RF Safestop Car Stopping Demonstration Question 7

2137-EX-ST-2018 Text Documents

Jankel Tactical Systems, LLC

2018-12-20ELS_221541

Typical RF SafeStop car stopping demonstration The following calculation provides an assessment of a typical RF SafeStop demonstration against the requirement of the “Control of Electromagnetic Field at work regulations 2016”. System performance parameters Magnetron power: 2.5MW Antenna gain: 23.4dBi System EIRP: 87.4dBW Duty cycle: 0.1% Pulse width: 1 to 3µs Frequency: 1.345GHz Assumptions 1) Any RF exposure is at the level of the peak RF power density associated with the RF transmitted beam, no account has been made for the RF spot size. 2) Full body surface area coverage is assumed. 3) Total energy absorption is taking place within the biological tissue (i.e. no RF reflection from the human body). RF electric field action level The RF electric field action level (AL) stated in the “Control of Electromagnetic Field at work regulations 2016” is for a continuous wave (CW) source rather than a pulsed source, at the operating frequency of the RF SafeStop system there is no AL limit for pulsed sources. Adhering to the CW source AL limit as an absolute peak RF field level would render the RF SafeStop completely ineffective against all target vehicles! It is not a legal requirement to adhere to the specified AL for RF electric field provided the appropriate exposure limit values (ELVs) are not exceeded. During a typical RF SafeStop demonstration e2v personnel will be exposed to RF field levels greater than the 110Vm-1 AL specified in the “Control of Electromagnetic Field at work regulations 2016”. The following sections provide evidence to show how e2v will adhere to the appropriate ELVs during a typical car stopping demonstration. Exclusion zone based on sensory effects For the sensory effects exposure limit value (ELV) one is concerned with the energy associated with a single high power RF pulse. For these calculations we have assumed the localised specific energy absorption (SA) level over the entire average human male. BA SA = PD × PW × BM The RF field strength associated with a given power density in free space is

V RF = PD × 377 Assuming the antenna is an isotropic radiator the RF power density decays as the square of the distance from the antenna and is given by the following relationship EIRP PD = 4π × R 2 Where SA = Specific energy absorption PD = Power density PW = Pulse width BA = Average body surface area of an adult male (1.9m2) BM = Average mass of an adult male (70kg) VRF = Root mean square of the RF field strength EIRP = Equivalent isotropic radiated power of the microwave source R = Range from microwave source For each system pulse width the range at which the sensory effects ELV of 10mJkg-1 will be exceeded is given in the following table Power density Range (personnel Pulse width RF field strength associated with pulse exclusion distance) 1µs 368.4kWm-2 11785Vm-1 10.9m 2µs 184.2kWm-2 8334Vm-1 15.4m 3µs 122.8kWm-2 6804Vm-1 18.9m Based on these calculations an exclusion zone 20m in front of the RF Safestop will be enforced, when the system is transmitting no personnel are allowed within this exclusion zone. A typical demonstration involves driving the target vehicle towards the RF SafeStop, whilst it’s transmitting, along a predefine course that directs the target out of the RF beam at either 20m or 25m from the RF SafeStop system. At these distances the specific energy absorption levels associated with each RF pulse width selection are given in the following table and are all below the occupational limit level of 10mJkg-1. Pulse width SA at 20m SA at 25m 1µs 3.0mJkg-1 1.9mJkg-1 2µs 5.9mJkg-1 3.8mJkg-1 3µs 8.9mJkg-1 5.7mJkg-1

Energy absorption associated with a target demonstration run For the specific absorption rate (SAR) ELV one must consider the cumulative absorption over any six minute period since the RF SafeStop is not operated continuously. The standard car stopping test run involves driving the target vehicle at 25mph towards the RF SafeStop along the antenna RF boresight. RF transmission commences when the vehicle is at 100m from the RFSS system and is terminated either when the vehicle stops or exits the main RF beam at either 20m or 25m from the RF SafeStop system. If the vehicle is stopped or effected the RF transmission may continue for up to 5 seconds whilst the confirmation of the success is relayed to the RF SafeStop operator. The RF specific absorption over a period of time is given by EIRP × D BA ARF = × × Time 4π × R 2 BM Where ARF = RF absorption D = RF SafeStop duty cycle Time = Exposure relative to a six minute period For the calculation of total RF absorption by the target driver during a single run the distance travelled by the target is subdivided into 5m intervals, at each interval the time is calculated from the vehicle speed and the absorption from the closest distance to the RF SafeStop system at the end of the 5m interval. For the following example it is assumed the target is stopped at the 20m exit point and hence sits at this distance for a 5 seconds confirmation time.

Distance from RF SafeStop Time interval RF absorption over time system interval 100 0 0 95 0.447 0.00016 90 0.447 0.00018 85 0.447 0.00020 80 0.447 0.00023 75 0.447 0.00026 70 0.447 0.00030 65 0.447 0.00035 60 0.447 0.00041 55 0.447 0.00049 50 0.447 0.00059 45 0.447 0.00073 40 0.447 0.00092 35 0.447 0.00120 30 0.447 0.00164 25 0.447 0.00236 20 5 0.04121 Total 11.705s 0.0513Wkg-1 For each demonstration run the target vehicle driver will be subjected to a SAR level of 0.0513Wkg-1 compared to the occupational limit level of 0.4Wkg-1. The specific absorption rate (SAR) ELV is averaged over a six minute period and as the RF SafeStop system is not used as a continuous transmitter it’s acceptable for the target vehicle driver to accumulate a SAR level over multiple demonstrations provided the ELV is not exceeded. Thus any individual driver can perform up to seven demonstartion runs in any six minute period and still remain within the occupational SAR ELV limit. Notes Although in the typical test scenario RF ground bounce will be present the effect on power density will be configuration dependent and will yield both reduction and enhance of the power density level over the distance travelled by the target vehicle. As a consequence the formula for an isotropic radiator has been used for all power density calculations, this is the approach adopted by the licensing authority when determining the general public and equipment safety distances.

RF field measurements within an unoccupied vehicle can yield localised points of RF field enhancement as a result of multiple signal reflections. However in all the above calculations the assumption is that total RF energy absorption is taking place when the signal is incident upon the human tissue (no signal reflection) and hence the RF field enhancement measured within an unoccupied vehicle will not be present. If any RF field enhancement is measured within the target vehicle when occupied would suggest that full RF absorption by the driver is not taking place and the above calculations are over estimating the driver’s exposure.


Document Created: 2018-08-10 16:48:15
Document Modified: 2018-08-10 16:48:15
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