RF Safety By Bob Buus, W2OD

Certification 6) Amateur Applicant/Licensee certifies that they have READ and WILL COMPLY WITH Section 97.13(c) of the Commission’s Rules regarding RADIOFREQUENCY (RF) RADIATION SAFETY and the amateur service section of OST/OET Bulletin Number 65. – FCC Form 605

Section 97.13(c) Before causing or allowing an amateur station to transmit from any place where the operation of the station could cause human exposure to RF electromagnetic field levels in excess of those allowed under § of this chapter, the licensee is required to take certain actions. (1) The licensee must perform the routine RF environmental evaluation prescribed by § (b) of this chapter, if the power of the licensee’s station exceeds the limits given in the following table: (2) If the routine environmental evalation indicates that the RF electromagnetic fields could exceed the limits contained in § of this chapter in accessible areas, the licensee must take action to prevent human exposure to such RF electromagnetic fields. Further information on evaluating compliance with these limits can be found in the FCC’s OET Bulletin Number 65, “Evaluating Compliance with the FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields.” Wavelength Band Evaluation required if power (watts) exceeds 160 m m m m m m m m m75 10 m50 VHF50 70 cm70 33 cm cm cm250 SHF250 EHF250 Repeater stations500 W ERP

The Human Body Heat Engine Absorption of RF Power causes heating Specific Absorption Rate (SAR) Watts/kg A SAR produces same heating regardless of frequency A human at rest produces about 1 Watt/kg A SAR of 4 W/kg will raise tissue temp. 1 degree – same as a brisk walk Avg. 4 W/kg or PK 20 W/kg in limbs safe.

Living Human Properties The human body is a marvel at regulating its internal temperature through blood circulation and perspiration. Sperm in the testes are very temperature sensitive. The lens of the eye lacks blood flow to keep it cool. If “cooked” at high temperature, cataracts will form.

RF Field Strength Electric E Field is in Volts per meter Magnetic H Field is in Amperes per meter Power Density, S is Watts per square meter S=ExH = E 2 /377 = 377 H 2 Watts/sq. meter 10 Watts/sq. m = 1 milliwatt/sq. cm For a point source, S = Power / 4πd 2

Field Strength Examples For a point source, S = Power / 4πd 2 Eff. Rad. PowerDistance, dStrength, S 1250 Watts1.0 Meter10 mW/sq. cm 125 watts1.0 meter1 mW/sq. cm 125 watts0.1 meter100 mW/sq. cm 125 Watts10 Meters.01 mW/ sq. cm

Conversion from SAR to RF Field Strength Exposing a human body to 10 mW/sq. cm Frequency of maximum absorption (about 70 mHz) Localized SAR values Average SAR is 1.88 W/kg From Proc. IEEE 68:27, 1980

FCC Requirements Most stringent between 30 and 300 MHz 20 mW/sq. cm deemed safe Controlled Environment less than 1 mW/ sq. cm (factor of 20) Uncontrolled Environment less than 0.2 mW/sq. cm (factor of 100) Evaluation must be done if output power exceeds 50 Watts at these frequencies.

Must do Evaluation if Power Exceeds 500 watts below 10 MHz 425 watts on 30 meters 225 watts on 20 meters 125 watts on 17 meters 100 watts on 15 meters 75 watts on 12 meters 50 watts on 10 meters and at VHF 70 watts on 70 cm 150 watts on 33 cm 200 watts on 23 cm 250 watts above 2 gHz 500 watts ERP for repeaters on all bands

Evaluation Procedure Limits of vary with frequency so you must evaluate for each frequency band you plan to use Controlled exposure limits are 5 times the Uncontrolled exposure limits Controlled exposure averaging time is 6 minutes Uncontrolled exposure averaging time is 30 minutes

Duty Cycle Advantage If we listen as much as we talk, the transmitter is on only 50% of the time For FM, PSK, and TTY, PEP is average transmitting power For CW, because of spaces between dots and dashes, average xmit power is 0.5 PEP For SSB with speech processing, the average power is about 0.2 PEP

Typical Duty Cycles DutyCycle ModulationAvg./PEPTalk/Listen FM PSK/TTY CW0.5 AM SSB Packet1.00.1

Radiated Power Calculation Average Radiated Power (6 or 30 min) = PEP output power (watts) Reduced by transmission line loss Times antenna gain Times ground reflection gain Reduced by Average/PEP ratio Reduced by Duty Cycle ratio

Final Calculation Must determine how far away you must be from the antenna to meet the FCC requirements for both the controlled and uncontrolled environments Using simple 1 / 4πd 2 gives conservative results. More complicated NEC analysis will permit higher power and/or closer distances

To Run RF Safety Program Run rfsafety.exe Input Call Change any or all of the defaults by selecting from drop-down menu or typing a numeric value followed by “enter”. Click “Calculate” and distance results are given for controlled and uncontrolled Clicking “Print” will give hard copy.

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