Thursday, March 7, 2019

Limiting RF exposure to Humans in Close Proximity to Satellite Transmitters

Satellite radios produce powerful Radio Frequency (RF) emissions. A method is provided for limiting the RF exposure to humans that are in close proximity to satellite transmitter antennas by establishing radial keep-out zones.


All calculation in this appendix are guided by OET Bulletin 65 and relate to FCC regulation only. Other national licensing agencies apply different standards, which may be more stringent.

Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, OET Bulletin 65 Edition 97-01, August 1997


The threshold for uncontrolled RF exposure is 1 mW/cm2 for up to 30 minutes.

From OET bulletin 65:

General population/uncontrolled exposure. For FCC purposes, applies to human exposure to RF fields when the general public is exposed or in which persons who are exposed as a consequence of their employment may not be made fully aware of the potential for exposure or cannot exercise control over their exposure. Therefore, members of the general public always fall under this category when exposure is not employment-related.

The threshold for controlled RF exposure is 5 mW/cm2 for up to six minutes.

From OET bulletin 65:

Occupational/controlled exposure. For FCC purposes, applies to human exposure to RF fields when persons are exposed as a consequence of their employment and in which those persons who are exposed have been made fully aware of the potential for exposure and can exercise control over their exposure. Occupational/controlled exposure limits also apply where exposure is of a transient nature as a result of incidental passage through a location where exposure levels may be above general population/uncontrolled limits (see definition above), as long as the exposed person has been made fully aware of the potential for exposure and can exercise control over his or her exposure by leaving the area or by some other appropriate means.

The Maximum Permissible Exposure (MPE) is a based on a combination of power density and time. It is possible to comply with higher power density and shorter exposure time. However, as a matter of practicality, persons should avoid exposure to power density above 5 mW/cm2 due to the short time frame permitted. Persons needing to work inside the controlled exposure limit should take action to turn off the satellite transmitter.

Transmit Control

The means to turn off the satellite transmitter are provided as a feature in ARINC Characteristic 791 and ARINC Characteristic 792. The Transmit Control discrete provides a ready means to shut down the satellite transmitter.

The reliance on the integrity of the transmit control function is elevated because of the potentially hazardous situation if the transmitter is not turned off when commanded off.  In particular, Transmit Control should not be a software function. Transmit control should be a hardware control function. By providing a transmit control input, the OAE can take suitable internal action to remove power from the HPA without necessarily shutting down any other part of the satcom system. Alternatively, Transmit Control can be implemented as a command to remove power using external relays.

ARINC Characteristic 791 defined Transmit Control with the ground state = INHIBIT transmission. When ARINC Characteristic 792 was in development, Transmit Control was defined with the opposite logic, ground state = ENABLE transmissions.

Typical discrete input circuits and associated wiring suffers two types of failures – open circuit and short to another wire or to ground.  The broken-wire scenario leaves the ARINC Characteristic 791 system enabled to transmit when the Transmit Control switch is thrown. In ARINC Characteristic 792, the same scenario leaves the satcom disabled in all cases.

Leaving the satcom transmitter on when commanded off can create excessive RF exposure to persons working nearby.  Leaving the satcom system disabled when it should be enabled will trigger immediate corrective action. Leaving the satcom system on when it should be disabled may not be discovered.

To enhance the ability to discover a Transmit Control fault, and to account for the action of disable rather than the command to disable, a Transmit Control Feedback discrete was introduced in ARINC Characteristic 792 and ARINC Characteristic 791 Part 1 Supp. 3. The feedback discrete should only be activated while the satcom transmissions are disabled.

A fault light can be driven by comparing the Transit Control discrete and the Transmit Control Feedback discrete. The logic is different between ARINC Characteristic 791 and 792.

TRUE= INHIBIT TRANSMITTER
FALSE = NORMAL MODE

Table 1 - ARINC Characteristic 791 Transmit Power Control Logic

A791 Transmit Control
A791 Transmit Control Feedback
INHIBIT
Gnd (True)
Gnd (True)
FAULT
Gnd (True)
Open (False)
FAULT
Open (False)
Gnd (True)
NORMAL
Open (False)
Open (False)

Table 2 - ARINC Characteristic 792 Transmit Power Control Logic

A792 Transmit Control
A792 Transmit Control Feedback
FAULT
Gnd (False)
Gnd (True)
NORMAL
Gnd (False)
Open (False)
INHIBIT
Open (True)
Gnd (True)
FAULT
Open (True)
Open(False)

Health to Humans

Every radio license requires an assessment for potential hazards of RF exposure to the health to humans along with any designations for regions with excessive power density.

