Winter 2.4GHz Propagation
in [Wireless Internet]
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From: msg on 5 Dec 2009 16:57 Every year when the first snow cover and temps below 32 degrees F occurs, I begin seeing a sharp increase in associations on my 802.11b access point (actually dhcp leases on the downstream router). The AP is tower mounted, with a switched duplexing amplifier and a 12dbi colinear whip at 30 foot elevation. The leaves have long since fallen so they are not a factor. I have been of the conviction that the improved propagation conditions are due to better noise figures in the receiver and preamp from the cold temps but I wonder how much the snow cover may be a factor. Anyone else have this experience? Michael
From: Tony Hwang on 5 Dec 2009 18:35 msg wrote: > Every year when the first snow cover and temps below 32 degrees F > occurs, I begin seeing a sharp increase in associations on my > 802.11b access point (actually dhcp leases on the downstream > router). The AP is tower mounted, with a switched duplexing > amplifier and a 12dbi colinear whip at 30 foot elevation. > The leaves have long since fallen so they are not a factor. > I have been of the conviction that the improved propagation > conditions are due to better noise figures in the receiver > and preamp from the cold temps but I wonder how much the snow > cover may be a factor. > > Anyone else have this experience? > > Michael Hi, Temperature inversion does wonders for long range reach.
From: Jeff Liebermann on 6 Dec 2009 00:28 On Sat, 05 Dec 2009 15:57:20 -0600, msg <msg@_cybertheque.org_> wrote: >Every year when the first snow cover and temps below 32 degrees F >occurs, I begin seeing a sharp increase in associations on my >802.11b access point (actually dhcp leases on the downstream >router). The AP is tower mounted, with a switched duplexing >amplifier and a 12dbi colinear whip at 30 foot elevation. >The leaves have long since fallen so they are not a factor. >I have been of the conviction that the improved propagation >conditions are due to better noise figures in the receiver >and preamp from the cold temps but I wonder how much the snow >cover may be a factor. > >Anyone else have this experience? > >Michael An increase in the number of DHCP leases can be caused by many things, none of which have anything to do with temperature. For example, one user that is playing with spoofing MAC addresses can eat up DHCP pool. Transient users, such as those driving by your AP in cars, with their PDA's, cell phones, laptops, game machines, iPod, etc, set to automagically associate with any AP, will eat up addresses. Address pool consumption is a good indication that something is happening, but it's not necessarily a change in sensitivity. There is no snow in this part of California, but I do have a bad guess if it really is a sensitivity change. The tower mounted bi-directional amplifier is the most likely culprit as it's about the only thing that could affect receiver sensitivity. My guess(tm) is that the bias on the front end GaFET or MMIC is drifting. Stabilizing the gain of these beasts over temperature is difficult. Looking at the data sheet of a typical MMIC amp: <http://pdf1.alldatasheet.com/datasheet-pdf/view/64744/HP/INA-51063.html> See Fig 5 for the NF (noise figure) versus Temperature. At 2.4GHz, it's about 4.0dB what I would guess would be 0C and 4.5dB at 85C. That doesn't look like much of a change, but it's enough to be noticeable when measuring sensitivity. At the same time, the gain drops from 18dB at roughly 0C to 16dB at 85C, which is the opposite of what you're seeing. Oh well. I checked some other common RF MMIC's and found similar changes. Unlikely, but still possible. Another dubious possibility is the antenna. It's made from copper sheet, maybe some brass, and pieces of coax cable inside. Some collinears have phasing coils. Some are just plated on a PCB (printed circuit board). A maker and model number would have been nice. I'm especially interested in a "whip" antenna at 2.4GHz. Perhaps you mean fiberglass radome? Anyway, the copper and brass move around with temperature. I would think that the antenna would have been optimized for room temperature or slightly above, but perhaps you ended up with one that is better at low temp. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
