How
do I eliminate wireless 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 wireless antennas and cables?
Rain will have no effect upon wireless 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 wireless antenna before and after installation?
The VSWR (Voltage Standing Wave Ratio) of a 2.4 GHz wireless
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
and Bird Electronics. 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 a wireless
antenna before installation, make sure that the wireless 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.
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