User's Manual

Trailblazer Installation and User Manual version 1.02 7
What’s an acceptable fade margin? A rule of thumb theory is that 10 dB of fade
margin will deliver about 90% reliability, and 20 dB will deliver about 99%
reliability. Because there is only a small ground wave component involved in the
radio propagation at 2.4 GHz, the above numbers presume both clear LOS (line
of sight) and clearing at least 60% of the first Fresnel zone. A fade margin of 16
to 20dB is suggested to overcome multi-path degradation and polarization.
2.1.6 Interference
The ISM (Industrial, Scientific, and Medical) band at 2.400-2.483 GHz is an
unlicensed, shared band. All users in the band must accept all other signals
within the band, interfering or not. There are only a few devices operating in this
band, but they could become a source of interference for the Trailblazer
products. If the interfering signal is stationary and has a bandwidth less than 3
MHz, it will not create any significant problem even if it is 100 times (20dB)
stronger than the receive threshold. If the interfering signal has a bandwidth
wider than 4 MHz and is 10 times (10dB) stronger than the receive threshold, it
can render channel(s) unusable. If the interfering signal is frequency hopping
throughout the band, such as the LAN bridge device, and is only 1/10 (–10dB) as
strong as the receive threshold, it can cause significant dropouts.
Some examples are:
Video Transmitters
These devices use analog radios which are usually fixed in frequency, utilize 6 to
10 MHz of bandwidth and are low power with an ERP of less then +10dBm.
Cordless phones
These devices are narrow band and fixed in frequency (during each use), utilize
1 to 2 MHz of bandwidth, are low power with an ERP of less then +10dBm. It is
strongly recommended to avoid the use of 2.4GHz cordless phones in the vicinity
of Trailblazer products.
Local Area Network (LAN) Bridges
These devices are true spread spectrum devices. They either frequency hop a 1
MHz bandwidth over the complete band or utilize CDMA of 16 MHz bandwidth
and are stationary. ERPs can range up to +30dBm.
Microwave Ovens
Microwave oven outputs have been measured at levels up to +20dBm ERP. The
circulators built into the ovens cause transmission of narrow pulses that sweep
the 2.4GHz band. The higher power spikes are mostly concentrated in the upper
half of the band between 2.450 and 2.485 GHz.
2.1.7 RF Cable Sizing and Recommendations
Before making any decisions as to which cable to use, first consider several
factors; the final length of the cable, the amount of power you want the cable to
tolerate, the loss factor of the cable at your chosen frequency and the outdoor
climate conditions the cable will be exposed to. For most general installations,