There is this thing called LUF (Lowest Usable Frequency). Not much in ham
literature about it but from Bill Orr's Radio Handbook it depends on
transmitter erp and atmospheric noise. Generally speaking the LUF can be
lowered (at HF) by approximately 2MHz for each 10dB increase in erp.
However it is obvious that the LUF changes throughout the day. Does it
follow the MUF? If it does then it will follow the sunspot cycle.
Are there any average graphs of LUF?
My understanding is that LUF and MUF are purely F-layer functions, so are
not relevant to us if you accept Alan's assumption that sky-wave propagation
is by D-layer with perhaps some E-layer. Also, is the MUF really measured? It
will change for every path, and for prediction purposes is usually calculated
from the Critical Frequency (ie the point at which the signals go straight
through the ionosphere). Perhaps LUF is also just a calculation.
Mike, G3XDV (IO91VT)