Can somebody tell me with the help of example that what is db that is mentioned on wireless antenna and wireless cards?

what is db, how it affect the performance to wireless card? higher db is better or lower db is better?

please tell me about this in detail....

thanks in advance!

dB is decibel. wireless output power is measured in dBm (decibel relative to miliwat), these values can be converted to eachother. for example 23dBm is 200mW. this is average transmit power. 30dBm is 1w which is very much for a wireless card. in theory this card can link at further distances, but the receiving end should be a similarly high speced card, with a high receive sensitivity (another parameter of wireless cards), and to talk back to your card, it also needs a high power transmitter.

how to can co-relate the dbi or dbm with the distance? please explain this or is there any formula that we can use?

dbi is decibels relative to an isotropic source (used in antenna specifications), also in same category is dbd which is decibel relative to a dipole antenna. Neither are related to a dbm, which as Nomis explained is a decibel relative to milliwatt... (used in power measurements)

What you are trying to understand is "free space loss" which is the amount a signal attenuates (lessens) by a certain distance... There is a mathematical constant in the formulae and also the formulae is specific to frequency.

You need to google it to understand the formulae.

But if you want a quick example. You do not need a program to work out path loss.

Its simple, you just have to remember 3 things only.

Mathematically

AT 2437 Mhz the free space loss at 1kM is -100dB
AT 5500 Mhz the free space loss at 1kM is -107dB

Every time you double the distance, add another -6dB to those figures.

Example.

2437 Mhz distance = 4kM then FSL = -112

5500 Mhz distance = 64kM then FSL = -143

Work as if you are dealing with binary, ie 1,2,4,8,16,32,64,128,256 etc etc.

each time you shift up the scale add -6dB...

Obviously this method is crude, but will give you an approximation within 1dB, this should be close enough for our purposes.

It will not take into account "fresnel zone"

But at least with this method, you only need to remember 3 sets of numbers, 100, 107 and 6.

So often on the forum I see the decibel misunderstood, on its own, the decibel means absolutely nothing!!!!!!! it is a mathematical reference only..

For example you need to say, its 10dB less than (x) or more than (x). For a dB to be used in a measurement, "it must have after it, the point of reference" eg dBd, dBi, dBm, dBu or dBW.. (these are all common values used in radio engineering).