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1. Thou shall know thy dBm and recall thy high school logarithms.
Radio Frequency (RF) power is measured in milli Watts (mW) or, more usefully, in a
logarithmic scale of decibels (dB), or decibels referenced to 1 mW of power (dBm).
Since RF power attenuates as a logarithmic function, the dBm scale is most useful.
Here are some examples of how these scales relate:
1mW = 0dBm
2mW = 3dBm
4mW = 6dBm
10mW = 10dBm
100mW = 20dBm
1W = 30dBm
A 2-fold increase in power yields 3dBm of signal.
A 10-fold increase in power yields 10dBm of signal.
A 100-fold increase in power yields 20dBm of signal.
2. Covet not high frequencies - as the lower the frequency, the more forgiveth the
laws of physics and propagation.
Industrial applications typically operate in "unlicensed" frequency bands, also
referred to as ISM (Industrial, Scientific and Medical). The frequencies and power
of these bands varies from country to country. The most common frequencies
encountered are:
* 2.4 GHz - nearly worldwide
* 915 MHz band - North America, South America, some other countries
* 868 MHz band - Europe
As frequency rises, available bandwidth typically rises, but distance and ability to
overcome obstacles is reduced. For any given distance, a 2.4 GHz installation will
have roughly 8.5 dB of additional path loss when compared to 900 MHz. However, lower
frequencies require larger antennas to achieve the same gain.
3. Honor thy receive sensitivity - as long-range performance is not a function of
transmit power alone.
The more sensitive the radio, the lower the power signal it can successfully
receive, stretching right down to the noise floor. There is so much variety in
"specsmanship" for radio sensitivity, that it is difficult to make a meaningful
comparison between products. The most meaningful specification is expressed at a
particular bit error rate and will be given for an ideal environment shielded from
external noise. Unless you're in a high RF noise environment (typically resulting
from numerous similar frequency radio transmitters located nearby), the odds are
good that the noise floor will be well below the receive sensitivity, so the
manufacturer's rated receive sensitivity will be a key factor in your wireless
system and range estimates.
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!
4. Thou shalt be wary of radio noise and recognize situations whereth radio noise
may hamper thine installation.
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.
5. Thou shalt always know thine fade margin - lest ye have a wireless link that
worketh not in rain, snow, or the presence of 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:
1. 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.
2. 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.
3. 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.
6. Thou shalt use thine given powers of mathematics and logic when specifying
wireless equipment.
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 established 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.
7. Thou shalt not allow leafy greens or mounds of earth between thine antennas; and
thou shalt elevate thy antennas towards the heavens; and thou shalt never, ever,
attempt a system at the manufacturer's maximum advertised distance.
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.
Radio manufacturers advertise "line of sight" range figures. Line of sight means
that, from antenna A, you can see antenna B. Being able to see the building that
antenna B is in does not count as line of sight. For every obstacle in the path,
de-rate the "line of sight" figure specified for each obstacle in the path. The type
of obstacle, the location of the obstacle, and the number of obstacles will all play
a role in path loss.
Visualize the connection between antennas, picturing lines radiating in an
elliptical path between the antennas in the shape of a football. Directly in the
center of the two antennas the RF path is wide with many pathways. A single obstacle
here will have minimal impact on path loss. As you approach each antenna, the
meaningful RF field is concentrated on the antenna itself. Obstructions located
close to the antennas cause dramatic path loss.
Be sure you know the distance between antennas. This is often underestimated. If
it's a short-range application, pace it off. If it's a long-range application,
establish the actual distance with a GPS or Google Maps.
The most effective way to reduce path loss is to elevate the antennas. At
approximately 6 feet high (2 m), line of sight due to the Earth's curvature is about
3 miles (5 km), so anything taller than a well-manicured lawn becomes an obstacle.
Weather conditions also play a large role. Increased moisture in the air increases
path loss. The higher the frequency, the higher the path loss.
Beware leafy greens. While a few saplings mid-path are tolerable, it's very
difficult for RF to penetrate significant woodlands. If you're crossing a wooded
area you must elevate your antennas over the treetops.
Industrial installations often include many reflective obstacles leading to numerous
paths between the antennas. The received signal is the vector sum of each of these
paths. Depending on the phase of each signal, they can be added or subtracted. In
multiple path environments, simply moving the antenna slightly can significantly
change the signal strength.
Some obstacles are mobile. More than one wireless application has been stymied by
temporary obstacles such as a stack of containers, a parked truck or material
handling equipment. Remember, metal is not your friend. An antenna will not transmit
out from inside a metal box or through a storage tank.
Path Loss Rules of Thumb:
* To ensure basic fade margin in a perfect line of sight application, never exceed
50% of the manufacturer's rated line of site distance. This in itself yields a
theoretical 6dB fade margin - still short of the required 10dB.
* De-rate more aggressively if you have obstacles between the two antennas, but not
near the antennas.
* De-rate to 10% of the manufacture's line of site ratings if you have multiple
obstacles, obstacles located near the antennas, or the antennas are located indoors.
8. Antennas
Antennas increase the effective power by focusing the radiated energy in the desired
direction. Using the correct antenna not only focuses power into the desired area
but it also reduces the amount of power broadcast into areas where it is not needed.
Wireless applications have exploded in popularity with everyone seeking out the
highest convenient point to mount their antenna. It's not uncommon to arrive at a
job site to find other antennas sprouting from your installation point. Assuming
these systems are spread spectrum and potentially in other ISM or licensed frequency
bands, you still want to maximize the distance from the antennas as much as
possible. Most antennas broadcast in a horizontal pattern, so vertical separation is
more meaningful than horizontal separation. Try to separate antennas with
like-polarization by a minimum of two wavelengths, which is about 26 inches (0.66 m)
at 900 MHz, or 10 inches (0.25 m) at 2.4 GHz.
9. Cable loss
Those high frequencies you are piping to your antennas don't propagate particularly
well through cable and connectors. Use high quality RF cable between the antenna
connector and your antenna and ensure that all connectors are high quality and
carefully installed. Factor in a 0.2 dB loss per coaxial connector in addition to
the cable attenuation itself. Typical attenuation figures for two popular cable
types are listed below.
Loss per 10 feet (3 meters) of cable length
Frequency RG-58U LMR-400
900 MHz 1.6 dB 0.4 dB
2.4 GHz 2.8 dB 0.7 dB
While long cable runs to an antenna create signal loss, the benefit of elevating the
antenna another 25 feet (7.6 m) can more than compensate for those lost dB.
10. Thou shalt recognize the issues of latency and packetization before thou issueth
purchase orders.
Before you lift a finger towards the perfect wireless installation, think about the
impact of wireless communications on your application. Acceptable bit error rates
are many orders of magnitude higher than wired communications. Most radios quietly
handle error detection and retries for you - at the expense of throughput and
variable latencies.
Software must be well designed and communication protocols must be tolerant of
variable latencies. Not every protocol can tolerate simply replacing wires with
radios. Protocols sensitive to inter-byte delays may require special attention or
specific protocol support from the radio. Do your homework up front to confirm that
your software won't choke, that the intended radio is friendly towards your
protocol, and that your application software can handle it as well.
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