RF Safety in Satellite Communications

Slide Number 1Rev -, July 2001

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

RF SafetySection 9


Vol 4: Earth Stations




IntroductionEarth Station antennas radiate electromagnetic energy.

Even though the transmission power level required in modern satellite communication is moderate when compared to many terrestrial broadcast towers, and the tightly focused antennas direct power away from the ground, this energy can still pose a hazard to workers and the public.

It is therefore important that satellite communication personnel be aware of the nature of RF energy and its impact on the human body.

This section of the course will serve as a brief introduction to the topic. For those who will work with RF energy on a daily basis, further training is advised.

4.9: RF RadiationVol 4: Earth Stations



Negative ConnotationsIn English, the word “Radiation” carries a heavy negative connotation. When it is heard, people immediately think of nuclear generating station disasters such as the explosion at Chernobyl in the Ukraine.

This “nuclear” type of radiation is not at all the same as the type used in satellite communication, and its effect on the human body is very different as well.

Nevertheless, some communication companies try to avoid using the word “radiation” in their written communication, just to avoid this negative association in the public mind.

If desired, the word “energy” can usually be substituted for “radiation” in written material or in formal, oral presentations.

4.9.1: Negative ConnotationsSec 9: RF Safety




Two Kinds of RadiationThere are two kinds of radiation: ionizing and non-ionizing.

The principle difference between the two is energy level: ionizing radiation contains more energy per packet, or quanta, than the non-ionizing kind.

The energy in ionizing radiation is sufficiently high to disrupt the atomic structure of human tissue.

Specifically, at these higher energy levels, electrons can be stripped from atoms, leaving them ionized. This can cause the immediate or long term cellular mutation effects we have come to associate with nuclear radiation.

Non-ionizing radiation cannot cause such effects.

However, non-ionizing radiation is also dangerous to human health!

4.9.2: Two Kinds of RadiationSec 9: RF Safety




RF Radiation is Non-IonizingAt the frequencies used in satellite communication, energy cannot reach ionizing levels. Such levels occur at the X-Ray and Gamma region.

RF radiation, therefore, exhibits primarily thermal effects. In fact, microwave ovens make use of the same section of the electromagnetic spectrum as radio frequency communications.

Recently, evidence has been growing that RF radiation is also capable of producing some non-thermal effects in humans. Since this evidence is currently controversial, this section of the course will focus on thermal effects only.

4.9.2: Two Kinds of RadiationSec 9: RF Safety




Effect of Non-Ionizing RadiationThe effect of microwave radiation on living tissue depends on the Specific Absorption Rate (SAR) of the exposed body.

SAR is a measure of how well a given substance can absorb the energy to which it is exposed. It is expressed in W/kg, with higher values indicating greater effect.

SAR depends on a variety of factors:• Strength of the field• Distance from the source• Exposure duration • Frequency• Type of modulation• Orientation• Body characteristics

4.9.3.1: The Specific Absorption RatePart 3: Effects of Exposure to Non-Ionizing Radiation

Vol 4: Earth Stations, Sec 9: RF Safety



Factors Affecting SAR4.9.3.1.1: Strength of FieldWe have already learned about electromagnetic fields and Power Flux Density (PFD).

Simply put, the greater the PFD the greater will be the affect of exposure on the human body per unit time.

4.9.3.1.2: Distance From SourceThis relates directly to the strength of field.

PFD drops off with the square of the distance from the source.

The effects of exposure will be lessened by increasing the distance from the source.





Factors Affecting SAR4.9.3.1.3: Exposure DurationThe longer a person is exposed to an electromagnetic field, the greater will be the overall effect.

Depending on strength of field and frequency, the exposed person might not become aware of the exposure.

Thus, exposure duration can often be hard to estimate in cases where an exposure claim has been made.

4.9.3.1.4: FrequencyThe frequency spectrum may be roughly divided into three:

• 0 to 3 kHz, Extremely Low Frequency (ELF)• 3 kHz to 300 MHz, Radio Frequency (RF)• 300 MHz to 300 GHz, Microwave (MW)





Factors Affecting SAR4.9.3.1.4: FrequencyAt ELF . . .

