impact of the loss tangent................

the loss tangent represents the loss in the dielectric

so if the loss tangent of the dielectric is high, the antenna efficiecny would be low as the losses are high

on the other hand if the loss tangent is low this will enhance the antenna performance in terms of GAIN & efficiency

loss tangent is the ratio of imag to real part of complex permittivity, It represents loss in the form of conductivity so high loss means the susbtrate is conductive instead of perfect insulator this will reduce the efficiency of any microwave structure whether its antenna or filter etc.