Talk:Wi-Fi How To
Back-to-back antennas
My first reaction to using wired back to back antennas to get around a brick wall is that it won't work. There is too much loss. I admit I have not done the calculations for this particular instance, but I have in other instances and found it always is too lossy. I am interested to know if anyone has really made this work. Maybe I should put some numbers down here.
+15 dBm transmit power, 28 mW, WRT54G stock
+7 dB transmit antenna gain, optional high gain antenna
- (+ 22 dBm ERP, exceeds 200 mW by 2 dB, may be an infraction)
-54 dB free space loss at 2.4 GHz and 5 meters
+8 dB receive antenna gain, could get a higher gain antenna
-1 dB coax between antennas and mismatch losses
+8 dB second transmit antenna gain, could get a higher gain antenna
-60 dB free space loss at 2.4 GHz and 10 meters
+2 dB second receive antenna gain, estimated notebook or PCI card type
-90 dB, Net Path Loss, sum of all gains and losses
-75 dBm receive power at receiver, transmit power + Net Path Loss
This system needs about 20 dB more receive power to be feasible. If one could change the back to back antennas to 18 dB antennas, you might make it.
Still, the two end terminals here could not be farther apart than the two distances shown, a total of 15 meters. Free space loss at 15 meters is 64 dB, compared to 70 dB for the 18 dB gain back to back antennas if you had them. So if you don't have good isolation from other extraneous reflections and refractions, you may get self-interference, maybe picket fencing, that is problematic. Oh Oh, the wikipedia reference does not explain the proper nature or origin of picket fencing.
A 2x2.5 ft passive repeater, properly aligned, should have a gain of about 50 dB, which is 16 dB better than the two 18 dBi back to back antennas. That 50 dB gain is just what I cranked out here in a spreadsheet, I would believe it when I see it. It assumes a zero degree angle between the incoming and outgoing path, which is obviously wrong. Need to correct by cos(alpha), the incident angle. :(
Other considerations: Use of such a high gain antenna facing past the wall means high directivity, so you won't get a wide coverage angle. Use of a reflector has the same problem, only more directive due to higher gain. And if you really are within 5 meters of the wall on the inside, better just move the router the 5 meters!
Now why am I writing this???? I should be going to bed!
- --Dave M. 07:57, 6 December 2008 (UTC)
Before I sign off, I am writing myself a note here. I need to put in a link to Passive Repeater Engineering, which is now available on-line. Thank you Valmont for making this old Microflect publication available.
And also a note to refer to page 37. One can start with an 8x10 passive and look up the gain at 2.6 GHz. Note that when you double of halve the size of the passive, you change it gain by 6 dB. So you can scale the 2.6 gig 8x10 down to 2x2.5 and estimate a number. Note that you are way off scale into the light pink color. That tinted area, Zone A, is a warning that you don't have enough gain to be sure you won't have multipath problems. --Dave M. 09:01, 6 December 2008 (UTC)
| Jeff's response: Revised number using most of his numbers and only correcting coax losses and
Doing the arithmetic, I get -79dBm which is low but usable to about 24
Mbits/sec. See my chart at Wi-Fi Receiver Sensitivity. Please post any further discussion there. Thanks. --John Navas 17:51, 6 December 2008 (UTC) |