• The SAR of the human body is poor.

• The effects of energy at this frequency are under debate. Consider, for instance, the debate on whether there are long term effects of living under power lines. At 60 or 50 Hz, power lines radiate in the ELF range.

• High power radiation in this range can result in shocks, burns and potentially damaging current flow in the limbs of a well grounded person.





Factors Affecting SAR4.9.3.1.4: FrequencyAt RF . . .

• Shocks, burns and limb current can still result.

• In this range the SAR of the human body begins to increase and deep thermal effects can take place.

• RF energy is capable of deep penetration of the human body, greatly affecting internal organs.





Factors Affecting SAR4.9.3.1.4: FrequencyAt MW . . .

• Continuing from RF, deep thermal effects continue.

• As frequency increases, however, SAR once again decreases, and thermal penetration decreases.

• Under these conditions, surface burning is the more likely effect.

• Satellite communication takes place in this range of frequencies and, therefore, this range is our primary concern.





Factors Affecting SAR4.9.3.1.4: FrequencyAt Resonance . . .

• Human bodies exhibit properties of electrical resonance.

• Depending on the height of the person and whether he or she is electrically grounded while exposed, resonance could occur somewhere between 30 and 300 MHz.

• At resonance, SAR is at its highest.





Factors Affecting SAR4.9.3.1.5: Type of ModulationPulsed modulation, such as in radar systems, requires special consideration.

Pulsed energy can have a greater effect than the same amount of energy steadily applied (the impact wrench principle).

In addition, some nervous system affects have been identified in animals exposed to pulsed radiation.

Most RF Safety Standards now place limits on peak power density as well as mean power density.





Factors Affecting SAR4.9.3.1.6: OrientationArrangement of the exposed body with respect to the polarization of the source will affect the SAR.

If the axis of polarization matches the long axis of a person, the absorption, and hence the damage, will be greater.

Absorption may be greater if incident radiation is met head-on rather than at an angle. At an angle, a greater percentage of the radiation may be reflected.





Factors Affecting SAR4.9.3.1.7: Body CharacteristicsThe height, shape and weight of the exposed person play a part in defining the SAR. We have seen that height is the key feature in determining the frequency of resonance.

The amount of water in given body parts is also a significant factor. The so called “soft tissues” and the internal organs are particularly vulnerable.

In addition to having a high moisture content, the eyes and testes are particularly susceptible. These organs do not have direct blood supplies and are therefore less able to self-regulate their temperatures.





4.9.3.2: Thermal EffectsPart 3: Effects of Exposure to Non-Ionizing Radiation


Thermal Effects of Exposure4.9.3.2.1: Deep Thermal EffectsThermal effects on the body result at the molecular level when molecules in the body attempt to follow the vibrations of the energy to which they are exposed.

This has a warming effect on the tissue.

As we have seen, at lower frequencies RF energy is better able to penetrate the body.

As a consequence, at lower frequencies it can be expected that the temperature of internal organs will be increased.

If this temperature increase is greater than the body can regulate, temporary or long-term damage can result.





Thermal Effects of Exposure4.9.3.2.1: Deep Thermal EffectsNote that deep thermal effects are the most insidious, since these may not be readily apparent. Unlike the skin, internal organs are not equipped with temperature sensors. Therefore, a person will not be directly aware of this temperature increase.

A person experiencing these deep thermal effects may begin by feeling a pleasurable, all-over warming.

It is a common rule of thumb amongst RF workers that if an inexplicable sensation of warmth suffuses the body, leave the area and have it tested for radiation.





Thermal Effects of Exposure4.9.3.2.1: Deep Thermal EffectsWith continued exposure, a person may experience akin to overexertion: perspiration, laboured breathing, elevated temperature.

If exposure continues, a condition very much like heat prostration results. There can be nausea, vomiting and fainting.