An example for calculating these limits is provided based on OET 65.

Antenna Factors

There are certain aspects that must be defined to assess radiation hazard.

1.     Maximum aperture extent (azimuth) (m)
2.     Maximum aperture extent (elevation) (m)
3.     Aperture Area (m2)
4.     Antenna Efficiency (%)
5.     Transmit output power (Watts)
6.     Transmit losses (dB)
7.     Frequency of highest EIRP (GHz)
8.     Maximum duty cycle of transmissions (%)
9.     Elevation sidelobe wrt to boresite (dB)

Ku band

For example, consider a mechanically steerable array with the following characteristics:

1.     Maximum aperture extent (azimuth) (0.65 m)
2.     Maximum aperture extent (elevation) (0.2 m)
3.     Aperture Area (1300 cm2)
4.     Antenna Efficiency (75%)
5.     Transmit output power (40 Watts)
6.     Transmit losses (-2 dB)
7.     Frequency of highest EIRP (14.5 GHz)
8.     Maximum duty cycle of transmissions (100 %)
9.     Elevation sidelobe wrt to boresite (-13 dB)

The 40 W HPA delivers 25,238 mW to the antenna “flange”.

Table 3 - Ku Amplifier Power
HPA
Watts
40
HPA
dBm
46.02
Cable Loss
dB
-2
Tx Power
dBm
44.02
Duty Cycle
Percent ON
100
Tx Power
mW
25238

The antenna gain is added to the HPA Tx power to arrive at 72,238 Watts (EIRP).

Table 4 - Ku EIRP
Antenna Gain
dBi
34.57
Gain Factor

2862
Sidelobe
dB
0
EIRP
dBW
48.6
EIRP
dBm
78.6
EIRP
Watts
72,238 

The antenna surface should never be approached with the transmitter on. Removing the radome should always be done only after removing all power from the satcom system.

Table 5 - Ku Antenna Surface Power Density
Surface Power Density
mW/cm2
77.66

From OET bulletin 65:
Near-field region. A region generally in proximity to an antenna or other radiating
structure, in which the electric and magnetic fields do not have a substantially plane-wave character but vary considerably from point to point. The near-field region is further subdivided into the reactive near-field region, which is closest to the radiating structure and that contains most or nearly all of the stored energy, and the radiating near-field region where the radiation field predominates over the reactive field but lacks substantial plane-wave character and is complicated in structure. For most antennas, the outer boundary of the reactive near field region is commonly taken to exist at a distance of one-half wavelength from the antenna surface.

Far-field region. That region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna. In this region (also called the free space region), the field has a predominantly plane-wave character, i.e., locally uniform distribution of electric field strength and magnetic field strength in planes transverse to the direction of propagation.

The near field and far field distances and power density show exposure above controlled limits.
  
Table 6 - Ku Boresite Near/Far Field Boundaries
Near Field Border Rnf
meter
5.11
NF Power Density Snf
mW/cm2
22.82
Far Field Border Rff
meter
12.25
FF Power Density Sff
mW/cm2
3.83

The power density in the near field is concentrated along the boresite beam and extends less than one diameter on each side.  The power density in the near field is assumed to be at a peak level anywhere in the region.

The power density in the far field marks the beginning of the radiated field, where power density falls with the square of the distance from the transmitter. The power density follows the antenna beam patter beyond the far field boundary.

The power density in between the near field and far field boundary is a transition zone. The power density in the transition zone is linear interpolated between the distance across the zone, from near field power density to far field power density.

Finding the uncontrolled and controlled boundary involves moving along from the near field, through the transition zone, to the far field until the power density falls below the specified value.

In this case, the Controlled limit is in the transition zone at 11.8m.
The Uncontrolled limit is in the far field at 24 m.

Table 7 - Ku Boresite MPE Boundaries
Transition Zone – 5 mW/cm2
meter
11.8
Transition Zone – 5 mW/cm2
feet
38.8
FF Zone – 1 mW/cm2
meter
24
FF Zone – 1 mW/cm2
feet
78.7

The satellite antenna points at the satellite in order to receive a signal and be enabled to transmit. It should be assumed that the satellite antenna is pointed at least 5 deg above the horizon and may be pointed in any direction relative the aircraft heading. The upper hemisphere from the base plate of the antenna is potentially illuminated.  Uncontrolled exposure is generally constrained to the length and width of the airplane, making a practical barrier.

