(antenna parallel to the floor) the doughnut is standing up like a giant automobile tire… great for the people on the floor above you and below you; great for the folks to the left and right of the access point; not so great for the folks on the same floor but in front of, or behind the access point.

Have you ever seen someone configure his or her access point antennae to look like a 1960ʼs television “rabbit ears” antenna? One antenna is angled to the left, and the other is angled to the right. Hopefully this isnʼt you. If it is, fix it before anyone finds out! Remembering how antenna diversity works, you can see that if the two antennae are at odd angles then the doughnut patterns for transmission and reception from the two antennae are tilted at opposing angles. This effectively defeats antenna diversity for most of the reception area. Itʼs best to orient the two antennae straight up. The fact that access points have flexible antenna couplings is to allow the access point to be placed horizontally, vertically, or even upside-down on the ceiling while retaining the vertical orientation of the antennae. Figure 4.1 (below) shows a good and a bad orientation for the access point antennae based on an understanding of general dipole radiation patterns, antenna reciprocity, and 802.11 antenna diversity.

Figure 4.1 Correct and Incorrect 802.11 Access Point Antenna Orientation

Transmitters and Receivers with Different Power Levels

Another application of the Reciprocity Theorem is in the design of an 802.11 network in which the clients and the access points use different transmit power levels. Remember that antenna reciprocity only exists if both sides of a communication link use the same input power to their antennae. Weʼve discussed the situation in which an access point with a highly directional antenna can push its signal out many miles to a client with a standard dipole antenna. Reciprocity tells us that the same characteristics that allow the directional antenna to get its signal to the client allow it to receive the clientʼs omnidirectional transmission. This reciprocity only works, however, if both client and access point have the same (typically 100 mW) input power.

Consider what happens if the access point has a 200 mW input power to a standard omnidirectional dipole antenna and the client has a 100 mW input to an identical omnidirectional antenna. The client could find itself in a location where it could receive the signal from the access point but could not, with its 100 mW transmit power, send a response back to the access point. In effect, using an access point that is more powerful than a client “breaks” reciprocity and causes chaos in the network. Clients attempt to associate with access points that they will never reach.

Consider what happens when the client is more powerful than the access point. Letʼs assume that weʼve configured our access point to use only 60 mW of output power. This may be in an effort to minimize bleed-over of the signal from the access point into an adjacent space (like a parking lot). The 100 mW client is now going to communicate to the 60 mW access point. This situation could actually be beneficial. True, reciprocity is broken, but this time in a way that augments, rather than detracts, from overall network performance.

Copyright 2003 - Joseph Bardwell