At this point there can be long-term damage.

4.9.3.2.2: Skin EffectsAt higher frequencies, typically above 3 GHz, electromagnetic energy does not penetrate as deeply into human tissue.At these frequencies, most of the energy that is to be absorbed will be absorbed near the surface of the body.





Thermal Effects of Exposure4.9.3.2.2: Skin EffectsEnergy absorbed in this way can cause long-term, subcutaneous cellular damage, or more localized, sometimes severe, burning.As with deep thermal effects, it is possible for people to be exposed to harmful levels without knowing it. This is especially true below 10 GHz. It is therefore good practice for RF workers not to count on the presence of any signs or symptoms, but to understand that they work in an environment in which they may be exposed to harmful radiation without knowing it.





Thermal Effects of Exposure4.9.3.2.3: Hot SpotsIn the case of both deep thermal and skin effects, it is possible for electromagnetic energy to create “hot spots.” Hot spots are areas in the body where temperature is elevated much higher than over the body as a whole.Obviously, this can result if the source is very close to the body or, if for some other reason, exposure itself is localized. In the case of deep thermal effect, hot spots can also develop when exposure is whole-body.This results when organs inside the body exhibit localized resonance and therefore absorb energy at a greater rate than the body as a whole.



SUMMARYSUMMARYPersons exposed to RF energy may feel:Persons exposed to RF energy may feel:



Or, persons exposed to RF energy may feel:Or, persons exposed to RF energy may feel:

Remember:Remember: There may be no warning of There may be no warning of exposure to RF energy.exposure to RF energy.



Vol 4: Earth Stations, Sec 9: RF Safety Slide Number 22Rev -, July 2001



Safety StandardsMost governments have safety regulations governing exposure to electromagnetic energy.

Many international bodies also publish standards and recommendations.

All these standards and recommendations have three things in common:

• They deal with thermal effects only

• They give exposure limits as a function of frequency

• They draw a distinction between the general public and the microwave worker

4.9.4: Safety Standards and Permissible LevelsSec 9: RF Safety




The General Public/Microwave Worker DistinctionIn all recommendations and standards, the guidelines for exposure of the general public are stricter than for microwave workers.

The idea here is that a microwave worker, if exposed unknowingly, is likely to be exposed over the eight-hour working day. A member of the general public, however, could be exposed over a 24-hour day.

In addition, an RF worker understands the nature of RF radiation and has chosen to work with it, whereas a member of the general public may be completely unaware.

Thus, exposure limits are reduced for the general public.

4.9.4.1: The Worker/Public DistinctionPart 4: Safety Standards and Permissible Levels




The IEEE C95.1-1999 StandardThe IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, is a typical standard.

It forms part of the effort of Standards Coordinating Committee 28 (SCC 28), which has a broader scope that includes standards for the effect of RF on volatile materials and electro-explosive devices.

The 1999 edition of this standard contains updates mostly in the area of contact currents and field measurement.

In C95.1 the worker/public distinction is termed controlled and uncontrolled environments.

As with other standards, the Maximum Permissible Exposure (MPE) limit is given as a function of frequency and time.

4.9.4.2: IEEE C95.1-1999Part 4: Safety Standards and Permissible Levels




The IEEE C95.1-1999 StandardAdditionally, measurement methods are included to assist RF workers in ensuring compliance with the standard.

These measurement methods are quite complicated. They involve distinctions between near-reactive, near-radiative, and far fields, require temporal and spatial averaging, and include body current measurements.

Thorough measurements of this kind may be necessary in site installation engineering or when questions arise concerning compliance to the standard.

Luckily, however, for normal satellite communications operation, such detailed measurements are seldom required.





The IEEE C95.1-1999 StandardSeveral factors make it possible to assure compliance with RF Safety Standards without detailed measurements.

Frequency:Satellite communication takes place primarily in the C- and Ku-Bands of frequency. In these bands, and at Ka-Band, standards are relatively uncomplicated. As seen in the next section, C95.1 simply calls for a frequency independent level of 10 mW/cm2 over 6 minutes at these frequencies.

