Properties of Radio Waves — Page 10, Lesson 13

Right, let's get into the properties of radio waves. We're starting with the frequency bands, which is the foundation for everything else in radio navigation. The radio part of the electromagnetic spectrum extends from 3 kHz to 300 GHz. That's a huge range, so for convenience it's divided into 8 frequency bands. Each band is related to its neighbour by a factor of 10 — so each band is ten times the frequency of the one below it. Let's go through them. First, Very Low Frequency, VLF, from 3 to 30 kHz, with wavelengths of 100 to 10 km. Civil aeronautical usage: nil. Next, Low Frequency, LF, 30 to 300 kHz, wavelengths 10 to 1 km. This is used for NDB and ADF — that's Non-Directional Beacon and Automatic Direction Finder. Then Medium Frequency, MF, 300 to 3000 kHz, wavelengths 1000 to 100 m. Used for NDB/ADF and long-range communications. High Frequency, HF, 3 to 30 MHz, wavelengths 100 to 10 m, used for long-range communications. Then we get to the ones you'll use most. Very High Frequency, VHF, 30 to 300 MHz, wavelengths 10 to 1 m. Used for short-range communication, VDF, VOR, ILS localizer, and marker beacons. Ultra High Frequency, UHF, 300 to 3000 MHz, wavelengths 100 to 10 cm. Used for ILS glide path, DME, SSR, satellite communications, GNSS, and long-range radars. Super High Frequency, SHF, 3 to 30 GHz, wavelengths 10 to 1 cm. Used for RADALT, AWR, MLS, and short-range radars. And finally Extremely High Frequency, EHF, 30 to 300 GHz, wavelengths 10 to 1 mm. Civil aeronautical usage: nil. Now, the key point here is the inverse relationship between frequency and wavelength — as frequency goes up, wavelength comes down. That's why VLF has 100 km wavelengths and EHF has 1 mm. Now let's move to phase comparison. Some radio navigation systems compare the phase between two signals to define navigational information. The first critical point: the two signals being compared must have the same frequency. If they don't, any phase comparison is meaningless. The second point: one signal is designated the reference signal, the other a variable signal, and the comparison must yield a positive result. To determine the phase difference between two signals, first identify the position of, for example, zero phase on each wave. Then move in the positive direction from the chosen point on the reference wave to measure the phase angle through which the reference wave has travelled before zero phase is reached on the variable wave. Let me give you the example. Starting at zero phase on the reference wave, point A, the reference wave travels through a phase angle of 270° before zero phase is reached on the variable wave, point B. So the phase difference is 270°. There's also a mathematical way. At the origin, the phase of the reference wave is 0°, which equals 360°, and the phase of the variable wave is 90°. Subtract the instantaneous phase of the variable wave from the instantaneous phase of the reference wave. So reference minus variable equals 360° minus 90°, which gives 270°. Same result. One important note: the phase difference must always be positive. So if your calculation yields a negative result, simply add 360° to get a positive answer. Let me show you this on the diagram. So the core idea is: phase comparison is how some navigation systems work, and the rule is same frequency, one reference and one variable signal, and always a positive result.