Application Notes: Using The Avcom PSA-1727A To Test Wireless LAN Antennas

By Steve Zahn

Problem:
How Well Does the Access Point Receive the Mobile PC?
A traditional test of a wireless LAN involves monitoring the change in signal quality (transmission rate) and signal strength as the PC moves farther from the access point. This procedure gives a relative measure of access point coverage. The assumption is that the signal from the access point propagates in the same path as the signal from the PC, on the return trip to the access point. The assumption is valid for quick checks of signal strength. This is the traditional method of measuring wireless coverage.

If signal coverage is unexpectedly poor, is it the fault of the access point antenna or is the position of the mobile PC beyond usable range?

Solution:
Looking at the Wireless LAN through the Access Point Antenna
The PSA-1727A provides a detailed view of the ISM Band spectrum. Disconnect the antenna from the access point and connect the coaxial able to the input of the PSA-1727A. You may require two adapters to complete the connection. For example, the Cisco Areonet 350™ access point employs TNC reverse polarity (RP) connections at the antenna port(s). A TNC RP to TNC standard adapter (Pasternack #PE9532) and TNC to N (Pasternack #PE9627) allows for direct connection to Cisco wireless antennas.

Tune the PSA-1727A to 2440 MHZ and adjust the SPAN for approximately 11 MHz and note the presence of broadcast signals, if any, from mobile PCs. Activate only one wireless PC and note its signal level from the antenna. Move the PC to the location(s) where networking performance is suspected of being poor. Note the resulting signal level from the access point antenna as the PC is moved to different locations.

In general, signal levels of –76 dBm or greater ensure top performance with mobile PCs. Signal levels below –90 dBm are not usable and the access point will not recognize the mobile PC.

Problem:
Improving Wireless LAN Security
In many situations, unauthorized LAN users may enter the wireless network just beyond a desired boundary. An office space meeting area may be deemed desirable for wireless access while rouge users can emerge just beyond these controlled boundaries. While encryption provides the greatest deterrent to unauthorized access, a simple method of keeping the wireless network signals from entering uncontrolled areas aids security measures.

Solution:
Substitute the PSA-1727A for the Access Point
As in the previous test procedure, the access point antenna is connected directly to the input of the PSA-1727A. Remove all wireless PCs from area and activate only one PC. Unlike where we were optimizing network performance with signal levels of –76 dBm or greater, we are now targeting signal levels at or below –90 dBm.

Tune the PSA-1727A to 2440 MHz and set the SPAN to 40 MHz per division. Move the PC outside the desired "wireless access" area and note the signal levels from the PC. The PC may attempt to broadcast acknowledgement signals on one or more wireless channels and it is necessary to see these signals in the display of the PSA-1727A. If the signal levels exceed –90 dBm, the access point could recognize the PC's attempt to access the wireless network. Reposition the access point antenna or add directional antennas to reduce signal levels from PCs outside the desired area.

Conclusion:
Wireless LAN Testing from the Access Point Antenna Out
The access point antenna receives a variety of signals from a growing number transmissions communicating within the ISM Band. Wireless telephones, wireless security systems, wireless speakers, and wireless baby monitors are just a few of the products sharing use of the ISM spectrum. These and future products sharing the ISM Band will challenge wireless networking.

It appears only logical that in order to know if the wireless access point is effectively communicating with the desired PCs, you need to "see" ISM Band spectrum from the access point antenna. Careful wireless network antenna placement minimizes the reception of undesired signals and optimizes performance for authorized users. The PSA-1727A confirms optimal antenna placement and helps identify problems before they affect wireless LANs.

Source: AVCOM Of Virginia Inc.