Conformity Magazine - April 2008 - (Page 24)

EIRP, ERP and Conducted Power Radiated power may be expressed in many different ways, such as power density, watts per sq. meter, or as field strength at some distance. Regulatory bodies use terms such as ERP (effective radiated power), EIRP (effective isotropically radiated power), or field strength. To confuse matters further, regulatory bodies sometimes use term e.i.r.p (equivalent isotropically radiated power) with small letters, or may use both terms interchangeably. In most cases, the output from low power transmitting devices is measured as field strength due to embedded antennas, even though the limit might be expressed as conducted power. In fact, field strength is the only physical quantity that can be measured outside the physical device (the rest are just calculations). Because SRD/LPD antennas are short compared to the wavelength λ, they are inefficient [9] and, their directivity or gain is also small [8]. For these reasons, ETSI combines their radiated power and the gain together as part of EIRP (equivalent isotropically radiated power, or e.i.r.p). It also includes the duty cycle of transmission in it. Thus, the ETSI expression for EIRP (e.i.r.p) is given as in [6] sec. 5.7.2.2, all in dB: P = A + G + log (1/x) where A is the average conducted power fed to the antenna, measured by an instrument, G the antenna gain, and x is the transmitter duty cycle (Ton/(Ton + Toff)). In Figure 1 above, electric field E and Pd at r-m from the antenna are related by another ETSI definition for the EIRP (effective isotropic radiated power) according to [5] Annex D as: EIRP = Pd x Area of Sphere (sphere of radius r, the distance from antenna) (2) (1) That makes perfect sense due the word ‘isotropic’ in its definition. Further, in this section, it is related to ERP as: EIRP = ERP / G Thus, EIRP = ERP / 1.62 (Effective Isotropically Radiated Power for a dipole) So, according to ETSI: EIRP = ERP – G (powers in dBm and gain G in dBi (2.14 dB)) On the other hand, the FCC uses EIRP terminology only for ultra-wideband systems at UHF and microwave frequencies, and defines it as Equivalent Isotropic Radiated Power, e.g., in Reference [1] sec. 15.501 para. (k), as follows: EIRP = Ps + G (G is any value of gain, all in dB) Where Ps is the power sent to the antenna (conducted power), and G is antenna gain (any value). The FCC again defines EIRP according to [10], where EIRP and ERP are related in dB, as: EIRP = ERP + G (G = 1.64 or 2.14 dB, the dipole antenna gain) Compare Equation 5 with the ETSI definition. Apparently, Ps and ERP are equivalent for a dipole antenna, but ERP is not considered the same quantity by the FCC and ETSI. And one may think of Ps = P2 in Figure 1, but antenna efficiency and VSWR are not parts of the equations. Therefore, the field created E may not be an exact indication of the power (5) (4) (3) Figure 1: A typical RF power link from transmitter to a reference point with its loss components. Antenna is ideal isotropic emitter with spherical field. 2 Conformity April 2008

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Conformity Magazine - April 2008 NewsBreaks Editor’s Note Mandatory Analog Cellular Service Ends FCC Releases Quarterly Reports on Consumer Inquiries and Complaints New Standards List Released for The EU’s Low Voltage Directive FDA Issues Notification on Unretrieved Device Fragments FDA Offers Guidance on Waivers for In-Vitro Diagnostic Devices ESD Open Forum Emerging Issues in Standards Challenges in Testing Design Fundamentals Focus on... New Products Product Reviews Premarket Approval Application Guidance Issued by FDA New Jersey Enacts Electronic Waste Recycling Regulations A New U.S. Standard for Workstation and Computer Design Might the CPSC Hold Retailers Accountable for Safe Products? CPSC Loses “Sole Toy Tester” CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - April 2008

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