Cambium Networks 54500 Point to Point Wireless Bridge User Manual Manual 1

Cambium Networks Limited Point to Point Wireless Bridge Manual 1

Manual 1

Analysis of operating and installation requirements to
satisfy FCC Regulations for RF exposure compliance
for the PTP54500 and PTP54300 Products
John Sharman
Ref: phn-1151_000v001
Abstract
This document analyses the operating and installation requirements to ensure
limits for RF Exposure compliance are not exceeded by the PTP54500 and
PTP58300 range of products with integrated antennas. The guidelines in FCC
Bulletin 65 are used for the analysis.
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Copyright Information
This document is the confidential property of Motorola, Inc. and without its prior written consent may not be copied or released
to third parties.
MOTOROLA, the stylized M Logo and all other trademarks indicated as such herein are trademarks of Motorola, Inc. ® Reg.
U.S. Pat & Tm. Office. PTP XXX is a trademark of Motorola, Inc. All other product or service names are the property of their
respective owners.
© 2006 Motorola, Inc. All rights reserved.
The parameters quoted in this document must be specifically confirmed in writing before they become applicable to any
particular order or contract. The company reserves the right to make alterations or amendments to the detail specification at its
discretion. The publication of information in this document does not imply freedom from patent or other rights of Motorola Point
to Point Wireless Solutions Group or others.
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Revision History
Version Date Comments Author
0.001 10 March 2008 Initial Issue JS
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OPERATIONAL PARAMETERS OF THE PTP54500/PTP54300 PRODUCT
1 Scope
The purpose of this brief working paper is to identify the RF power produced by the
PTP54500/PTP54300 equipment under various operating conditions. This mean RF
power plus the antenna gain used in specific installations identifies the effective power
density (dBm/cm2) that is to be compared against allowed limits for human exposure.
The two part numbers in the heading above reflect changes in product branding only;
the product performance is not affected.
2 References
Evaluating Compliance with FCC Guidelines for Human Exposure to Radio frequency
Electromagnetic Fields:
OET Bulletin 65, Edition 97-01, August 1997 [1]
3 Background
Reference [1] identifies how the radiated power density should be calculated for
different distances from the antenna. The variables used are Radiated Power Density
(S), conducted power (P), Antenna Gain (G) and distance (R) . The formula given is
S= (P * G)/(4 * Pi * R2)
The limit allowed for S depends on whether the exposure risk is to a member of the
public or not. The PTP54500/PTP54300 products are approved by the FCC under the
UNII Rules (Part 15.407). This requires (Part 15.407(f) that all products meet the radio
frequency radiation requirements for the “general population/uncontrolled
environment” case. At the frequency of operation of these products, this requires that
the value of S to be used is 1mW/cm2.
It is clear from [1] that the power to be used should be the maximum transmitted
power, subject to any allowance for source-based time-averaging.
Notes
a) the FCC require that the power density be calculated at a minimum distance of 20cm
b) the value of P * G is the same as the transmitted EIRP.
4 PTP54500/PTP54300 Specific Issues
4.1 FCC Regulations
The PTP54500/PTP54300 is approved under FCC Part 15.407 and this regulation
limits the maximum EIRP to 28.76dBm in a channel bandwidth of 15MHz. The
PTP54500/PTP54300 products ensure that this power cannot be exceeded.
4.2 Dual Polarisation
The PTP54500 and PTP54300 products use an integrated dual polarised antenna, with
each polarisation connected to an identical transceiver circuit inside the unit. In order
to comply with the FCC EIRP limits, the design of the products reduces by 3dB the
conducted power and EIRP of each of the two individual polarisations.
4.3 Power Control
The power levelling loops in the product measure the transmitted power on each
polarisation at all times and limit each to the Maximum Transmit Power –3dB. The
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Maximum Transmit Power during the transmit period (total for both polarisations) for
PTP54500/PTP54300 equipments in production is set to not exceed 5dBm to ensure
that the Maximum EIRP requirement during the burst (27.76 dBm) is not exceeded
when the product is used with the supplied integrated antenna (G = 23dBi).
The PTP54500/PTP54300 equipments operate on a TDD basis using the same
frequency for up/down link. The transmit duty cycle resulting from the TDD operation
is fixed at <50%. .
The FCC regulations allow source-based time averaging to be used in working out the
EIRP value for the exposure calculation. This reduces the effective mean conducted
power and EIRP by at least 3dB from the levels of conducted power and EIRP that
would be applicable if the products were to transmit with a duty cycle of 100%.
It should be noted that this is very much a worst case as the product operates Receiver
driven Transmit power control.
4.4 FCC Testing
The testing has confirmed that the maximum conducted power and EIRP limits were
not exceeded by the product. The effect of the dual polarised antenna on the EIRP was
calculated by taking the linear sum of the results for each polarisation.
5 Analysis
5.1 Transmitted Levels
The Radiated Power Density can be assessed on the basis of the antenna gain for each
polarisation and the linear sum of the transmitter powers on the two polarisations.
More simply it could be based on the total EIRP allowed by the FCC regulations (as
the test reports show that the linear sum of the EIRPs from each of the two
polarisations on the product is less than this limit). This would present a conservative
assessment as the calculated power density levels would be less using the actual test
results. This is the approach that is used for the analysis below.
Example
Channel BW = BW
Allowed Conducted Power = 11dBm + 10*Log(BW) - (Actual Antenna Gain -
6dBi)
Antenna Gain = Antenna Gain of each polarisation
Allowed EIRP = Allowed Conducted Power + Actual Antenna Gain
EIRP used for Radiated Power Density Calculation = Allowed Conducted
Power - Allowance for source-based time-averaging in the product TDD
structure
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In the case of PTP 54500/PTP54300
Channel BW = 15 MHz.
Allowance for source-based time-averaging: Maximum Tx duty cycle = 50% or -
3.0dB
Allowed EIRP = 11 + 11.76 - Actual Antenna Gain + 6 + Actual Antenna Gain
= 28.76 dBm
EIRP used for MPE calculation = 27.78 -3.0 = 25.76 dBm or 376mW
5.2 Radiation Levels
The table below shows the result of calculating the radiated power density using the
formula given in Ref [1] at a distance of 20cm from the antenna and confirms that the
power density level is below the limit given in Ref [1] for general population/
uncontrolled environments at that distance.
Total EIRP in burst 25.76 dBm
Less TDD duty cycle -3.0 dB
Total Mean EIRP 376 mW
Power Density Limit 1 mW/cm2
Radiated Density at 20cm 0.0748 mW/cm2
6 Conclusion
The equipment meets the limit for general population exposure at a distance of 20cm
with a margin of over 10dB. This margin has been calculated using the FCC’s
maximum EIRP limit and the PTP54500/PTP54300 do not exceed that limit.
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