Additionally, at C- and Ku-Band frequencies, separate measurement of E and H fields are not required. Instead, only the E field is measured, and its values represent the combined power density.





The IEEE C95.1-1999 StandardAntennas:Unlike terrestrial communication, the antennas used in satellite communication are aimed upward and are highly directional.

While there is some spillover to the rear and some cast out to the side, the level of this unwanted power product is far below that of the main transmission path.

Care must be taken, however, with the clearance of the main lobe over buildings and hills, as anyone on top of these structures could be exposed.



Power:Again unlike terrestrial communication, the power levels required in modern satellite communication are moderate.



The IEEE C95.1-1999 StandardProactive Corporate Policy Based On Leakage:Regardless of the Standard officially adopted by the corporation, it is a good idea to set the corporate policy lower still.

Telesat, for example, is covered by Health Canada’s Safety Code 6, which sets the exposure limit at 5 mW/cm2 over 6 minutes. To meet this obligation, Telesat has enacted an internal corporate policy, based largely on leakage tests, which puts the limit at 0.1 mW/cm2.

Thus, if an RF leak is discovered that exceeds 0.1 mW/cm2, then the condition must be repaired immediately.

Setting the corporate limit this low helps ensure that the Safety Standard is met without requiring complicated field measurements.





The IEEE C95.1-1999 StandardProactive Corporate Policy Based On Leakage:In addition, for desired radiation in the propagation path, antenna pattern specifications and installation data can be used to determine if there is any likelihood of exposure. These calculations are made based, once again, on the reduced exposure level of 0.1 mW/cm2, providing a comfortable safety margin.

With all this said, RF engineers are reminded that they could be called upon to show compliance with applicable standards through comprehensive measurement.

Measurement procedures can be found in IEEE Standard C95.3-1991 or other suitable texts.





4.9.4.3: Exposure LimitsPart 4: Safety Standards and Permissible Levels


Exposure Limits in IEEE C95.1-1999





Exposure Limits in Canada’s Safety Code 6





Exposure Limits Graphically Displayed

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C-BandIEEE C95.1-1999

Canada Safety Code 6

RF Worker10 mW/cm2 5 mW/cm2

General Public4 mW/cm2 1 mW/cm2

Ku-Band

10 mW/cm2

9 mW/cm2

5 mW/cm2

1 mW/cm2

Exposure Limits Compared

As can be seen from a comparison of just these two, there can be significant differences between standards. This is another good reason for a corporation to adapt an even tighter policy limit.



The following specific prevention measures are recommended:

• Immediate correction of all unwanted RF emmision above 0.1 mW/cm2

• Periodic and ongoing testing (the Radiation Survey)

• Signage guidelines in the event of a leak

• Training

4.9.5: An Earth Station RF Safety PlanSec 9: RF Safety


An Earth Station RF Safety PlanSince exposure to RF energy can often go undetected by the person exposed, the best route to RF Safety for a working Earth Station is Prevention.



Establishing an Earth Station RF Safety PlanRF exposure is not a “clear and present danger” in the same manner as fire. As a result, it can be taken for granted by non-vigilant RF workers and managers.

To ensure that this does not happen, every Earth Station should establish a safety policy. This policy should be written, endorsed by upper management, posted, and adhered to by RF workers without exception.

The policy should contain procedures to be followed:• When an RF leak is suspected or discovered• At scheduled maintenance periods• When known leaks cannot be repaired immediately• When visitors are on site• When training is required

4.9.5.1: Establishing a Safety PlanPart 5: An Earth Station RF Safety Plan




TrainingNewly hired RF workers or engineers must be given training designed to introduce them to the nature of RF Energy and to RF Safety practices.

The training should include a theoretical portion detailing the effects of RF on the body, and a practical component in which the students conduct Radiation Surveys with appropriate test equipment.

