On Wed, May 29, 2002 at 05:00:30PM -0600, Mike Avery wrote: > On 29 May 2002 at 11:02, Tim Pozar wrote: > > > On Wed, May 29, 2002 at 10:44:59AM -0700, Tim Pozar wrote: > > > As an example... > > > > If we had a 24 dBi antenna on both ends of a point-to-point link > > > with the legal max power of .25 Watts (FCC 15.247) we would have a > > > fade margin of 47.775 dB (with a 80 dBm RX threshold) over a 1 mile > > > link. If we had a passive repeater connecting two 1 mile links we > > > would have to multiply the pass loss of one link (104.2 dB) by 2 and > > > the antenna gain (48 dBi) by 2. This would have a total path loss > > > of 108.4 dB and would put the RX signal at -88.42 dBm. > > > Whoops... That should be 208.4 dB. > > I believe you were closer the first time at 108.4 dB. dB > are not a linear scale, and a doubling or halving of signal is > a 3 db change. So, if you lose 104.2db on each leg, the > total loss would be 107.2 db because you have doubled the > loss - or added another 3 db of loss.
Hmmm... I may not have described how this works very well. Sorry gang... Say if I have a link that goes through a passive repeater, each link has path loss. Path loss it pretty easy to calculate (check out http://www.lns.com/papers/pathcalc) and for a 1 mile 2.4 GHz path it is just slightly more than 104.2 dB. Since you have two links, you just add these number together with other loss factors such as coax or connector loss. On the "gain" side of things you have the transmitter power and antenna gain. You would add these number together. An sample calculation (from pathcalc.pl - see above)... Frequency = 2.4 GHz TPO = .25 Watts TPO = 23.9794000867204 dBm TX line loss = 3 dB TX ant gain = 24 dBi Path length = 1 miles Path loss = 104.204224834232 dB RX ant gain = 24 dBi RX line loss = 3 dB RX signal = -38.2248247475117 dBm RX threshold = -80 Fade margin = 41.7751752524883 dB Since we have two links, we need to double the line loss, antenna gain and path loss numbers as we have two links and 4 antennas. So... TPO = 24 dBm TX line loss = - 3 dB TX ant gain = + 24 dBi Path loss = - 104.204224834232 dB Repeater RX line loss = - 3 dB Repeater RX ant gain = + 24 dBi Repeater TX ant gain = + 24 dBi Repeater TX line loss = - 3 dB Path loss = - 104.204224834232 dB RX ant gain = + 24 dBi TX line loss = - 3 dB ======== - 100.4 dBm Since most radios want about - 80 dBm for a signal, this path won't work as it is 20 dB below the RX threshold. (This time I calculated transmission and connector line loss.) Tim -- general wireless list, a bawug thing <http://www.bawug.org/> [un]subscribe: http://lists.bawug.org/mailman/listinfo/wireless
