determination of the limits of usefulness for feed array compensation.

Cherrette, A. R.↗
NASA NTRS1986

Near-field spillover from a subreflector: Theory and experiment

In a dual reflector antenna, the spillover from the subreflector is important in determining the accuracy of near-field measurements. This is especially so when some of the feed elements are placed far away from the focus. In this paper, we present a high-frequency GTD analysis of the spillover field over a plane just behind the subreflector. Special attention is given to the field near the incident shadow boundary and the role played by the slope diffraction term. Our computations are in excellent agreement with experimental results.

Lee, S. W.↗
NASA NTRS1986

Near-field spillover from a subreflector: Theory and experiment

In a dual reflector antenna, the spillover from the subreflector is important in determining the accuracy of near-field measurements. This is especially so when some of the feed elements are placed far away from the focus. A high-frequency GTD analysis of the spillover field over a plane just behind the subreflector is presented. Special attention is given to the field near the incident shadow boundary and the role played by the slope diffraction term. Computations are in excellent agreement with experimental results.

Lee, S. W.↗
NASA NTRS1986

Strategy for reflector pattern calculation - Let the computer do the work

Using high frequency approximations, the secondary pattern of a reflector antenna can be calculated by numerically evaluating a radiation integral I(u,v). In recent years, tremendous effort has been expended to reducing I(u,v) to Fourier integrals. These reduction schemes are invariably reflector geometry dependent. Hence, different analyses/computer software development must be carried out for different reflector shapes/boundaries. It is pointed out, that, as the computer power improves, these reduction schemes are no longer necessary. Comparable accuracy and computation time can be achieved by evaluating I(u,v) by a brute force FFT described in this note. Furthermore, there is virtually no restriction on the reflector geometry by using the brute force FFT.

Lam, P. T.↗
NASA NTRS1985

Directivity optimization of a reflector antenna with cluster feeds - A closed-form solution

The directivity of a reflector antenna deteriorates as the feed moves away from the focal point for beam scanning. This deterioration can be substantially reduced if a cluster feed instead of single feed is used to control a beam. A closed-form solution is presented for the cluster excitation to achieve the optimum directivity. For an offset 108 lambda parabolic reflector scanning 10 beamwidths, the optimum directivity achieved by a 19-element (seven-element) cluster is 12 dB (8 dB) higher than that of a single element. Comparison of the optimum-directivity design and the popular conjugate field matching design is made. When the cluster spacing d is greater than 1 lambda, it is found that the optimum directivity is higher than that of conjugate field matching (CFM) scheme by an insignificant amount, although the excitations of two designs can be quite different. For d less than 0.5 lambda, the optimum design may exhibit the supergain phenomenon, namely extremely high directivities achieved by an oscillatory cluster excitation.

Lam, P. T.↗
NASA NTRS1985

Strategy for reflector pattern calculation: Let the computer do the work

Using high frequency approximations, the secondary pattern of a reflector antenna can be calculated by numerically evaluating a radiation integral I(u,v). In recent years, tremendous effort has been expended to reducing I(u,v) to Fourier integrals. These reduction schemes are invariably reflector geometry dependent. Hence, different analyses/computer software development must be carried out for different reflector shapes/boundaries. it is pointed out, that, as the computer power improves, these reduction schemes are no longer necessary. Comparable accuracy and computation time can be achieved by evaluating I(u,v) by a brute force FFT described in this note. Furthermore, there is virtually no restriction on the reflector geometry by using the brute force FFT.

Lam, P. T.↗