Archived from groups: alt.internet.wireless (
More info?)
Roby <roby@no-address.net> wrote:
>Jo wrote:
>> Having problems with wireless signal from a LINKSYS wireless router.
....
>> Why are there 2 aerials anyway?
>
>Only one antenna is used to transmit. Both are used to receive: the router
>compares signal strength from each antenna and uses the stronger of the
>two. Might even be smart enough to properly combine the two for maximum,
>but I doubt it. In any event, adjusting antenna orientation may help.
It switches between the two antennas for both transmit and
receive. An inherent problem exists because the received signal
strength is used to determine which antenna is selected, and
that may not be the best antenna for the next transmission.
In receive mode the radio periodically switches antennas to
determine if there is a better received signal. The next
transmission, even if it is to a different client, will be
on the last antenna selected for a good received signal.
One example that demonstrates the potential problem, would be if
two highly directional antennas are used, one pointed North and
one pointed South, to two different clients. If the last
received signal is from Client North and the next transmission
goes to Client South... the data packets sent to Client South
will not be received until Client South, for whatever unrelated
reason, happens to make a transmission (which will cause the
receiver to lock onto the South antenna for the next
transmission).
Such a system will work very well for periods of time (while all
data goes to one of the two clients), and will have dropouts and
long delays at other times (any time data is being sent to both
clients).
A couple of conclusions about wireless network topology and
configuration can be drawn from the above. One is that if there
are multiple clients, the two antennas should be identical.
Another is that if there is only one client (e.g., a roving
laptop at home), then there could be two very different antennas
to provide coverage in two very distinct areas of the house.
Changing the orientation of the antennas probably amounts to
having two very distinct antennas, because the difference
between horizontal and vertical polarization can be much more
than 25 dB of signal strength (which is a *lot*). But, by the
same token... in most locations there are so many objects which
reflect 2.4GHz signals that the actual difference between
horizontal and vertically polarized signals by the time they get
from the transmitter to the receiver is probably less than 6 dB.
And that means having one antenna vertical and the other
horizontal might actually provide just about the same benefits
as having them separated by 6 inches! Don't do that with a
point-to-point link, but it is worth trying in a home or office
setting.
>Changing the router position - even by a few inches - may help as well.
>Radio energy at these frequencies bounces off obstacles and often
>re-combines with itself to produce local hot or deadspots. It would be
>dandy to have magic goggles that could display field strength. Maybe Jeff
>will loan you his.
All one has to do is try it, and use software that shows signal
strength. Just keep in mind that it takes 20-40 seconds for the
effects to register because the radio reports a running average
rather than an instantaneous signal level.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
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