7.5 Radio Waves, Frequency Bands and Propagation
Key Takeaways
- The aviation-relevant bands run MF, HF, VHF and UHF; VHF and UHF are line-of-sight while HF can refract off the ionosphere for long-range work.
- Lower frequencies penetrate obstacles better but carry less data; higher frequencies carry more data but are blocked more easily.
- Terrain, buildings, ionospheric conditions, sunspot activity, electrical equipment and thunderstorms all affect propagation.
- A pre-flight range test confirms the C2 link at the intended geometry, and is the only reliable way to know your effective range at a given site.
Radio Waves and Their Characteristics
A radio wave is electromagnetic energy travelling at the speed of light. Three properties describe it:
- Frequency — cycles per second, in hertz. Aviation and RPAS frequencies run from hundreds of kilohertz to several gigahertz.
- Wavelength — the physical distance of one cycle, inversely proportional to frequency: where c is the speed of light. A 121.5 MHz signal has a wavelength of about 2.5 m; a 5.8 GHz signal about 5 cm. This is why an aviation VHF antenna is long and a 5.8 GHz drone antenna is a stub.
- Polarisation — the orientation of the wave's electric field, set by the antenna's orientation. Mismatched polarisation between transmitter and receiver loses a large fraction of the available signal.
The Band Ranges
| Band | Frequency range | Propagation | Aviation and RPAS use |
|---|---|---|---|
| MF — medium frequency | 300 kHz – 3 MHz | Ground wave by day, sky wave at night | Non-directional beacons (NDB) |
| HF — high frequency | 3 – 30 MHz | Sky wave: refracts off the ionosphere for very long range | Oceanic and remote-area long-range communications |
| VHF — very high frequency | 30 – 300 MHz | Line of sight | Aviation voice radio (118.000–136.975 MHz), 121.5 MHz emergency |
| UHF — ultra high frequency | 300 MHz – 3 GHz | Line of sight, shorter range, higher data capacity | GNSS (~1.2–1.6 GHz), transponders, 433/900 MHz telemetry links |
| SHF — super high frequency | 3 – 30 GHz | Strict line of sight | 5.8 GHz video and C2 links, weather radar |
RPAS command-and-control and video links live mostly in the ISM (industrial, scientific and medical) bands — 2.4 GHz and 5.8 GHz — with 900 MHz and 433 MHz used for long-range telemetry. These are unlicensed shared bands: convenient and cheap, but you share them with Wi-Fi, cordless equipment and every other drone at the site.
The frequency trade-off
There is a consistent trade running through the whole table:
- Lower frequency → longer wavelength → better penetration of foliage, buildings and terrain, longer range for a given power, but less data capacity and a physically larger antenna.
- Higher frequency → shorter wavelength → more data capacity (high-definition video), smaller antenna, but more easily blocked by obstacles and shorter effective range.
This is why a long-range telemetry link runs at 900 MHz while the HD video runs at 5.8 GHz on the same aircraft: each frequency is chosen for what it must carry.
Line of Sight and Effective Range
VHF and above are line-of-sight propagation: the signal travels in a straight line and does not follow the curvature of the earth or bend around a hill. For an RPA this has two consequences.
First, effective range is a geometry problem, not a product specification. A manufacturer's "8 km range" figure is measured in open terrain with clear line of sight and no interference. Put a ridge, a tree line or a steel-framed building between the ground station and the aircraft and the usable range collapses — sometimes to a few hundred metres.
Second, height helps. Raising the ground antenna, or flying the aircraft higher, extends the radio horizon. A pilot standing in a gully with the aircraft behind a rise has no path at all; the same pilot moving 50 m onto the ridge line may have a solid link.
Factors Affecting Propagation
Schedule 4 topic 7(b)(iii) lists five factors explicitly. Each has a practical RPAS consequence.
Terrain. Hills, ridges and cuttings physically block line-of-sight paths. Terrain also reflects signals, producing multipath — the receiver gets the direct signal plus a delayed reflection, and the two can partially cancel. Multipath is why a link can be poor at one position and solid five metres away.
The ionosphere. Layers of ionised gas 60–600 km up refract HF signals back to earth, allowing long-range HF communication far beyond line of sight. The ionosphere varies with time of day (more layers by day, fewer at night), season, and solar activity. It does not usefully refract VHF and above, which pass straight through — which is exactly why GNSS works and why VHF is line-of-sight.
Sunspot activity. The 11-year solar cycle changes ionospheric ionisation. High solar activity improves long-range HF propagation but also produces solar flares and geomagnetic storms that degrade GNSS accuracy and can cause brief outages. Space-weather warnings are worth checking before a precision-survey flight.
Interference from electrical equipment. High-voltage powerlines, substations, welding equipment, industrial motors and switch-mode power supplies all radiate broadband electrical noise that raises the receiver's noise floor and cuts effective range.
Thunderstorms. Lightning produces enormous broadband radio noise, heard as crackling on an HF or AM receiver. A storm within tens of kilometres can degrade radio links — and long before that becomes the limiting factor, the storm itself is a reason not to be flying.
Digital Versus Analogue
Schedule 4 topic 7(c) asks about digital and analogue signals and about shielding and optimisation.
- Analogue signals degrade gracefully. Analogue video gets progressively noisier as signal weakens — snow, then rolling bars, then nothing — giving the pilot warning.
- Digital signals degrade abruptly. A digital link is clean right up to the point where error correction can no longer keep up, and then it drops out entirely — the "digital cliff". A pilot watching perfect HD video has very little warning before the picture and the link disappear.
That difference matters operationally: on a digital system, watch the link-quality indicator, not the picture. The RSSI or signal-bar reading is the early warning that a clean picture will not give you.
Shielding reduces radiated interference between components on the airframe: braided or foil shields on power leads, ferrite chokes, physical separation between the ESCs and the GNSS/compass module, and grounding of the shield at one end only. It is why the GNSS antenna on most commercial multirotors sits on a mast or on top of the shell, as far from the power electronics as the airframe allows.
Range Testing
The only reliable way to know your effective range at a given site is a range test, and Schedule 4 names it (topic 7(c)(ii), command and control link range testing).
A practical procedure:
- Power up the aircraft and the ground station and let the link establish.
- With the aircraft on the ground and not armed, walk away from it while watching the link-quality reading, or use the manufacturer's dedicated range-test mode.
- Note the distance at which link quality begins to degrade. Ground-level testing is pessimistic — the link will usually be better in the air — but it establishes a relative baseline.
- Repeat with the antennas in the orientation you will actually use.
- In flight, fly a short, close orbit before transiting downrange, and confirm link quality stays solid through all headings — this detects airframe shadowing, where the aircraft's own body blocks the antenna at certain angles.
Do the test at the site, not once in the car park at home. Range is a property of the environment, not of the aircraft.
An RPA carries a 900 MHz telemetry link and a 5.8 GHz video link. Why are two different frequencies used?
A remote pilot is flying a digital C2 and video system. The picture is perfect, but the link-quality indicator has been falling steadily. What should the pilot do?
Which factor affects GNSS accuracy through changes in the ionosphere over an approximately 11-year cycle?