Training should also include the use of appropriate signs, when to limit access to specific areas, and when (and where) to shut down transmission in an emergency.

Finally, training must include the repair of RF leaks.

It is recommended that this training be instructor led.

4.9.5.2: TrainingPart 5: An Earth Station RF Safety Plan




TrainingIn addition to training newly hired RF employees, refresher training must be offered.

This training can be a brief repetition of the training given to newly hired workers. It should definitely include a review of all procedures.

This training need not be instructor led. A training film, document, or Computer Based Training (CBT) could be provided.

4.9.5.2: TrainingPart 5: An Earth Station RF Safety Plan




4.9.5.3: The Radiation SurveyPart 5: An Earth Station RF Safety Plan


The Radiation SurveyA radiation survey is an evaluation of the actual or potential (due to malfunction) radiation levels in any area, specifically in the vicinity of microwave devices.

This differs from a comprehensive field strength survey that incorporates time and spatial averaging.

The Radiation Survey required for day-to-day Earth Station operations is part leakage test, part field strength test, where the measurements are of instantaneous levels.

These levels are then compared to the more stringent corporate guideline, with corrective action taken as required, thus ensuring that the working environment is safe and that the applicable Government Specification is met.





4.9.5.3.1: When to Conduct a Radiation SurveyRadiation surveys should be performed:

• For all new transmitting RF installations cabable of greater than 5 watts

• Following any repairs or changes to: waveguide joints and flanges on transmit runs, waveguide, combiner networks, power amplifier outputs, or protective shielding

• When any malfunction of operating equipment that may affect the radiation levels is suspected

• When the output power of an HPA is significantly increased

• In any case, periodically, at intervals less than two-years





4.9.5.3.2: How to Conduct a Radiation SurveyFor radiation surveys of this nature, hand held field strength meters equiped with E-field probes are usually all that is required.

These meters most often give readings directly in mW/cm2.

All manufacturer’s guidelines on the use of the instrument must be followed, and the calibration status of the instrument must be maintained.

To conduct a survey, begin by performing a detection test on the instrument. This is done by exposing the probe to a known RF source. This is not a calibration check, but merely a rough check to ensure that the meter is indeed functioning prior to beginning.





4.9.5.3.2: How to Conduct a Radiation SurveyFollow the manufacturer’s instructions to ensure that the meter is properly zeroed and ready for use. If scale selection is manual, select a scale in which the standard to be met is near the full scale deflection for the instrument.

Checking for LeaksNow, holding the sensor probe away from the body, slowly move it through the area to be tested. Observe the instrument readout and note the maximum value to be seen.

When conducting a leakage test, begin at the output of the HPA (transmitting at nominal power) and “sniff” around all waveguide runs, flanges, diplexers, switches and transitions until the antenna is reached.

To “sniff”, hold the probe 2 or 3 inches away from the potential RF source, without actually touching it.





4.9.5.3.2: How to Conduct a Radiation SurveyIf any RF energy is detected by the meter, move the probe around in that area to try to determine where the energy is coming from. Move the probe close to the leak, and farther back, and try different probe angles until the largest field strength is observed. Record this value.

At the antenna, test all transmit waveguide up to the orthocoupler at the rear of the antenna. For larger antennas with hubs at the rear that house the orthocouper, check all aound inside the hub.

Do not stand in front of a transmitting antenna at any Do not stand in front of a transmitting antenna at any time during the performance of this test!time during the performance of this test!





4.9.5.3.2: How to Conduct a Radiation SurveyHaving checked for leakage in this manner, the final check is a survey of the general area.

Holding the probe away from the body, move slowly though areas where RF workers or other people might be expected to stand or move.

Inside, check all hallways and the areas to the rear of equipment racks.

Outside, check pathways and the areas in and around antennas, including to the rear of the antennas. Again, do not stand immediately in front of transmitting antennas to perform this action.

If applicable, check rooftops, parking lots, or pedestrian sidewalks—anywhere a person might stand or walk in the vicinity of transmitting antennas.





