MUF and LUF, Bob, Week 15

Today's topic covers a set of General Class questions regarding MUF and LUF. For those that have the General Class Exam Question Pool, they are questions G3B01-G3B12.

The reason I am using the abbreviations at first is that two of the questions ask you to identify what MUF and LUF stand for. MUF stands for Maximum Usable Frequency and LUF stands for Lowest Usable Frequency. More specifically, they define the maximum (highest) and lowest frequencies that can be used to communicate between two stations. These terms refer to atmospheric conditions so it is assumed that we are talking about communications requiring skip. We don't talk about MUF or LUF for direct point to point communications. In the process of discussing MUF and LUF we are going to sneak in a bit of general radio wave propagation theory so hang on to your hats. Here we go.

When we talk about skip or skywave communication you will often hear that the radio wave was reflected back to earth or "bounced" back. These are handy terms to use but are not exactly accurate. What really happens is that the radio wave is refracted, which is to say bent or curved. To use an analogy with beams of light, the atmosphere does not act like a mirror, but instead acts like a lens or a prism. This is important to know for MUF because this bending of the radio transmission changes with its frequency. That is, the higher the frequency, the less the radio beam is bent. The MUF can be thought of as the frequency that just barely gets curved enough to make it back to earth. If the frequency is a little higher the transmission would still be bent but would not quite be bent enough to come back down and would go off at an angle that would just miss the surface of the earth and keep going into space.

The LUF has less to do with the bending of the signal as it does the absorption of the signal. This is not such a hard limit as the MUF since what you are measuring is how much you can have the signal be absorbed and still be readable above the noise floor. Mode of operation figures into this as well since, as we know, some modes like CW can be understood much further down in the noise than others like SSB. This absorption is a fairly continuous thing, that is to say, the lower the frequency the more the absorption. So the best, least absorbed, transmission occurs just below the MUF.

Since we want to be as close as we can to the MUF without going over, a bit like the game of 21, this is the frequency most often reported. So let's talk a bit about what affects this frequency and how it is measured. The MUF is determined by the characteristics of the ionosphere and by the location of the two stations. The ionosphere is not a simple static layer. It is constantly changing although much of the time the changes are fairly predictable and not great over small periods of time. The sun has the biggest effect on the ionosphere. At sunrise the ionosphere changes such that the MUF goes up significantly and quickly and stays high throughout the daylight hours. The MUF then gradually declines throughout the night, reaching it's lowest point just before dawn. Anything that affects the ionosphere, affects the MUF, so solar flares can significantly change your operating characteristics.

In order to be sure you aren't losing some of your transmission by straying over the MUF as it changes, you actually want to be a bit below the MUF for reliable working conditions. In one reference I found they defined the Optimum Working Frequency (OWF) as 85% to 90% of the MUF. In the same reference they also differentiate between the Operational MUF and the Basic MUF. The Basic MUF is more of a theoretical value while the Operational MUF takes into account the antennas, operating mode, power, etc.

Let's return to one of the points I made above, namely that the MUF depends on the locations of the two points. While this is true, this statement refers to changes in the MUF over great distances. You can be pretty confident, for example that the MUF between Seattle and Chicago is about the same as between Olympia and Chicago. But going around the globe the MUF varies by quite a bit and also, of course varies over time. In one of the references below is a near real-time map of the MUF around the world. But ultimately the MUF for you to communicate with another station is specific to your particular circumstances.

Finally, how do you find out what the MUF is? Until very recently, the best way to tell was to listen to beacons at various frequencies and locations to see if you could hear them. This is still a perfectly good way to do it but now there are many online sources to get up-to-the-minute values for MUF and there are also many programs for predicting it. The only web sites I know about are the ones I used to research this lesson and I haven't looked into any of the programs at all. Maybe there are others on frequency that can share some of the web sites or programs they use.

A near real-time MUF map of the world.

OWF and Operational vs. Basic MUF