Let’s do a level set on basic HF radio propagation through the ionosphere and around the world.
As you know, HF or high frequencies are that sliver of electromagnetic spectrum between 3-30 MHz. Our ionosphere is a sliver of our atmosphere between 60-500 kilometers above Earth surface. And, these two slivers is where the magic happens.
Every day, solar radiation heats the ionosphere, creating a plasma which refracts radio waves. As we try to communicate, we are bracketed by two parameters, MUF and LUF. Both are driven by sunlight.
Maximum Useable Frequency tells us which radio signals will bend over and return to earth, which is what we want. MUF is a function of the critical frequency of each ionosphere layer. In turn, the fc is the natural frequency of the plasma in each layer of the ionosphere.
On the other hand, Lowest Useable Frequency tells us the point below which signals will be absorbed (and disappear) in the ionosphere. In a quiet geomagnetic period, both MUF and LUF are driven by solar radiation and peak during the middle of the day.
Because these parameters vary across the earth’s surface, the propagation window between MUF and LUF is different for each transmission path.
Basic HF Radio Propagation in the Ionosphere
MUF relies on the higher E and F layers to refract signals. LUF is driven by the lower D layer, which basically absorbs signals.
You may have heard of Solar Flux, which is a measurement of our sun’s ionizing radiation, mostly ultraviolet. As our SF increases, our ionosphere becomes more dense. At higher E and F layers, increased density means more refraction at higher frequencies. But, at lower D layer, increased density means more absorption at higher frequencies. More on this later.
How do we determine the critical frequency fc of each layer? Simple. We transmit a signal straight up, by sweeping an ionosonde transmitter across all of HF. We listen for reflections, like radar. When the reflections stop coming back, we have found fc.