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Publication Number: 192659 Rule Parts: 15.247 Publication Date: 04/09/2007
Keyword: Determining Antenna Gain for RF ID Systems Operating Under 15.247

First Category: Radio Service Rules
Second Category: Part 15 Intentional Radiators

Question:   How does the Commission determine the antenna gain for RF ID systems operating under Section 15.247 that employ both vertical and horizontal radiating elements?

Answer:   Some RF ID systems transmit simultaneously on both a vertical and a horizontal antenna to improve the read rates for tags that have unpredictable orientations.
For such systems the Commission will use the highest linear vertical and horizontal gain to determine compliance with Section 15.247.  Thus, for example, an RF ID tag reader that employs a 6dBi gain vertical antenna and a 6 dBi gain horizontal antenna will be treated as having a 6 dBi gain.  An RF ID tag reader that employs a 9 dBi gain vertical antenna and a 6 dBi gain horizontal antenna will be treated as having 9 dBi gain.
Section 15.247 limits the conducted output power to 1 Watt.  Therefore, for systems that employ a single transmitter to feed both the vertical and horizontal antenna, the total power may not exceed 1 Watt.  Similarly, if separate transmitters are used to feed each antenna element, the aggregate conducted output power may not exceed 1 Watt.  Note that Section 15.247 requires a reduction in conducted output power for antenna gains in excess of 6 dB for certain frequency bands.
We recognize that, from a technical standpoint, simultaneous transmissions on both vertical and horizontal radiating elements effectively yeilds a circularly or elliptically polarized antenna that will have higher gain than that of the individual vertical and horizontal antenna elements.  However, it does not appear necessary or appropriate to treat such antennas as circularly or elliptically polarized for purposes of determining the compliance with Section 15.247.  We note, for example, that the frequency bands covered by Section 15.247 typically do not employ circularly or elliptically polarized antennas and therefore would not "see" any increase in signal levels.


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