Hi Don,
The answer below (from the FCC Knowledge base) was as close as I could get
when doing similar investigation some time ago. The last paragraph might
partially answer your question.
Hope it helps,
Bob Sykes
EMC Engineer
First Category:
Radio Service Rules
Second Category:
Part 15 Intentional Radiators
Third Category:
Radiated emission limits
Question: Determining Antenna Gain for RF ID Systems
Operating Under 15.247: 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.
_____
From: [email protected] [mailto:[email protected]] On Behalf Of UMBDENSTOCK,
DON
Sent: Wednesday, May 24, 2006 10:31 AM
To: [email protected]
Subject: antenna gain
The FCC specifies antenna gain in rules such as 15.247. Gain can be measured
in different ways. So I asked the FCC directly, what method do they use? The
response was "industry standard". Does anyone have any idea what the
"industry standard" is for determining the gain of a 900 MHz patch antenna?
Does it make a difference whether it is linear or circular polarized for the
"industry standard"?
Don Umbdenstock
Manager Compliance Engineering
Tyco Safety Products / Sensormatic
6600 Congress Avenue
Boca Raton, FL 33487 USA
Phone: 561.912.6440
<mailto:[email protected]> [email protected]
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