I'm not assuming you are an engineer, nor am I trying to turn you into one. So, I've tried to simplify the following description as much as I can. The good news here is that you don't need to remember this. It's nice to know, and you'll amaze your friends, but you can survive and extend your network without it.
You may be familiar with the term decibel as a measurement of noise level. Engineers express the strength of a signal in decibels (dB). In the early days of telephones, decibels were created as a way to measure the performance of telephone equipment and cables (and named after Alexander Graham Bell, the inventor of the telephone).
A decibel is a relative measurement between a reference input signal level and the output signal level. A bel is equal to 10 times the input signal level, and 1 decibel is l/10th of a bel. One bel is the minimum level of change in sound levels that a human ear can discern, which means that your ear can hear the difference between a sound level of 1 bel and a sound level of 2 bels, but you can't hear the difference between 5 decibels and 7 decibels (although your cat or dog probably can). Many people experience pain when sound levels reach 130 dBs (unless they go to a lot of rock concerts and have become used to loud noises). Decibels are the measurement that you will most likely encounter when you are extending your WLAN because the gain of an antenna system is typically expressed in decibels.
Note:Gain is the amount that the power of a signal is amplified. For antennas, the gain is expressed in decibels. For example, if the signal output strength of an antenna is increased tenfold over the input power (10:1 ratio), you express the gain as 10 dB. If the output level of an antenna is equal to the input level (1:1), there is no gain in signal strength and you express this as 0 dB.
When you shop for a Wi-Fi antenna, you must select an antenna with the right amount of gain for your situation. Consider the coverage area that you need, as well as the distance between WLAN clients and the access point. Select an antenna with enough gain for a strong signal, but avoid using too strong of a signal, which may actually create problems due to reflections of the RF signal.
The problem caused by these reflections is called multipath-interference, or simply multipathing. Reflected signals arrive at the antenna at different times and at different strengths, canceling each other out and resulting in a weaker signal. So, using too strong a signal in a small area can cause more problems than it would solve.
The antennas that come installed on many access points for home and SOHO WLANs are usually around 2 dB. For the average home WLAN, you'll seldom need more than a 6 dBi antenna dBi maximum (see the related Insider Insight for an explanation of dBi). You can buy higher gain antennas; these are usually meant for outdoor use and longer distance links. For example, some 18 dBi panel antennas are rated for distances of up to 10 miles. So, an 18 dBi antenna is probably overkill between your house and garage.
Insider insight: When you shop for Wi-Fi equipment particularly antennas and connectors, you may see some measurements listed in dBm or dBi. The dBm measurement indicates decibels relative to one mill watt (l/1000th of a watt), and dBi represents the gain on an antenna system relative to an isotropic radiator. Often, you'll see dBm used to measure signal loss from a connector.
An isotropic radiator is a theoretical ideal, a device that creates radio waves at 100 percent efficiency equally in all directions or (for reception) is equally sensitive in all directions. It¡¯s used as a standard test reference in laboratories, and products are compared against the performance of an isotropic radiator or point source.
The measurements dBi and dBm aren't terribly important where we are concerned; you really don't need to understand dBm and dBi unless you're designing antenna systems or if you're perhaps building your own "home brew" antenna.
The frequency of Wi-Fi equipment also affects the range of the signal. A high-frequency signal requires more power to cover the same distance than a low-frequency signal. This can be seen in the range difference between 802.11b/g devices versus 802.11a devices. Because manufacturers don't want to increase power consumption of mobile devices, the result is a shorter range for the 5.0 GHz 802.11a signal.
High-frequency signals also are more susceptible to interference from obstacles than are low-frequency signals. The signal from 2.4 GHz and 5 GHz Wi-Fi devices are negatively affected by walls and other solid objects. The 5 GHz 802.11a signal has more problems with these obstacles than 2.4 GHz 802.1 lb/g signals do. Compared to Wi-Fi, low-frequency television signals have few problems going through walls and ceilings to reach your television.