At the attenuators were used so far, the input and output impedances are equal. However if different impedances must be adapted somehow, it can be also used resistors for it. The calculation of the loss arising inevitably with it, shows this webpage. Since no coils or capacitors are used, the matching takes place frequency-independently. Attenuators are suitable therby for the wide-band matching.
If you want to know, how looks the equations used into the script, then see at the mathematical background: loss, resistors.