Jul 212012
 
Radio waves do not penetrate obstacles very well.  High-gain antennas have a flat radiation pattern: So, a larger antenna will only help for long range; it will not help if you have differing elevations. Use high power antennas to send a signal long distances to a very specific and focused point. The higher the gain, flatter the signal radiation pattern.Low power antennas send their signals to higher and lower elevations in local area: Everywhere within their 100-200 foot range.Antenna gain acts on both the transmit power and receiver sensitivity, so you are not only sending your signal further but you are also able to receive much weaker signals within its line of site. Gain of an antenna is a measure of its enhancement of signal strength:  Gain is measured by the dBi rating. 
Calculating Gain from addition of an antenna:
  3 dB rule”: For every 3 dB increase in level, the power is doubled. For every 3 dB decrease, the power is cut in half.

  10 dB rule”: Every 10 dB increase in level results in 10 times the power, and every 10 dB decrease in level results in 1/10 the power.

  See more detail:  Table of power increase multiples from increase in dB by various amounts:  
See table on right side of this page.
  Signal loss in cable:  If a cable connects your device to the antenna, take signal-loss in the cable into consideration

Antennas generally have greater coverage at the expense of range, and greater range at the expense of width of coverage area (“half-power beam width”).   Therefore, omni-directional antennas, which radiate the beam in all directions (on a horizontal plane) generally have gains no higher than 12dBi, while directional antennas can have gains up to 30dBi.  As a general rule of thumb:  The higher the gain, the narrower the beam width (coverage area). 
Half-power beam width is the specification of the antenna’s coverage area.  It is measured relative to the points at which the antenna’s radiation drops to half its peak value.

BELOW DIAGRAM:  Approximate “Line of Sight” antenna range in free space (outside) from a standard 100mw power Wireless device with antenna both ends at given gain level.   See also: Ubiquiti gear range


Antenna gain

Wireless range

The table is only a rough guide as conditions from site to site vary wildly. This assumes absolutely no objects between both ends of the range – no trees, buildings or hills. Once you add obstructions the range drops dramatically. For instance, within a building with concrete walls the range can drop to 30-50m.

100mW device + antenna
0dB 200m
4dB 440m
7dB 620m
10dB 1.2Km
13dB 2.8Km
16dB 5.0Km
20dB 12.5Km
24dB 31Km

 INDOORS / BUILDINGS  See also this page on dipole antennas

Metal reflects the signal. Modern houses may also have foil backed plasterboard to meet fire regulations and modern glass has a metal content.

This said, as a general rule 802.11g/ WiFi devices will cover a house quite well, not guaranteed though.  Bear in mind if your WiFi antenna is placed down behind the computer case it already has two layers of steel and a wall to penetrate first.

Indoors coverage:  We recommend using the low power 2dB antennas and using more wireless repeaters spaced every 100 feet to properly cover an area.

Laptops only transmit 100 feet so there is nothing gained by having a high gain antenna on one end when the laptop can’t even talk back more than 100 feet.

Making sure that all repeaters are no more than 100 feet from each other ensures that laptops can always see a repeater or two.

  2 Responses to “Range & Radiation Patterns of WiFi Antennas”

  1. pls i want to send wireless signal over 25 to 30 km, what radio or wireless divice do i need @ both ends? Pls its urgent.

  2. Ubiquiti RocketM5 with RocketDish30dbi, on both ends. That’s all you need.

Ubiquiti in stock Alfa USB adapters and upgrades to extend range; tech support Longest Range WiFi USB Adapters USB cables to better position your antenna Mounts for Antennas POE injector, Power over Ethernet WiFi Bridges for Point to Point wireless links
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