Complex sentence: Tremendously high frequency a frequency band so high that it's hard to consider it as a radio band, however it's used by some authors to designate the decade band next to EHF in increasing frequency order.
Split into simpler sentences: Tremendously high frequency is a frequency band so high that it is hard to consider it as a radio band. It is however used by some authors to designate the decade band next to EHF in increasing frequency order.