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Sporadic E or Es can allow VHF signals (30-300 MHz) to propagate over long distances via a reflection from the E layer. Sporadic E is generally classified into three types by, Mid-latitude, Equatorial and Auroral. Sporadic E or Es (E Skip) can allow VHF signals (30-300 MHz) to propagate over long distances via a reflection from the E layer. Sporadic E is generally classified into three types by, Mid-latitude, Equatorial and Auroral. From the month of May through to August is best for Sporadic E (Es) which can affect signals on all bands from 14-144MHz, although it is most commonly noticed on 28MHz and 50MHz. Openings for 2m may happen twice a year. The cause of Es ionization is not precisely known. Maybe it has to do with wind shear in the upper atmosphere or storm activity. Sporadic E is easily detected by monitoring 28MHz and 50MHz. === MUF ===Maximum Usable Frequency. Operational MUF is the highest frequency you can operate with your specific working conditions and equipment to communicate with another specific station. MUF is dependent on ionospheric radio propagation. Frequencies including the Critical Frequency; Lowest Usable Frequency, LUF; Maximum usable frequency, MUF; and the Optimum Working Frequency, OWF are all relevant when determining which frequencies will provide the best performance for a short wave radio, and HF 2-way radio communications. Know your international / DX beacons that you can tune to so that you can determine the current MUF.The MUF is a moving target. It depends on ionization with the factors mentioned previously. Above that MUF threshold the RF is lost to space. Just below it waves are subject to scatter effect.The MUF is typically in the HF range. It is infrequent that the MUF will elevate up to the VHF range, and rare for it to be in the UHF range.