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Just wanted to put in a caveat here about the use of the word ?aperture?. It can cause a lot of confusion.

If we consider an outdoor antenna of the parabolic type, and look at it ?straight-on?, we will see ?a circle?. This is the classic representation of a parabolic antenna when viewed down the boresight. There is a formula for calculating the gain of an antenna such as this which involves the square of the diameter of the antenna ( the ?circle?).

The ?size of the circle? is often referred to as the aperture (or ?opening?). For example, think of a camera aperture. As the size of the aperture gets larger, more light is let in, as the size of the aperture gets smaller, less light is let in ( think of the light as being signal).

With a parabolic antenna, the larger the aperture, the larger the gain (all other things being equal). Parabolic antennas can vary in size from tiny little VSAT dishes to large tracking antennas used by NASA (over 200 feet across).

http://en.wikipedia.org/wiki/Parabolic_antenna

http://media-2.web.britannica.com/eb-media/82/72982-004-9191583E.gif

http://www.youtube.com/watch?v=-D7dlg0Zl2Q

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http://www.bcee.concordia.ca/index.php/Image:Font_Romeu_France.jpg
http://sciencestage.com/v/589/sience-james-may-big-ideas-solar-furnace-power-melting-steel-fuel-air-hot-dogs-burn-fast-sun-high-te.html

So how does changing the size of the aperture affect the gain ?

There are different types of parabolic antennas (Prime Focus, Cassegrain etc). The type commonly used in Wi-Fi (mostly outdoors) is the Prime Focus type. This device works similar to how a car reflector works in our headlights ( but in reverse).

We have a curved surface (viewed from the side) and a focal point. A parabolic reflector has the property that if parallel signals (relative to the center line of the parabola) enter the parabola (or ?dish?), they ?bounce?off the metal surface (could be plastic coated with metal) and end up ?gathered?at the focal point. This is where we have the concentration of energy. Quite simply, the bigger the dish, the more energy is collected and the higher the RSSI and SNR (assuming no extraneous inteference etc). This signal is then collected and sent off to the Wi-Fi gear (there may be a pre-amplifier prior to the Wi-Fi gear in some cases).

In a nutshell, the bigger the antenna, the bigger the signal.

But what about conventional Wi-Fi antennas of the dipole type ? The term aperture is used to some degree there as well, but not in the sense of a parabolic.. An electromagnetic signal develops a potential difference (voltage) across the antenna. Without going into all the details, ignoring the differences between quarter wavelength, half wavelength antennas etc, you do not see the massive difference in sizes between dipoles which utilize the same frequency.

I won?t get into the technical details, but the method by which a dipole ?creates a useful signal?is not the same in some respects as to how a parabolic does this. The parabolic antenna is acting as a ?gatherer of energy?and that is where the term aperture should really be used. It?s a small point, but important.

To make things even more confusing, a parabolic antenna often has a small dipole like ?pick-up?in the feed of the antenna.

We have to be very careful with this whole FSPL/Attenuation business as different texts use different defintions.

Whilst it is true that if we double the distance that a 5 Ghz signal travels, and at the same time double the distance that a 2.4 Ghz signal travels, both will suffer a 6 dB loss. However, if we have two links with a 2.4GHz signal and a 5GHz signal running parallel, and we check the FSPL at say 100 meters out, we will find that the FSPL of the 5GHz signal WILL be greater. It has to be, as, irrespective of whether we measure in meters or feet, the formula for FSPL is given as:

A CONSTANT + 20 Log D + 20 Log F.

D = distance from transmitter

F = frequency of operation

At (say) 100 meters out, the CONSTANT is, well?.a constant. No change there. The only thing that changes is F. This causes the FSPL value to be higher for the 5 GHz signal than from the 2 GHz signal.

Now, as regards attenuation, that depends on how it is defined. We should also talk about power flux density etc.

I?d be interested to hear or read exactly what Joe said. This is an area where precise defintions are very important.

Dave