Navigation Radios - General Discussion
Before we get into the meat of navigation radios, there are few general things we should know about them. First off, the easiest way to understand how a radio station works is to imagine a light bulb. The light bulb, of course, emits visual beams of light, whereas the radio station emits radio signals...but the mental concept is exactly the same.
So let's imagine the light bulb in the center of an open football field, and it's not much of a reach to understand that anyone in the field --no matter how many people, or where they are standing-- will be able to see the light, correct? If we're all pointing in the same direction, the the light would strike us from different angles (directions), but the light would strike us all, nonetheless. The exact same thing holds true for certain types of navigation radio systems, primarily the ADF/NDB system...where NDB means Non Directional Beacon. That's an oxymoron, because NON, in this case, actually means ANY or ALL...the NDB sends out signals in ALL directions at the same time, just like a light bulb does with light beams. In the picture below, every aircraft within range of the radio station is able to "see" the radio signal.
Now, let's build a wall around our light bulb, and then we'll go poke a very tiny hole in the East side of the wall. As you'd imagine, the only way we'll ever see light from the bulb is if we align ourselves perfectly with the hole in the wall and the light bulb, correct? Correct, and that's just peachy if I only want to see the light from one single direction, but that's not the case: I'd like to be able to see that light from any direction I choose...not all directions at once, but any specific direction. Meaning, I'd like the ability to move the hole... and we'll call this a radial.
To see this more clearly, let's go back and give our wall the ability to rotate very fast and continuously. In addition to that, we'll change light bulbs to a different color every time the wall rotates one degree, and we'll maintain the same color scheme with every full rotation. Eventually, we'll learn that "green" is the 270-degree radial, and if I want to see it (or fly toward it), first I must align myself with that 270-degree radial, and then turn to a heading of 270 degrees. And there you have a clear mental picture of precisely how a Variable Omnidirectional Radio (VOR) system works; a unique Radial signal is transmitted by the station for each degree of the compass...the aircraft radio is able to interpret which one it is seeing.
Another important factor to understand about radio navigation systems is that they operate in the VHF (Very High Frequency - 108.00-117.95 Megahertz) range, and in the LF range (Low Frequency - 550-1700 Kilohertz, also the AM radio band) Thus, navigation radios are limited to "line-of-sight" capabilities...meaning, the receiver in the aircraft must be able to "see" the transmitter, or nearly so. Given this knowledge, we can assume correctly that radios positioned in the flatlands will have a far greater range than those nestled in between mountains and valleys.
Along with this, I'll point out that navigation radio systems (with exception to the ILS Glideslope system, which we'll discuss later on) are pretty much impervious to elevation (altitude), unless your elevation interferes with the line-of-sight between aircraft and radio station. Meaning...an ADF or VOR signal will tell you the exact same thing whether you are at an altitude of 5,000 feet, 8,000 feet, or 2,000 feet, as long as you are within range of the station and no serous obstructions are between you and the tower.