Antenna Diversity: Beating Multipath Fades
See how a second antenna fills in multipath fades, how far apart diversity antennas need to be, and what selection diversity and maximal ratio combining each gain, in a calculated indoor multipath field.
How to use this tool controls and what to look for
Press Play to move two antennas through a multipath field and watch the second antenna fill in the first one's fades.
Controls to change
- Band: choose 2.4, 5 or 6 GHz.
- Surroundings: compare Cluttered, no line of sight with Line of sight.
- Antenna spacing: move the antennas closer together or further apart, in wavelengths.
- Antenna B cross-polarized: turn antenna B sideways for polarization diversity.
- Position: move along the track by hand.
- New random room: try another multipath field.
What to look for
- Deep fades are only a fraction of a wavelength apart, so a small move changes the signal a lot.
- Below about half a wavelength apart, the two antennas fade together and diversity helps little.
- Cross-polarized antennas fade independently even when they are very close together.
- Compare selection diversity and MRC: about 10 and 12 dB less fade margin with two antennas.
About this tool
Indoors, reflections make the signal rise and fall by 20 dB or more over a few centimetres. This tool walks two antennas through a calculated multipath field and shows how a second antenna fills in the fades, how far apart the antennas need to be, and how much selection (switched) diversity and maximal ratio combining (MRC) each gain, including polarization diversity.
What you'll learn
- Why multipath creates deep fades only a fraction of a wavelength apart
- Why antennas need to be about half a wavelength apart, or differently polarized, to fade independently
- How selection diversity and MRC compare: about 10 and 12 dB less fade margin with two antennas, 19 dB with four
- How Wi-Fi uses diversity: MIMO receive chains, transmit diversity, antenna mounting and client antenna placement
Common questions
What is antenna diversity?
Using two or more antennas so that when the signal fades at one, another is likely still strong. The radio either picks the better antenna (selection or switched diversity) or combines all of them (maximal ratio combining), which greatly reduces deep multipath fades.
How far apart should diversity antennas be?
About half a wavelength or more: roughly 6 cm at 2.4 GHz, 2.7 cm at 5 GHz and 2.3 cm at 6 GHz. Closer than that, they sit in the same part of the fading pattern and fade together. Antennas with different polarizations fade independently even side by side.
Is MIMO the same as diversity?
MIMO radios use the same antennas either for diversity, combining them for a more robust single stream, or for spatial streams, sending different data on each for more speed. They switch per frame: streams close to the AP, diversity at the edge of the cell.
Free for private and educational use. This interactive model is a simplified teaching aid and may contain mistakes. It is provided as is, without warranty. Use it to understand the concepts, not as a substitute for a site survey, vendor documentation or professional design advice.