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wireless solution

Posted on 2004-04-04
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please can you provide me information about 2.4 ghz wireless
i have no idea about the gain and dbi , why we use omni directional antenna or yagi antenna , etc ,
thanks
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Question by:skynoc
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6 Comments
 
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Expert Comment

by:slink9
ID: 10751473
This sounds a bit like homework.  You need to learn this stuff on your own.
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marius_petrescu earned 2000 total points
ID: 10751517
Hi skynoc,

First of all, a wireless equipment has in it's construction 2 blocks: a modem, which converts data into a analog signal, and a radio transceiver which carries that signal to the receiving partner and receives data from it. To unterstand the db thing, there are 2 theoretical elements to define:

- first, an ideal emitter-receiver antenna, called an isothropic antenna, which has no gain (0 dB) and emits all its energy equally in all directions, whith a 100% efficiency. Something like this doesn't exist, it's only on paper. All antenna radiation gain parameters can be compared to this thing and are noted as "dBi" where that "i" states that this isotropic antenna is used as reference. For example a half wavelength dipole anntenna has approx. 2.7 dBi gain,  and this because it radiates better in some direction (90 deg fom its axis) and it doesn't radiate along it's axis. When using a dipole antenna as a reference, the gain is expressed in "dBd" where "d" stands for dipole (for quick conversions 3dBi = 0 dBd, eg. 13 dBi = 10 dBd).

-   now the dB.... the gain expressed in dB is 10 times the decimal logarithm of the fraction of the 2 elements that have to be compared. So, if a amplifier amplifies 100 times, the input versus output ratio is 100, the logarythm is log(100/1) = 2 and we multiply the result by 10, so that amplifier has a gain of 20 dB. Note that if you cascade elements, the dB's get added or substracted, not multiplied. If you cascade a 10 dB amplifier and a 6 dBd antena, the overall gain is 16 dB compared to a system with no amplifier and dipole antenna.

In the same way, if using a antenna, the signal is let's say 10 times bigger than the one which would be obtained by using a dipole, we have 10 * log(10/1) = 10* 1 = 10 dBd, which is approx. 13 dBi.

The difference between omnidirectional and directional antennas is simple. A omni antenna radiates equally in all directions from the point of view of it's horizontal pattern (so you use it in locations where users from different directions need to gain acces to the system, let's say at a central access point). The directional antenna (a Yagi, a logperiodic or a parabolic dish for example) has a radiation pattern in one direction only, but offers higher gains, by channeling the radio energy in that direction only, and receiving only from that direction (so you use it mainly in end user setups, and they point towards an acess point).
Note that omni antennas have gain by radiating horixontally and not vertically, the greater the gain, the more flattened the radiation pattern of that antenna is.

So, finally, to estimate the performance of a wifi setup, you must take in acount the output power (which is expressed in dBm - the reference is a output power of 1 mW, so 17 dBm means 50 mW), the loss of the coaxial cable (which gets substracted from the overall gain) and the gain of the antenna, and you obtain the Effective Radiated Power (ERP), which is what you actually are using.
E.g. 17 dBm output power, -5 dB from let's say 2m of coax cable, and 13 dBi from a Yagi antenna gives you 25 dBm ERP.
Anothe example: 17 dBm output, 20dB booster, -25 dB from 10 m cable, 6 dBi omni antenna gives gives 18 dBm ERP
The above examples show you that a good antenna and short cable are more effective than a 5W booster with a long cable and low gain antenna (and it's also ilegal because the power is limited afaik to 50mW/17dBm)!!!

Greetings, MP
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Author Comment

by:skynoc
ID: 11355636
what is the difference between the DBI and DBD
thanks
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LVL 2

Expert Comment

by:marius_petrescu
ID: 11355877
- dBi is antenna gain compared with an isotrope radiator
- dBd is antenna gain compared with a free space radiating dipole

greetings and thanx.
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LVL 23

Expert Comment

by:slink9
ID: 11356190
If I ever go back to school, I will be sure to ask my homework questions here.  It is easier than studying!!!!
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