Properly designed radio links can provide low cost, effective and flexible data gathering systems that operate for many years with very little maintenance.

Narrowband FM or Spread Spectrum
Until recently, narrowband FM was the dominant choice for wireless telemetry.   A licensed, coordinated frequency was applied for and received assuring any additional future users would be responsible to avoid interfering.

Now, spread spectrum is a popular choice where licensed narrowband frequencies are not available. No license to apply for, simply purchase equipment that is compliant with the FCC part 15 rules.   

The key word here is choice. Both narrowband FM and Spread Spectrum provide a wireless solution.   But, making the right choice for your telemetry system depends on your application and the tradeoffs you are willing to make.

The Risk of Interference
One concern is interference and the noise floor.   Narrowband is a licensed frequency coordinated to give you a channel free from interference.    Spread Spectrum is unlicensed using frequency hopping technology  to avoid interference.  In highly
populated areas interference may be a problem while narrowband frequencies are shared frequencies, the rules state that any new users must take measures to avoid interference to all existing users on that frequency.

Licensing
Another concern is the licensing hassle.   A common misconception regarding the narrowband licensing process is that it takes 3 to 4 months before a system can be operational.   That’s not true.   Once the application is filed with the Federal Communications Commission (FCC), a system can be on the air two weeks later. There is no need to wait for the official license grant to be sent from the FCC.

Recognizing the need for more licensed narrowband spectrum, the FCC has taken steps to ensure increased spectral efficiencies.   All new narrowband products must be designed to operate at 12.5 kHz supporting 9600 bits per second or at 25 kHz supporting 19200 bits per second.   Thus, creating a pool of radio modem products requiring narrower bandwidths in which to operate. 

The Need for Range
Next consider the range required by your system.   Typically a narrowband system will operate more reliably over a longer range than a spread spectrum system.   The operating range of a radio system is determined by various factors.

First is RF power.   A narrowband FM system can operate at a higher power level. Narrowband licenses will typically allow 2 to 5 watts power output.   By their nature, narrowband radio modems concentrate the power over a very narrow bandwidth where spread spectrum is limited to 1 watt power output that is spread over a very large bandwidth.

Next is receiver sensitivity.   The more sensitive a receiver is the more range it is possible to communicate over.   In theory, doubling the receiver sensitivity has the same effect as doubling the RF power in relation to range.   One caveat regarding receiver sensitivity.   In a noisy environment, the receiver will not be able to function at its ideal sensitivity level.

The frequency selected also has a bearing on range.   Some frequencies are more susceptible to man made noise, atmospheric conditions, absorption by trees and foliage or multipath reception.

VHF frequencies are popular for their long range.   However, man-made noise, skip interference and ducting effects were a concern.

UHF frequencies are free of most man-made noise and atmospheric conditions.    At UHF, absorption by trees and foliage causes a greater path loss, but penetration into buildings is better because the short wavelength signal has the ability to reflect off conducting objects.

At 900 MHz, skip Interference and ducting are insignificant.    However, foliage absorption of the short wavelength is greater which reduces range.   In addition, moving objects in the communications path can cause fading due to multipath reception.

Antenna gain will affect the range as it has an impact to radiated power and receiver sensitivity.   The greater the antennas gain the greater the range.   However, simply using the highest gain antenna is not the complete answer.   As antenna gain increases so does its size and you loose antenna beam width.   A more practical approach is to minimize the antenna size to reach an acceptable beam width both vertically and horizontally.    Vertically acceptable as not to over shoot sites close to the transmitter site and horizontally acceptable as to not miss sites outside the antennas beam angle.

Range losses can also occur through the feedlines and connectors.   At lower frequencies, the attenuation loss in the feedlines and connectors is reduced. Remember every 3 dB of loss is roughly cutting your power output in half thus limiting the amount of RF energy reaching your receiver and decreasing your range.

At the remote site,   a sensor or sensors are typically the data source.   The output of the sensor(s) is usually converted to digital data by a small computer device or RTU (Remote Terminal Unit). The RTU is interfaced to a radio modem device.   The modem converts the digital data into an analog signal that can be transmitted over the air.   The radio transmitter then transmits the signal to the host site radio receiver.   Now the process is reversed.    The modem takes the analog signal received and converts it back to a digital form that can be processed by the data recovery equipment.

In a typical application, the base or host site requests data from the remote site(s).   The base transmits a request to the remote unit telling it to send its data.   The base reverts to a receive mode and awaits the transmission from the remote site.   After the remote sends it’s data, it goes back to a receive mode waiting for further instructions to come from the base.   Once the base receives the remote site information, it may send additional instructions to that site or continue on to request data from the next remote site.   This polling process continues until all the remotes in the system have sent their data.

System Design Criteria
There are a few steps that can ensure a well-designed radio system.

System Architecture
Typical telemetry system architecture will involve a base or host location and one or multiple remote locations.   The base will be centrally located with the remotes dispersed around it. In some cases, it will be necessary to insert a repeater to re-transmit a signal. (See figure 1 and 2 below).

Figure 1 Typical Point to Multi-Point System               Figure 2  Point to Multi-Point System with a
                                                                                                   Repeater Link

Site Selection

Antenna Selection

 Installation
Proper installation can mean the difference between a marginal telemetry system and a reliable one.   Beyond the requirement for a reliable power source and protection from the elements, proper installation requires attention to grounding and lightning protection.   Proper grounding guards against unwanted noise entering your system and will help prevent loss to communications equipment in the event of a lightning strike.   Lightning protection can prevent a catastrophic failure just when you need your telemetry system communications the most to warn about a power failure at a mission critical site.

In Summary
In the end, it comes down to making the right choice for your telemetry system.   Proper up front planning of sites will ensure good network coverage.   Be aware of the tradeoffs between narrowband and spread spectrum technology.   If your application requires mission critical information to be transmitted over a distant and varying terrain, licensed narrowband can be the best choice.

Look for suppliers who specialize in products designed for transmitting data.   Can they support you with system design, technical service and the longest warranty? What about providing  diagnostic information on the health and well being of your radio network?   Can their product make your job easier?   After all, isn’t that the choice you’re looking for?  

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