Controlled exposure, notably for deicing, does intrude. In this case, the controlled limit extends about 12m radially from the antenna.  Any encounter beyond the 12m controlled limit is suitable for up to 6 minutes.  Any encounter within the 12m controlled limit is not permitted without shutting down the satellite transmitter.

The previous discussion was done with respect to the azimuth, the widest part of the aperture. The elevation side of the equation is only evaluated with respect to a far field sidelobe, for fear of exposure to persons working on the ground near the antenna. The near field power density would not follow downwards below the antenna as it follows a column along the boresite about the size of the antenna itself.
Table 8 – Ku Sidelobe Near/Far Field Boundaries
Far Field Border
meter
1.16
FF Power Density Sff
mW/cm2
21.4

The thresholds for the side lobe are in the far field. Whether these limits extend meaningfully into any further restriction must be evaluated in the context of a particular installation.

Table 9 - Ku Sidelobe MPE Boundaries
FF Zone 5 mW/cm2
meter
2.4
FF Zone 5 mW/cm2
feet
7.9
FF Zone 1 mW/cm2
meter
5.4
FF Zone 1 mW/cm2
feet
17.6


Ka band

Conducting the analysis for the same antenna but operating in Ka band changes the results.

1.     Maximum aperture extent (azimuth) (0.65 m)
2.     Maximum aperture extent (elevation) (0.2 m)
3.     Aperture Area (1300 cm2)
4.     Antenna Efficiency (75%)
5.     Transmit output power (10 Watts)
6.     Transmit losses (-3 dB)
7.     Frequency of highest EIRP (30 GHz)
8.     Maximum duty cycle of transmissions (100 %)
9.     Elevation sidelobe wrt to boresite (-13 dB)

5012 mW of transmit power.
Table 10 - Ka Amplifier Power
HPA
Watts
10
HPA
dBm
40
Cable Loss
dB
-3
Tx Power
dBm
37
Duty Cycle
Percent ON
100
Tx Power
mW
5012

61,406 Watts EIRP.
Table 11 - Ka EIRP
Antenna Gain
dBi
40.9
Gain Factor

12,252
Sidelobe
dB
0
EIRP
dBW
47.9
EIRP
dBm
77.9
EIRP
Watts
61,406

The antenna surface must never be approached with the transmitter on.

Table 12 - Ka Antenna Surface Power Density
Surface Power Density
mW/cm2
15.4

The near field azimuth power density does not exceed the 5 mW/cm2 controlled threshold; there is no limit on this aspect.
The uncontrolled limit lies in the transition zone at about 25 m.

Table 13 - Ka Boresite Near/Far Field Boundaries
Near Field Border Rnf
meter
10.6
NF Power Density Snf
mW/cm2
4.53
Far Field Border Rff
meter
25.4
FF Power Density Sff
mW/cm2
0.76

Table 14 - Ka Boresite MPE Boundaries
Transition Zone – 5 mW/cm2
meter
N/A
Transition Zone – 5 mW/cm2
feet
N/A
Transition Zone – 1 mW/cm2
meter
24.4
Transition Zone – 1 mW/cm2
feet
80.1

The elevation plane near field does exceed the controlled power density. While it is debatable whether the side lobe power density is as powerful as shown, it offers a conservative element for prescribing about 2.3 m for controlled exposures.
Whether these limits extend meaningfully into any further restriction must be evaluated in the context of a particular installation.
  
Table 15 – Ka Sidelobe Near/Far Field Boundaries
Near Field Border Rnf
meter
1.0
NF Power Density Snf
mW/cm2
11.6
Far Field Border Rff
meter
2.4
FF Power Density Sff
mW/cm2
4.25

Table 16 - Ka Sidelobe MPE Boundaries
Transition Zone – 5 mW/cm2
meter
2.3
Transition Zone – 5 mW/cm2
feet
7.4
FF Zone – 1 mW/cm2
meter
5
FF Zone – 1 mW/cm2
feet
16.2

Ka band with 30% Duty Cycle

Taking the same example as above but account for only 30% duty cycle. Duty cycle reduces the transmission power as a scaler (30% on, 70% off).

