When you think of higher frequencies like 100MHz it would seem hard to build a spectrum analyzer. This would require some calculus to do this is in regular way. Special hardware may cost decent amount of money. But Alexandru (aka. OZ9AEC) demonstrates that this can be achieved with minimal investment in hobby level. All you need is a Beaglebone, graphical LCD and RTL2832U Based DVB-T USB dongle (rtl-sdr).
Of course you cannot expect precision with such tool, but it may serve purpose in many cases.