FAQs - Troubleshooting

How do I eliminate interference from a new competitor's unlicensed system?

The best way to reduce the interference is to work with him and agree upon polarizations, channels and coverage areas. One of you should use vertical polarization and the other horizontal polarization for PtMP. If you are using DSSS channel 1 for a certain area, then he should use channel 6 or 11. Both of you should be using sector panel antennas with good F/B. If you are using DSSS and he is using FHSS, then you must rely upon polarization, distance and sectorization for isolation between the systems. You may be able to place a null in your AP antenna pattern toward his nearest AP location. If possible, place your AP as far away from his AP as possible, so that your customers can use the directivity and F/B of the CPE antenna for isolation. If both of you are using FHSS, then you should agree to use separate non-interfering sets. Wireless video (ENG) systems should also be avoided. The Broadband Wireless Alliance has offered to coordinate frequencies for interested parties.

What effect does rain and ice have on antennas and cables?

Rain will have no effect upon antennas protected within radomes. The radomes must also have a drain hole for condensation drainage. However, Yagi antennas without radomes are highly vulnerable to rain, as the rain drops will accumulate on the elements and detune the performance. (The droplets actually make each element look longer than it really is!) Water intrusion in coaxial cable will increase the cable losses significantly and raise the VSWR at the transmitter. (See the next question for weatherproofing suggestions) If the link does not come back up after the rain evaporates, then you probably have a water-intrusion problem in the cable. Sometimes you can open both ends of a cable and measure a very small voltage across the center conductor to shield (< 100 mV) if water is inside the cable. This is caused by galvanic action between dissimilar conductors with water as the electrolyte. Ice accumulation on exposed elements can cause the same detuning effect as rain, however it stays around longer. Radomes will protect the radiator from most of these effects, however if the radome surface is very close to the radiator and/or the ice is very thick, then the VSWR may be impaired. Ice can also damage antennas if it falls on the antenna from a higher structure or tree.

How can I check the VSWR of my antenna before and after installation?

The VSWR (Voltage Standing Wave Ratio) of a 2.4 GHz antenna may be checked with most HP/Agilent or Anritsu RF Network Analyzers that have a maximum frequency of 3 GHz. Lower-cost hand-held units are also available from Anritsu , Bird Electronics, and Praxsym. The Anritsu S332B Sitemaster / Spectrum Analyzer combo has both VSWR and Spectrum Analyzer features in one unit. It is also possible to use an IFR spectrum analyzer for return loss (VSWR) measurements. The WLANexpert also has VSWR measurement capabilities for PRISM chipset-based cards. Antennas at this frequency may be checked with an attached transmission line no longer than: 25 feet (LMR-400 & 600), or 5 feet (LMR-195 & RG-58). Longer cables will make the VSWR appear much lower than it really is. When testing an antenna before installation, make sure that the antenna is outdoors and pointing away from the ground and any metallic objects. A VSWR of less than 1.5:1 is excellent, and less than 2:1 is acceptable. Most antenna manufacturers spec their antennas for either 1.5:1 or 2:1 across the bandwidth.

How can I tell if my Access Point antenna is working correctly?

There are 2 main properties that you can check if you have the proper equipment. The first property is the antenna's VSWR (voltage standing wave ratio). See the separate FAQ on how to measure this. The second main property is the radiation pattern. For an omnidirectional antenna, the received signal strength at a client should be similar for all angles at a fixed radius from the AP. Since the client antenna may have directional properties and terrain & obstacles may affect the AP coverage area, the received signal may vary as much as 6 to 10 dB over different paths at a fixed distance from an AP. For a directional or sector antenna, the recieved signal strength at a client should be at least 15 dB stronger off the front side of the antenna than off the back side at a fixed distance. If it isn't, then the antenna may be defective or damaged. Another way to check to see if an antenna is working is to unplug the coaxial cable from the antenna. If the received signal off the front side of the antenna doesn't change significantly, then the antenna may be defective or damaged. This may also indicate a problem with the cable or connectors too.

How can I check my coax cable assemblies?

The easiest and quickest way is to use a multimeter (ohmmeter). For each cable assembly, touch each of the 2 multimeter probes to the center conductor at each end of the cable. The multimeter should indicate a "short" or less than 1 ohm resistance. Extremely long cables will show more resistance. Also, touch each of the 2 probes to the sheild conductor at each end of the cable. The multimeter should indicate a "short" again. Lastly, touch 1 probe to the center conductor and 1 probe to the sheild at one end of the cable. The other end must be left unconnected. The multimeter should indicate an "open" or greater than 10,000 ohms.

Why is the range so poor for my Access Point?

There can be many reasons why you are seeing reduced range from an AP. First, make sure that you are getting at least 30 dBm EIRP from your AP and antenna combination. The maximum allowed by the FCC is 36 dBm. Use a short length of low-loss cable between the AP and antenna, and if you are just using one antenna, make sure that it is connected to the TX/RX port. If you have to use an omnidirectional antenna, make sure that it has at least 7 dBi gain. 10 dBi is better. On the client end, outdoor antennas with at least 10 dBi gain should be used if possible. To obtain distances over 6 miles, the client EIRP should be at least 36 dBm. If the clients use a laptop with a built-in antenna, they should use systems with at least 100 mW transmit power. Laptops with built-in antennas and 30 mW or less should not expect more than 300-500 feet range from an AP.

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Technical FAQ Categories

Basics, Access Point, Antenna Properties, Canadian RSS-210, Client Equipment, FCC Part 15, Point to Multi-Point, Point to Point, Pre-installation and Site Preparation, Radio Propagation, Reference Material, Troubleshooting.