1504 mW power. 
Table 17 - Ka (30% Duty Cycle) Amplifier Power
HPA
Watts
10
HPA
dBm
40
Cable Loss
dB
-3
Tx Power
dBm
37
Duty Cycle
Percent ON
30
Tx Power
mW
1504

18,422 Watts EIRP
Table 18 - Ka (30% Duty Cycle) EIRP
Antenna Gain
dBi
40.9
Gain Factor

12,252
Sidelobe
dB
0
EIRP
dBW
42.7
EIRP
dBm
72.7
EIRP
Watts
18,422

The Surface power density is below the controlled limit. Because of many reasons, power must always be off when working on the antenna with the radome removed.

Table 19 - Ka (30% Duty Cycle) Antenna Surface Power Density
Surface Power Density
mW/cm2
4.6

Near Field and Far Field power density do not exceed 5 mW/cm2.

The uncontrolled limit is in the transition zone.
  
Table 20 - Ka (30% Duty Cycle) Boresite Near/Far Field Boundaries
Near Field Border Rnf
meter
10.6
NF Power Density Snf
mW/cm2
1.36
Far Field Border Rff
meter
25.4
FF Power Density Sff
mW/cm2
0.23

Table 21 - Ka (30% Duty Cycle) Boresite MPE Boundaries
Transition Zone – 1 mW/cm2
meter
15.3
Transition Zone – 1 mW/cm2
feet
50.1

Conducting the analysis for the elevation sidelobe.

The elevation plane near field does not exceed the controlled power density.
The uncontrolled limit is 2.7 m. Whether this limit extends meaningfully into any further restriction must be evaluated in the context of a particular installation.

Table 22 – Ka (30% Duty Cycle) Sidelobe Near/Far Field Boundaries
Near Field Border Rnf
meter
1.0
NF Power Density Snf
mW/cm2
3.47
Far Field Border Rff
meter
2.4
FF Power Density Sff
mW/cm2
1.28

Table 23 - Ka (30% Duty Cycle) Sidelobe MPE Boundaries
FF Zone – 1 mW/cm2
meter
2.7
FF Zone – 1 mW/cm2
feet
8.9

Flat Panel Phased Array

A phased array antenna has a steerable boresite, unlike those antennas discussed above that operate with a fixed boresite and mechanical steering. The phased array antenna gain is a function of scan angle, and this creates a variation in power density that varies with pointing angle. In addition, the aperture minor axis varies with scan angle.

Analyzing a phased array antenna is more complicated than for an antenna with a fixed radiation pattern. One approach is to characterize the antenna as a family of antennas, to account for various scan angles. The composite results provide the control limits.

Summary

In summary, for a sample antenna the limits for exposure are shown below. Every antenna must be evaluated with its particular characteristics. These results are only to provide a computational example and should not be construed to be indicative of any particular antenna.

Table 24 - Summary MPE Boundaries

Main Beam
Elevation Sidelobe

Controlled
Uncontrolled
Controlled
Uncontrolled

meter
meter
meter
meter
Ku band
12
24
2.4
5.4
Ka band
N/A
25
2.3
5
Ka band (30%)
N/A
16
N/A
2.7

Table 25 - Combined MPE Boundaries

Resultant limit

Controlled
Uncontrolled

meter
meter
Ku band
12
24
Ka band
2.3
25
Ka band (30%)
N/A
16

Formula

Rnf Near Field: D2/4𝛌
D is the maximum extent for that plane.

Power Density at Near Field
Snf = 16*𝞮*𝞹/pD2
P is the power to the antenna flange (mW)
𝞮 is the efficiency of the antenna (%)

Rff Far Field: 0.6*D2/𝛌

EIRP = Power at flange + Antenna gain
Antenna Gain = 10 * log(𝞮*4*𝝅*A/𝞴)

Power Density at Far Field
Sff = EIRP / (4*𝝿*R2)
R = distance from the antenna (R>Rff)

Power Density at the Aperture Surface = 4P/A

Power Density in the near field = Snf

Power Density in the transition zone = (((R-Rnf)/Rff-Rnf)*(Sff-Snf)) + Snf



Stay tuned!


Peter Lemme

peter @ satcom.guru
Follow me on twitter: @Satcom_Guru
Copyright 2019 satcom.guru All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

No comments:

Post a Comment