DBM question

Questions about the operation or expectations of the NetStumbler software

DBM question

Postby miked2000 » Mon Nov 24, 2003 12:26 pm

I have been reading and searching to understand the dB better. It is all as clear as a muddy pool but on a calm day so it is getting better.

One thing I am having trouble with and hope someone can help out with is that received singnal strenghts read in negative numbers because, if I understand it right, the losses of the signal. That 0 dbm is equal to 1 milliwatt so the negitive numbers indicates that the power received is less than 1 milliwatt.

Ok, so if a Orinoco Classic card has a typical output of 15 dbm (and if I am correct) that would be 31.6 Milliwatts of output. I have not been able to find the Netgear ME102 Access point output specifications but assume it is at least 15 dbm if not higher??

Now, here is where I am having trouble understanding. If I set my notebook with the Orinoco card five (5) feet from the ME102 Access Point using the built in anteann and look at the information from Netstumbler I see my singal is -37, noise -97 and SNR -60.

My question is that if I am putting out 31.6 milliwatts or higher output do I see less than 1 milliwatt at the card? Could there be that much loss in that short of path? In my mind, a short distance like that should result in a positive number of something over 1 milliwatt being received.

I would not think (but have been wrong before) that the path loss would be that great. Using this conversion (http://www.ydi.com/calculation/free-space-loss.php) the loss should be 43.7 dB but there is where I am stuck trying to figure out do I take the 15dB output and subtract 43.7 dB loss?

Thoughts?
miked2000
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