Appendix G: Radio Frequency Basics
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Manual Documentation Number: ZXTx-IO-x-2310m
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 around. 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.
Fade Margin
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 has greater attenuation at higher frequencies. Specifications vary by type and manufacturer so check
them yourself but, at 900 MHz, 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.
Remember Your Math
Contrary to popular opinion, no black art is required to make a reasonable prediction of the range of a given radio signal.
Several simple concepts must be understood first, and then we can apply some simple rules of thumb.
The equation for successful radio reception is:
TX power + TX antenna gain – Path loss – Cabling loss + RX antenna gain – 10dB fade margin > RX Radio sensitivity
or (less commonly) RF noise floor
Note that most of the equation’s parameters are easily gleaned from the manufacturer’s data. That leaves only path loss and,
in cases of heavy RF interference, RF noise floor as the two parameters that you must establish for your particular installation.
In a perfect world, you will measure your path loss and your RF noise conditions. For the majority of us that don’t, there are
rules of thumb to follow to help ensure a reliable radio connection.
RF Attenuation and Line of Sight
In a clear path through the air, radio signals attenuate with the square of distance. Doubling range requires a four-fold
increase in power, therefore:
Halving the distance decreases path loss by 6dB.
Doubling the distance increases path loss by 6dB.
When indoors, paths tend to be more complex, so use a more aggressive rule of thumb, as follows:
Halving the distance decreases path loss by 9dB.
Doubling the distance increases path loss by 9dB.
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