10 Tips to Remember About Wireless
1. Basic Logarithms
Radio Frequency (RF) power is measured in milliwatts (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:
2. The higher the frequency. the more multi-path propagation you will encounter.
Industrial applications typically operate in “license free” frequency bands, also referred to as ISM (Industrial, Scientific and Medical). The frequencies and power of these bands vary 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
3. Long range reception is not completely dependent on transmit power.
The more sensitive the radio, the lower the power signal it can successfully receive.
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. 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). It can be as much as 6 to 12 dB.
4. Knowing the noise floor value can be as useful as knowing the receive sensitivity.
RF background noise can come from sources like solar activity, high frequency digital products or competing forms of radio communications. The background noise establishes a noise floor at which the desired signals are lost in the background ruckus. The noise floor will vary by frequency.
The noise floor will be often be lower than the receive sensitivity of your radio, in which case, it would not be a factor in your system design. But, if you are in an environment where high degrees of RF noise exist in your frequency band, use the noise floor figures rather than the radio receive sensitivity to make your calculations. Doing a simple site survey to determine the noise floor value can pay off down the road.
Sources of interference are not always obvious. When in doubt, look about. Antennas are everywhere. They are on the sides of buildings, water towers, billboards and chimneys. Some are even disguised as trees.
5. Everything from rain to solar flares can affect wireless communications.
Establishing a fade margin of no less than 10dB in good weather conditions will help assure that the system will continue to operate effectively when conditions degrade due to weather, solar, and RF interference. There are some creative ways to estimate the fade margin of a system without investing in specialty gear. Pick one or more of the following and use it to ensure that you have a robust installation:
a. Some radios have programmable output power. Reduce the power until performance degrades, then dial the power back up to a minimum of 10dB. Remember that doubling output power yields 3 dB and an increase of 10dB requires a ten-fold increase in transmit power.
b. Invest in a small 10dB attenuator. (Use the correct one for your radio frequency.) You do
c. Antenna cable is lossy and, more so, at higher frequencies. Specifications vary by type and manufacturer. So check them yourself. At 900MHz, a coil of RG58 in the range of 15 to 30 m (50 to 100 ft) will be 10dB. At 2.4GHz, a cable length of 6 to 12 m (20 to 40 ft) will yield 10dB. If your system still operates reliably with the test length of cable installed, you have got at least 10dB of fade margin.
6. Simple math can help you specify the right wireless equipment.
Predicting the range of a given radio signal is not a black art. There are 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
Note that most of the 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 yourself.
7. Antenna placement makes a big difference.
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:
• Doubling the distance increases path loss by 6dB.
When indoors paths tend to be more complex, so use a more aggressive calculation:
• Doubling the distance increases path loss by 9dB.
Radio manufacturers advertise “line-of-sight” range figures. Line-of-sight means that you can see antenna B from antenna A. Just being able to see the building that contains antenna B does not count as line-of-sight. For every obstacle in the path, de-rate the line-of-sight figure specified for each obstacle. The type of obstacle, the location of the obstacle, and the number of obstacles will all play a role in path loss.
Visualize the lines radiating between the antennas as an elliptical path in the shape of a football. The center of the RF path is wide, with many pathways. A single obstacle here will have minimal impact on path loss. But, as you approach each antenna, the RF field narrows. Obstructions located close to the antennas can cause dramatic path loss.
It is easy to underestimate the distance between antennas. If it is a short-range application, pace it off. If it is 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 2 meters (6 ft) the line of sight about is only 5 kilometers (3 mi) due to the curvature of the earth. Anything taller than a well-manicured lawn will be an obstacle.
Weather conditions matter. Increased moisture in the air increases path loss. The higher the frequency; the higher the path loss.
Beware of foliage. A few mid-path saplings are tolerable but, it is very difficult for RF to penetrate significant woodlands. If you are crossing a wooded area, your antenna must be higher than the treetops.
Industrial installations often include many reflective obstacles. These will create numerous paths between the antennas. The received signal is the vector sum of each of these paths. Depending upon 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. Plan for that.
Remember that metal is not your friend. An antenna will not transmit from the inside of a metal box or through the walls of a storage tank.
Path Loss Rules of Thumb:
- Never exceed 50% of the manufacturer’s rated line-of-sight distance. This alone yields a theoretical 6dB fade margin – a big step on the way to 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 if the antennas are located indoors.
8. Competing antennas can interfere with your own.
Proper antenna choice and location will have a big impact on your wireless connectivity. Antennas can increase their effective power by focusing the radiated energy in a desired direction. Using the correct antenna not only focuses power, it also reduces the amount of power broadcast into areas where it is not needed.
But, one symptom of the increasing popularity of wireless is the fact that everyone seeks out the highest convenient places to mount their antennas. It is not uncommon to arrive at a job site and find that other antennas are already sprouting all over your installation point. Even if you suspect that these systems are spread spectrum and likely to be using other ISM or licensed frequency bands, you will still want to maximize the distance between their antennas and your own. 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 0.66 meters (26 in) at 900 MHz, or 0.25 meters (10 in) at 2.4 GHz.
9. Use good cable in the shortest possible segments.
High frequencies do not propagate well through cable and connectors. Use high quality RF cable between the antenna connector and your antenna. Ensure that all connectors are also high quality and that they are 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.
While long cable runs to an antenna create
10. Consider latency and packetization before issuing purchase orders.
Bit error rates for wireless communications are orders of magnitude higher than those for wired communications. Most radios quietly handle error detection and retries for
-- by Mike Fahrion, Chief Technology Officer, B+B SmartWorx / Advantech