This paper is concerned with the analysis of phase gratings as passive quasi-optical multiplexing devices. One important application of such components is in the local oscillator injection chain of heterodyne array receivers. Gaussian beam mode analysis can be applied as a powerful tool when modelling the optical performance of phase gratings in a real submillimeter system of finite throughput and bandwidth. In our experimental investigations we have concentrated on the Dammann Grating (DG) which is a binary optical component and thus straightforward to manufacture. A number of quartz gratings were fabricated and carefully tested to evaluate the practical limitations of such quasi-optical components. Because of its convenient refractive index quartz can be used to produce gratings with very low reflection losses. The results presented confirm DGs to be particularly suitable multiplexers for sparse arrays of finite bandwidth.