From: msg on 6 Dec 2009 13:37 Jeff Liebermann wrote: > On Sat, 05 Dec 2009 15:57:20 -0600, msg <msg@_cybertheque.org_> wrote: > >> Every year when the first snow cover and temps below 32 degrees F >> occurs, I begin seeing a sharp increase in associations on my >> 802.11b access point (actually dhcp leases on the downstream >> router). The AP is tower mounted, with a switched duplexing >> amplifier and a 12dbi colinear whip at 30 foot elevation. >> The leaves have long since fallen so they are not a factor. >> I have been of the conviction that the improved propagation >> conditions are due to better noise figures in the receiver >> and preamp from the cold temps but I wonder how much the snow >> cover may be a factor. >> >> Anyone else have this experience? <snip> > An increase in the number of DHCP leases can be caused by many things, > Address pool consumption is a good indication that something is happening, > but it's not necessarily a change in sensitivity. <snip> The change is dramatic; all throughout the warmer seasons I see perhaps one or two stranger leases per quarter (90 days); when the temp first dips to 20 degrees F (and there is a snow cover -- this seems to be essential as 20 degree days prior to the snow show no change) I see twenty or more in a single day. Last night with temps dipping to near zero degrees F, there were a dozen more leases added. > > There is no snow in this part of California, but I do have a bad guess > if it really is a sensitivity change. The tower mounted > bi-directional amplifier is the most likely culprit as it's about the > only thing that could affect receiver sensitivity. My guess(tm) is > that the bias on the front end GaFET or MMIC is drifting. Stabilizing > the gain of these beasts over temperature is difficult. Looking at > the data sheet of a typical MMIC amp: > <http://pdf1.alldatasheet.com/datasheet-pdf/view/64744/HP/INA-51063.html> > See Fig 5 for the NF (noise figure) versus Temperature. At 2.4GHz, > it's about 4.0dB what I would guess would be 0C and 4.5dB at 85C. That > doesn't look like much of a change, but it's enough to be noticeable > when measuring sensitivity. At the same time, the gain drops from > 18dB at roughly 0C to 16dB at 85C, which is the opposite of what > you're seeing. Oh well. I checked some other common RF MMIC's and > found similar changes. Unlikely, but still possible. Too bad I didn't photograph the board before installing it so that now I could know what parts it contained. These PDFs describe this amp (I am using the 2400cx): http://ftp.cybertheque.org/pub/tmp/wifi/2400cData-Sheet.pdf http://ftp.cybertheque.org/pub/tmp/wifi/2400cAppNotes.pdf > > Another dubious possibility is the antenna. It's made from copper > sheet, maybe some brass, and pieces of coax cable inside. Some > collinears have phasing coils. Some are just plated on a PCB (printed > circuit board). A maker and model number would have been nice. http://ftp.cybertheque.org/pub/tmp/wifi/omni12.pdf > I'm especially interested in a "whip" antenna at 2.4GHz. Perhaps you mean > fiberglass radome? When is a vertical a whip? This one flexes a little ;) > Anyway, the copper and brass move around with > temperature. I would think that the antenna would have been optimized > for room temperature or slightly above, but perhaps you ended up with > one that is better at low temp. Too bad there is no real temp data for this antenna; I also wonder about moisture freezing and changing characteristics.
From: Jeff Liebermann on 7 Dec 2009 20:33
On Sun, 06 Dec 2009 12:37:02 -0600, msg <msg@_cybertheque.org_> wrote: >Too bad I didn't photograph the board before installing it so that now >I could know what parts it contained. These PDFs describe this >amp (I am using the 2400cx): > http://ftp.cybertheque.org/pub/tmp/wifi/2400cData-Sheet.pdf > http://ftp.cybertheque.org/pub/tmp/wifi/2400cAppNotes.pdf "The requested object does not exist on this server." >Too bad there is no real temp data for this antenna; I also wonder about >moisture freezing and changing characteristics. With only 12dBi of gain, I don't think the internal parts are that critical. Thermal expansion isn't going to move things around that much. It's possible that the receive amp is unstable and oscillating. Maybe lowering the gain with the lower temperature returned it to sanity. Difficult to tell from here with near zero hardware info. However, if it is for real, then the receive amp is the most likely culprit. -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com jeffl(a)cruzio.com # http://www.LearnByDestroying.com AE6KS |