Glossary

Fresnel Zone A simple and quick explanation of Fresnel ellispsoid role in radio propagation is to see the thing like a virtual "pipe" where most of the energy travels between a transmitting and receiving site. So in order to avoid losses there should be NO obstacles inside this zone (forbidden region) because an obstacle will disturb "the energy flow". (the explanation is really simplified !).
For example, if half of the forbidden zone is masked (antenna at the limit of line of sight), there will be a signal power loss of 6 dB (power loss of 75 %) [source].

As distance increase, the Frensel Zone gets bigger. For longer ranges, the height of the antennas should increase for the Frensel Zone to clear obstacles [picture].

SWR (Standing Wave Ratio) This is a measure of the amount of energy absorbed and radiated by an antenna compared to the amount it reflects back to the transmitter. An SWR value of 1:1 is perfect (no reflected energy), while a WLAN antenna should have an SWR less than 1.5:1 [source].

Impedance Every part must have impedance of 50 ohm. The impedance of coaxial cable depends of ratio D/d where D is inner dia of the shield tube and d is dia of inner wire. The ratio is about 2.4 in the 50 ohm air insulated coaxial. Other insulate materials decreases the ratio with so called velocity factor [source].

The exact equation of Impedance versus diameter ratio is:

[source]

Resonance Any piece of metal will serve as an antenna. If some bits of the metal are the same size in one dimension as the radio wave it's catching will the wave to be felt stronger by the metal, resulting in the a stronger electrically induced current. It actually works well if you are some even multiples and fractions of the wave's size (1/2, 1/4, 1/8). Quater wavelength (1/4) segments are very common and useful. [source]

Wavelength The wavelength is the distance your signal goes in one cycle. Electromagnetic waves travel at the speed of light and that means the velocity is about 300 000 kilometers/sec. And since the frequency of the wireless network equipment is on the 2.4 Gigahertz band it's easy to calulate the wavelength: 300 000 km/s / 2400 = 125 mm.

One complete cycle of a wave

Range / distance This table can be used ash a general guideline, and assumes the transmitter are a standard 30mW, the receiver has typical WiFi sensitivity, and no cable losses. To use this table, find the intersection or the two antennas ate each end or the risky. The number in the square are the expected range (free line or sight) concerning which you should expect to be able to make a reliable risky. For example, an AntCaptenna will work concerning approx. 7km to a 180 degree Waveguide antenna (assuming clear line or sight, and a 30 mW WiFi transmitter).

Don't forget that every extra 6dB or cable loose reduces the distance by 50%.

[source]

Antenna Antenna gain is normally given in decibels over an isotropic antenne [dBi]. It's the power gain in comparison to a hypothetical isotropic (all directions equal) antenna. The more gain an antenna has, the more it is directive (energy sent in a specific direction), the less local (noise) signals are picked up. This improves the signal to noise ratio. [source]

dBi is a logarithmic measurement, so every 3 dBi is a doubling of gain [WiFi Toys book].

Polarization A dipole transmits a vertically polarized signal. This means that the electrical component of the energy, the so-called "E-field," is parallel to the dipole element and perpendicular to the floor. By turning the dipole 90 degrees (so its axis is horizontal) it will radiate a horizontally polarized signal, where the E-field vector is parallel to the ground [source].

Plane Radiation plots are most often shown in either the plane of the axis of the antenna or the plane perpendicular to the axis and are referred to as the azimuth or "E-plane" and the elevation or "H-plane" respectively [source].

Power (dBm) Power is expressed in watts or milliwatts. Power can be expressed on a logarithmic scale relative to 1 mW, in dBm. ('deci-Bell relative to one milliwat) . In that case, the output is compared to one milliwatt.

[source]

A drop of 1 dB means that the power is decreased to about 80% of the original value while a 3 dB drop is a power decrease of 50% or one-half the power. A 10 dB drop is considered a large drop, a decrease to 10% of the original power level.
A doubling of power is 3 dB while a quadrupling is 6 dB. Therefore, if the antenna gain is doubled (3 dB) and the transmitter power is quadrupled (6 dB), the overall improvement is 9 dB. Likewise, dB can also be subtracted [source].

It is hard to compare dBm and mW. Here is some help: 30dBm is exactly 1000mW or 1 full Watt, 20dBm is exactly 100mW, 17dBm is about 50mW, 15dBm is about 32mW. Because antenna and distance calculations use dB it might be better to use dBm for all power ratings.

A dBm <-> mW calculator can be found here, for those that are wondering about exact mW/dBm correlations. Power in dBm is simply 10*log(power in mw) so, for example, 100mw expressed in dBm is 10*log(100)= 10*2= 20dBm. [source]

Receiver sensitivity (dBm) Receiver has a minimum received power threshold (on the card connector) that the signal must have to achieve a certain bitrate. If the signal power is lower (-94 is lower than -82) the maximum achievable bitrate will be decreased or performance will decrease. So we have better use receiver with low threshold value, here are some typical receiver sensitivity values:

• Orinocco cards PCMCIA Silver/Gold : 11Mbps => -82 dBm ; 5.5Mbps => -87 dBm; 2Mbps=> -91 dBm; 1Mbps=> -94 dBm.
• CISCO cards Aironet 350: 11Mbps => -85 dBm ; 5.5 Mbps => -89 dBm; 2 Mbps => -91 dBm; 1 Mbps => -94 dBm.

[source]

Receive Sensitivity is how much signal a card needs to receive in order to work at that speed level. A 3 dB difference is double the power. You can now see some cards are getting much better distances. The difference between a Cisco 350 and a Addtron at 1 Mbps is 32x times the sensitivity. This means that the Cisco needs 1/32 as much signal strength as the Addtron does to work at the same rate. Also, a lower receive sensitivity number is better (IE: -95 is better than -80).
Receive sensitivity is measured in dBm @BER 10E-5 or (or 8% FER). [source]

SNR (Signal to Noise Ratio) Receiver sensitivity is not the only parameter for the receiver, we have also to take into account the signal to noise power ratio. It's the minimum power difference to achieve between the wanted received signal and the noise (thermal noise, industrial noise due for example to microwave ovens, interering noise due to other WLAN on the same frequency band). It is defined as:

If the signal is more powerful than the noise, signal/noise ratio (also called S/N ratio) will be positive. If the signal is buried in the noise, the ratio will be negative. In order to be able to work at a certain data rate the system needs a minimum S/N ratio:

• Orinoco PCMCIA Silver/Gold: 11Mbps => 16 dB ; 5.5 Mbps => 11 dB ; 2 Mbps => 7 dB ; 1 Mbps => 4 dB

[source]

Beamwidth When the beamwidth is getting narrower the gain and the range in a particuliar direction usually increases. If the beamwidth is wider, i.e. 360 degrees, it covers a bigger area but the distance the signal can travel decreases. High-gain vs low-gain respectively [by me].

Most antenna users are interested in the directivity or beamwidth of the antenna. As mentioned earlier, this is usually referred to as the "half-power" or 3 dB beamwidth, the points between which half the power is radiated or concentrated, and specified in degrees. As an example, the typical half-power beamwidths of a 3, 6 and 10 element Yagi are 60, 40 and 30 degrees respectively.

[source]

Lobes There are three 'kinds' of lobes; main-, side- and rearlobes or beams. The mainlobe is most important and radiates in the direction the antenna is pointing. The two other lobes are less important and usually unwanted. They radiate from 'blind' spots or flaws in the antenna design and decreases the maximum antenna gain (main lobe) [by me].