4.9.5.3.3: Record KeepingIf, at any time during a radiation survey, the meter detects the presence of microwave energy, this value must be recorded.

A Microwave Radiation Survey log should be kept at all Earth Stations.

The log should include the date and time of the measurement, the amount and location of any leaks, the transmit power of any HPA’s involved in the transmit path, the name of the tester, and the reason for the test.

Even if no microwave energy is noted, this log should still be filled out with zeros as entries. The log will be kept as a station record, demonstrating compliance with the station’s procedures.

If a level greater than that established in the Earth Station’s procedure is discovered, it must be corrected at the earliest possible moment.





ELECTROMAGNETIC RADIATION SURVEY RECORD

EARTH STATION: LOCATION: SYSTEM: SUBSYSTEM: OPERATING FREQUENCY: TEST EQUIPMENT: MODEL #: SERIAL #:

MODEL #: SERIAL #:

LAST CALIBRATION DATE:

TEST#

LOCATION OF TEST OBSERVED RADIATIONLEVEL (mW/cm2)

OPERATINGPOWER LEVEL (W)

REMARKS

TESTS PERFORMED BY: WITNESSED BY: DATE:

Page of Figure 4.9.5.3.3 A Typical Radiation Survey Form



Corrective ActionRadiation in excess of the value set in the Earth Station’s procedures must be corrected as soon as possible.

If transmission from the associated HPA can be inhibited, do so immediatley.

Never work on leaks, even small ones, when the source of the leaking RF is still present. This is because, when the cause of the leak is disturbed, the amount of leaked enegry could increase greatly. Also, when you work on the leak, your eyes, which are especially vulnerable to RF energy, will be close to the source of the leak.

If the origin of the leak is a waveguide juncture or flange, check to ensure the flange fasteners are correctly tightened. If tight, then disassemble the waveguide connection, clean the flanges, and reassemble.

4.9.5.4: Corrective ActionPart 5: An Earth Station RF Safety Plan




Corrective ActionNow re-test to ensure the hazard was corrected.

If the hazard persists replace the applicable waveguide or other component.

If Transmission Cannot be Inhibited ImmediatelyIt is recognized that transmission cannot always be terminated immediately due to the service-carrying nature of the HPA.

In this case, interim measures may be put in place while an equipment release-from-service is obtained. These measures will also be employed when a part required to repair the problem is not immediately available.

It is now very important to know the MPE limit of the governing safety standard and to ensure that personnel do not exceed this value.





Corrective ActionBegin by errecting appropriate warning signs. Use as many signs as required to ensure that there is no way to physically approch the danger area without a sign being visible. The signs are shown on the last few slides.

Along with the signs, post a form stating the location and power level of the leaking microwave energy. The sign should also indicate the MPE limit of the governing standard as a reminder to all personnel. The date of the discovery of the leak, and the name of the appropriate managerial authority could also be included on this information posting.

If necessary—and especially if the area of the leak will be visited by those who are not RF workers—erect a physical barrier to prevent indiscriminate access to the area of the hazard. Caution tape or pylons suit this purpose.





Other Microwave Safety Guidelines• Terminate the inputs of all RF equipment.

• Do not allow open waveguide, even in unused waveguide runs. Always terminate with a load or shorting plate.

• Always check and double check that the RF is OFF when exposing waveguide connected to power amplifiers. Employ a lockout-tagout procedure, just as you would with high voltage work, to ensure that transmission STAYS off.

• Never stand in front of a transmitting antenna's beam.

• If any potential exists for danger to public safety, erect a physical barrier.

• Don't allow a fault to remain.



Negligible Hazard

Minor Injury Possible from Misuse

Area of Unrestricted Occupancy

Microwave Radiation

Moderate Hazard

Serious Injury Possible fromMisuse

Area of Limited Occupancy

Microwave Radiation

Serious Hazard

Critical Injury or Death Possible from Misuse

Area of Denied Occupancy

Microwave Radiation

Engineering

RF Safety in Satellite Communications