RF Exposure

FCC ID: SEAMOD31

RF Exposure Info

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FCCID_4333482

                                                                      UBISENSE AMERICA LLC
                                                                      999 18th Street
                                                                      Suite 901
                                                                      Denver
                                                                      CO 80202
                                                                      United States




                                                                       24th May 2019



Dear Sir / Madam,

Per KDB inquiry consultation with FCC, we have prepared answers to the
reviewer’s questions (their list item #3), in order to develop an exposure
exclusion analysis for the 6.5GHz transmitter of the UBIMOD31 device:

      3) Considerations for exclusion from MPE evaluation measurements and simulations
      were mentioned at e.g. pg 16 of Nov. 2017 FCC-TCB conference notes

      One approach to start with might be per said pg 16:
      For certain products with simple antenna configurations that transmit uncorrelated
      signals and low exposure is justified
      – due to low power or certain inherent operating conditions
      – the power density at close proximity to users may be estimated according to the
      maximum power available at the antenna aperture and applicable beam width

      In reply herein please provide details about device design and operating
      configurations, features if any providing inherent spacing to persons, and antenna
      element(s) layout, feed power, etc.


Summary statement:
        The 6.5GHz transmitter of the UBIMOD31 device has an
           extremely low measured average antenna feed power of
           2.6 microwatts (-25.9dBm)
        The transmitter has a simple single monopole antenna
           configuration
        The device does not incorporate features providing significant
           inherent spacing to persons (although typical integrations are
           likely to result in some antenna-person separation)
        However, by virtue of the extremely low power output, even with
           zero separation to the person the estimated power density at the
           antenna is far below the 1mW/cm2 limit for any relevant spatial
           averaging area.


Device design

The device is a small (1” x 1”) module intended to be integrated into host
equipment for the purpose of location tracking of that equipment. It is shown
below:




               6.5GHz monopole
               antenna


                                                                          2.4GHz ceramic
                                                                          antenna


      Power connector




The device has two low-power transmitters – a 6.5GHz Part 15.250
transmitter which is the primary subject of our KDB enquiry, and a 2.4GHz
Part 15.249 transmitter1. Note that the two transmitters are never active
simultaneously.

The 6.5GHz transmitter has a single top-loaded monopole quarter-wavelength
antenna, as shown in the diagram below – it is a traditional monopole antenna
with additional capacitive loading disc in order to widen its operating
bandwidth:
                                              5mm



                                  4.25mm




The transmitter does not use multi-antenna, beam-forming or coherent
transmission methods.


Operating configurations and output power

The average output power of the 6.5GHz transmitter is directly proportional to
the rate of transmissions (i.e. the number of location-determining packets it

1
 As explained in our initial KDB enquiry, SAR exemption calculations for the Part 15.249
2.4GHz transmitter are straightforward (its maximum peak radiated output power after tune-
up is only 0.56mW)


transmits per second). That transmission rate cannot in any circumstance be
higher than 167 location packets per second due to limitations in the device’s
microcontroller.

Direct measurements were made of the full-bandwidth power delivered to the
antenna using a wide-bandwidth thermocouple-based average RF power
meter (Agilent E4416A+8481A), which is modulation-independent. The
average power measurements were made with the device set to beacon
continuously at its highest possible transmission rate (167 maximum-payload
location packets per second), and were taken over a signal averaging period
of 20s (to comply with the interim guidance on MPE time averaging in the “RF
Exposure: Order/NPRM Issues” TCB Workshop notes).

After correction for cable losses, the source-based, time-averaged feed power
in the worst-case configuration (167 maximum-payload packets/sec.) was
found to be -25.9dBm = 2.6µW.

