Waveguiding mechanism and modal characteristics of hollow core fibers consisting of a single or a regular arrangement of dielectric tubes are investigated. These fibers have been recently proposed as low loss, broadband THz waveguides. By starting from a description in terms of coupling between air and dielectric modes in a single tube waveguide, a simple and useful model is proposed and numerically validated. It is able to predict dispersion curves, high and low loss spectral regions, and the conditions to ensure the existence of low loss regions. In addition, it allows a better understanding of the role of the geometrical parameters and of the dielectric refractive index. The model is then applied to improve the tradeoff between low loss and effectively single mode propagation, showing that the best results are obtained with a heptagonal arrangement of the tubes.