Properties of Radio Waves — Page 10, Lesson 13

Let’s start with the big picture. The radio part of the electromagnetic spectrum runs from 3 kHz all the way up to 300 GHz. That’s a huge range, so for convenience we split it into eight frequency bands. And here’s the neat part — each band is related to its neighbour by a factor of 10. So if one band starts at 3 kHz, the next starts at 30 kHz, then 300 kHz, then 3 MHz, and so on. That factor-of-10 relationship holds all the way up. Now, let’s walk through the bands in order, because each one has a name, a frequency range, a wavelength range, and a specific use in civil aviation. First is Very Low Frequency, or VLF. That runs from 3 to 30 kHz, with wavelengths from 100 km down to 10 km. In civil aviation, its use is listed as nil — nothing uses it. Next is Low Frequency, LF, from 30 to 300 kHz, wavelengths 10 km to 1 km. This is used for NDB/ADF — that’s Non-Directional Beacon and Automatic Direction Finder. Then Medium Frequency, MF, from 300 to 3000 kHz, wavelengths 1000 m to 100 m. This is used for NDB/ADF again, plus long-range communications. High Frequency, HF, from 3 to 30 MHz, wavelengths 100 m to 10 m. This is used for long-range communications. Very High Frequency, VHF, from 30 to 300 MHz, wavelengths 10 m to 1 m. This is used for short-range communication, VDF (VHF Direction Finding), VOR (VHF Omnidirectional Range), the ILS localizer, and marker beacons. Ultra High Frequency, UHF, from 300 to 3000 MHz, wavelengths 100 cm to 10 cm. This is used for the ILS glide path, DME (Distance Measuring Equipment), SSR (Secondary Surveillance Radar), satellite communications, GNSS (Global Navigation Satellite System), and long-range radars. Super High Frequency, SHF, from 3 to 30 GHz, wavelengths 10 cm to 1 cm. This is used for RADALT (radar altimeter), AWR (Airborne Weather Radar), MLS (Microwave Landing System), and short-range radars. Finally, Extremely High Frequency, EHF, from 30 to 300 GHz, wavelengths 10 mm to 1 mm. And like VLF, its civil aviation use is nil. So that’s the full spectrum — eight bands, each a factor of 10 apart, each with its own job. Now let’s move to the second big idea in this section: phase comparison. Some radio navigation systems work by comparing the phase of two signals to define navigational information. There are two critical rules here. First, the two signals being compared must have the same frequency — otherwise any phase comparison is meaningless. Second, one signal is designated the reference signal and the other the variable signal, and the comparison must yield a positive result. To find the phase difference between two signals, you first identify the position of, say, zero phase on each wave. Then you move in the positive direction from the chosen point on the reference wave, and 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 from the figure. Starting at zero phase on the reference wave — call that 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 to get the same result. At the origin, the phase of the reference wave is 0°, which is the same as 360°. The phase of the variable wave is 090°. You 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°. And note — the result must always be positive. So if your calculation ever yields a negative result, you simply add 360° to get a positive answer. That’s the whole idea: same frequency, one reference and one variable signal, and a phase difference that’s always expressed as a positive angle.