Note that this extremely low average power level is inherent to the source
because of:

   1) The low signal power spectral density levels and limited bandwidth
      permitted by Part 15.250
   2) The extremely short (<=384 pulses) and sparse (~1ns pulses
      transmitted at 1µs intervals) nature of the individual location packets
   3) The relatively low packet transmission rate, which is at most 167
      packets/sec. (Of course, when lower packet rates are used, the source-
      based, time-averaged feed power will fall commensurately further from
      this worst-case value)

The worst-case average power level is not in any way dependent on the
pattern of exposure to the bystander, thus justifying the use of source-based
time averaging.


Features providing inherent spacing to persons

The 6.5GHz antenna is exposed on the top surface of the device. Therefore,
in principle it would be possible to use the device with the antenna in
extremely close proximity to persons, and therefore we cannot rely on the
physical nature of the device alone to assure separation.

In practice, however, typical host installations will provide some separation
from nearby persons. The 6.5GHz antenna is fragile and host integrators will
place it behind a protective enclosure (typically made of plastic) to avoid
damage. For reference, two proposed integrations for this device are shown
below, together with the distances (measured from the centre of the 6.5GHz
antenna) to the closest exterior point on the enclosure:


    Example
    proposed host
    integration #1

    (minimum distance
    from mid-point of
    antenna to external
    surface = 7.5mm)




    Example
    proposed host
    integration #2

    (minimum distance
    from mid-point of
    antenna to external
    surface = 4.4mm)




Both examples place the UBIMOD31 device inside a plastic enclosure with a
battery power supply to form a location tracking tag which can be attached to
objects or carried by a person – other similar integrations are possible.


Estimation of power density in close proximity to persons

Assuming zero separation between the 6.5GHz antenna and the person, then
we can use the physical area of the antenna components to determine the
maximum possible power density next to the antenna, assuming that the
whole feed power is distributed over the whole (or even only a subset) of the
antenna bounding surface. We have computed power densities from the
antenna feed power and antenna geometry for three scenarios below:
Scenario    Diagram                       Area over which         Power density over area   Comments
            (entire feed power radiated   antenna feed power is
            over red surface)             distributed
1                                                                                           Most realistic, given
                                          5mm x π x 4.25mm =      2.6µW / 0.67cm2 =         standard far-field radiation
                                          67mm2 =                 4µW/cm2 =                 pattern of monopole –
                                          0.67cm2                 0.004mW/cm2               effective antenna aperture
                                                                                            is cylindrical
2                                                                                           Represents region which
                                          π x (5mm)2 / 4 =        2.6µW / 0.2cm2 =          might typically be closest
                                          20mm2 =                 13µW/cm2 =                to a person, but unrealistic
                                          0.2cm2                  0.013mW/cm2               given that the disc lies in
                                                                                            the null of the far-field
                                                                                            radiation pattern
3                                                                                           Power density calculated
                                          0.67cm2 + 0.2cm2 =      2.6µW / 0.87cm2 =         over entire antenna
                                          0.87cm2                 3µW/cm2 =                 bounding surface
                                                                  0.003mW/cm2


As can be seen, by virtue of the very low average antenna feed power, the
power density at the (bounding) surface of the antenna is very far below the
1mW/cm2 power density limit for general exposure in each case.2

We would be grateful if the OET would consider this exposure exclusion
analysis for the UBIMOD31 device.

Regards,




Dr Andy Ward
Chief Technology Officer, Ubisense
24th May 2019




2
  In fact, because the antenna input feed power is only 2.6 microwatts, and because power
density cannot be negative in any area, it is impossible even in principle for the power density
to exceed 1mW/cm2 over any circular area with diameter greater than 0.6mm. Since field
probes which might plausibly be used for direct power density measurements (e.g.
https://speag.swiss/products/dasy6/probes/new-eummwvx-vector-e-probe/?pdf=view) have
sensor elements which are physically larger than this diameter, and would therefore
effectively average the power density over a larger area, it seems that measurements using
these probes could never result in a reading which exceeded the 1mW/cm2 limit



Document Created: 2019-06-07 14:08:57
Document Modified: 2019-06-07 14:08:57

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