Fading · Entry 06.2
Multipath
Reflections off terrain and buildings, and what they do to FM in a moving car.
When the same signal arrives twice
A transmitter sends one signal. By the time that signal reaches a moving car, it may have arrived along several different paths simultaneously — direct line-of-sight, a reflection off a hillside, a bounce off a glass tower block, a diffracted wave around a ridge. Each copy travels a different distance. Each therefore arrives with a different phase. At the receiving antenna they add together, and the result depends entirely on whether they reinforce or cancel.
At any given instant the combination may be close to perfect addition. Half a wavelength of path difference later — which, at FM frequencies near 100 MHz, is roughly 1.5 metres — the reflected copy arrives in exact opposition to the direct wave and the two null each other. A car moving at motorway speed can sweep through several of these constructive-and-destructive interference zones every second, producing the rapid flutter that FM listeners recognise on elevated stretches of road with hard surfaces on both sides.
| What | The consequence |
|---|---|
| Path difference | the extra distance a reflected copy travels versus the direct wave, measured in wavelengths |
| Half-wavelength cancellation | the geometry that produces a null; at 100 MHz this is roughly 1.5 m of path difference |
| Delay spread | the spread of arrival times across all multipath copies; the parameter digital guard intervals are sized against |
Terrain is the primary reflector in rural areas: a limestone escarpment, a metal-clad barn roof, a flat reservoir surface. In cities, glass and steel facades are efficient reflectors, and the geometry changes continuously as the vehicle moves. This is the mechanism behind selective fading, which affects different frequencies unequally: when reflected copies arrive late enough, their cancellation falls at some frequencies within the FM channel and not others, so the signal is distorted rather than simply weakened.
The effect on FM is more damaging than it first appears. FM's capture effect protects it well against a weaker interfering transmitter — the stronger signal dominates. But multipath is not an interferer; it is the station's own signal, arriving late and out of phase. Capture cannot help when the signal is fighting itself. The result in the audio is stereo pilot distortion, high-frequency smearing, and in bad cases a momentary dropout as the null deepens.
Reception diversity — two antennas separated by enough distance that both cannot sit in the same null at the same moment — is the standard engineering counter-measure, including for mobile receivers. Vehicles use electronic switching or blending between a screen-printed rear-window antenna and a short-mast front antenna for this reason. Digital radio systems handle multipath more elegantly: DAB and HD Radio are built on multi-carrier formats whose guard intervals are specifically sized to absorb the delay spread caused by reflections, turning a destructive interference problem into one the codec can simply ignore.
The multipath environment a station faces is inseparable from its terrain. A transmitter sited on a clean hilltop with open ground in all directions generates far fewer reflection paths than one embedded in a valley surrounded by buildings — a fact that any field strength survey route will quickly make visible.
For reference
How it shows up in practice
- Rural multipath: escarpments, reservoir surfaces, metal roofs — large, stable reflectors; null zones are predictable in location
- Urban multipath: glass and steel facades, continuously changing geometry as a vehicle moves
- Symptom in analogue FM audio: stereo pilot distortion, high-frequency smearing, momentary dropouts