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Spread-spectrum radio communications, long a favourite technology of the military, is in the midst of explosive commercial development. Driving cost down has always been a top goal of any wireless system designer. The following paragraphs detail the FCC rules and expand on the options available to the system operator or integrator.
Where does Part 15 fit into SCADA?
The Code of Federal Regulations (CFR), the codification of the general and permanent rules and regulations published in the Federal Register, is divided into 50 titles that represent broad areas subject to Federal regulation.
Title 47 of the CFR addresses Telecommunications and is administered by the Federal Communications Commission (FCC). One of the most commonly referenced parts of Title 47 is Part 15, which covers the regulations under which an intentional, unintentional, or incidental radiator that can be operated without an individual license. This covers as well the technical specifications, administrative requirements and other conditions relating to the marketing of FCC 15 devices.
FCC Part 15 Subpart C governs intentional radiators. The various types of intentional radiators covered by Subpart C include cordless telephones, remote control and alarm transmitters, and spread-spectrum systems for wideband data transmission. Intentional radiators governed by Part 15 FCC must either incorporate a permanently attached antenna or provide a unique coupler to prevent the use of un-authorized antennas.
The FCC rules for operation of radio transmitters are, for the most part, very detailed regarding fundamental field strength, power and/or power density, frequency accuracy, and permitted harmonic and spurious emissions.
Part 15 FCC regulations emphasize that the Part 15 device manufacturer must design a system that will not cause interference with, and be able to tolerate, the noisy primary users of the band. This is where spread spectrum systems excel because of their low noise transmissions and ability to operate in an adverse environment. The responsibility for resolving any harmful interference to radio services that might occur from their use lies with the operator of those devices.
How to choose an external antenna
FCC regulations dictate that effective isotropic radiated power (EIRP) at transmitters cannot exceed 36dBm per system when using a 1 Watt radio, based on a 6 dBi antenna. 1 watt transmit power (+30 dBm) plus 6 dBi antenna gives the +36 dBm EIRP limit. More transmit power and less antenna gain, or the opposite, is ok as long as you don’t exceed that number.
They don’t talk about losses in cables, filters or connectors but those must be included. As an example, if you had 3 dB of losses and a +30 dBm transmitter, then you could use a 9 dBi antenna. If you want a higher gain antenna for more directivity, say 12 dBi, then you need to reduce the transmit power to +27 dBm.
For anyone selling spread spectrum wireless systems, these limitations are a key factor in overall system design. Key resources include the radio manufacturer itself; the top manufacturers are quite prepared to make recommendations for any given application. Path profiles or actual site surveys provide the best methods for determining appropriate antenna gain values.
For typical oil & gas SCADA applications, the most common choices for remote sites remain unity gain omni-directional, or 8 dBi and 12 dBi yagi (directional) antennas. Typical master omni-directional base station antennas are usually 5 dBi or 8 dBi models.
Good SCADA automation can become GREAT when optimized antenna systems are incorporated into the design from the start. Visit http://wavelinkantenna.com to see what great antenna options Wavelink Antenna Systems can offer you for your spread spectrum application. Gary Tootle advises and supports wireless communications professionals throughout USA and Canada at http://wavelinkantenna.com/contact.php
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