B&B Electronics ZXT9-MB2A-KIT - Manual - Manual (Page 160)

Document Number: ZXTxRM-5110m
Page
160
You can often improve your receive sensitivity, and therefore your range, by reducing data rates
over the air. Receive sensitivity is a function of the transmission baud rate so, as baud rate goes
down, the receive sensitivity goes up. Many radios give the user the ability to reduce the baud rate
to maximize range.
The receive sensitivity of a radio also improves at lower frequencies, providing another significant
range advantage of 900 MHz (vs. 2.4 GHz) - as much as six to twelve dB!
D.4 You must consider RF noise
RF background noise comes from many sources, ranging from solar activity to high frequency
digital products to all forms of other radio communications. That background noise establishes a
noise floor which is the point where the desired signals are lost in the background ruckus. The
noise floor will vary by frequency.
Typically the noise floor will be lower than the receive sensitivity of your radio, so it will not be a
factor in your system design. If, however, you’re in an environment where high degrees of RF noise
may exist in your frequency band, then use the noise floor figures instead of radio receive
sensitivity in your calculations. If you suspect this is the case, a simple site survey to determine the
noise floor value can be a high payoff investment.
When in doubt, look about. Antennas are everywhere nowadays - on the sides of buildings, water
towers, billboards, chimneys, even disguised as trees. Many sources of interference may not be
obvious.
D.5 Fade Margin is critical for reliable operation in adverse weather
and interference
Fade margin is a term critical to wireless success. Fade margin describes how many dB a received
signal may be reduced by without causing system performance to fall below an acceptable value.
Walking away from a newly commissioned wireless installation without understanding how much
fade margin exists is the number one cause of wireless woes.
Establishing a fade margin of no less than 10dB in good weather conditions will provide a high
degree of assurance that the system will continue to operate effectively in a variety of weather,
solar, and RF interference conditions.
There are a number of creative ways to estimate fade margin of a system without investing in
specialty gear. Pick one or more of the following and use it to ensure you’ve got a robust
installation:
Some radios have programmable output power. Reduce the power until performance degrades,
then dial the power back up a minimum of 10dB. Remember again, doubling output power yields 3
dB, and an increase of 10dB requires a ten-fold increase in transmit power.
Invest in a small 10dB attenuator (pick the correct one for your radio frequency!). If you lose
communications when you install the attenuator installed in-line with one of your antennas, you
don’t have enough fade margin.
Antenna cable is lossy, more so at higher frequencies. Specifications vary by type and
manufacturer so check them yourself but, at 900MHz, a coil of RG58 in the range of 50 to 100 feet
(15 to 30 m) will be 10dB. At 2.4GHz, a cable length of 20-40 feet (6 to 12 m) will yield 10dB. If your
system still operates reliably with the test length of cable installed, you’ve got at least 10dB of fade
margin.