Chapter 10 – Electrical, Antenna Structure and RF Safety Practices

1. Chapter 10 – Electrical, Antenna Structure and RF Safety Practices • Open and Short Circuits • Electrical Safety • Grounding • RF Environmental Safety Practices • RF Awareness Guidelines • Maximum Permissible Exposure (MPE) Limits • Limiting RF Exposure

2. Normal, Open and Short Circuits • Normal Circuit • When normal current is flowing through the circuit • Open Circuit • When the current flow is interrupted by switch or fuse • Circuit break presents an extremely high resistance. • Short Circuit • When the current flowing through the circuit is following a “shorter” low resistance path between the power source terminals. • Allows high current to flow in the circuit

3. SW SW + + R R E E I=E/R I=0 Normal Circuit Open Circuit Normal, Open and Short Circuits SW Short circuit is a very low resistance path across voltage source. Current can be very high and possibly result in a fire. A fuse in the circuit can protect against a short circuit condition by forming an open circuit. + R E I Short Circuit

4. Fuses • A device made of metal that will heat up and melts when a certain amount of current flows in a circuit. • A fuse creates an open circuit when blown. • A fuse should never be used in the neutral or ground line of a ac power circuit. • In a mobile installation, fuses should be installed in both negative and positive supply leads as close to the battery as possible.

5. Electrical Safety Guidelines • A main station power switch should be used to turn off all equipment at once. • Never operate equipment without proper shields installed over all circuit components. • A safety interlock can be used to automatically turn off power when a shield or cover is removed. • High voltage power capacitors may remain charged even if power has been turned off. • Should be manually discharged before servicing equipment.

6. Electrical Safety Guidelines (Cont’d) • Electrical codes require three-wire power cords and plugs on many tools and appliances. • The “hot” wire is usually black. • The “neutral” wire is usually white. • The frame/ground wire is usually green or bare wire. • Do not install higher current capacity fuses in an existing circuit.

7. Electrical Safety Guidelines (Cont’d) • Antenna/tower safety • Always wear a safety belt in good condition, a helmet and safety glasses when climbing a tower. • Do not stand under a tower when someone is climbing • Keep antennas and towers away from electrical power lines. • Always respect electricity • As little as 100 mA of current can be fatal. • The minimum voltage that can be dangerous to humans is 30 volts.

8. Grounding • All station equipment should be connected to a good ground. • The best ground is provided by ground rods located near the station. • All rods must be connected together to form a single grounding system. • All antennas, feed lines and rotor cables should be grounded for effective lightning protection • The best protection is to disconnect all cables and ground the cables.

9. Grounding

10. RF Environmental Safety Practices While Amateur radio is a safe activity, there has been considerable discussion and concern in recent years about the possible hazards of electro-magnetic radiation (EMR) including both RF energy and power frequency (50-60 Hz) electromagnetic fields.

11. RF Environmental Safety Practices • RF Energy • RF energy is electric and magnetic energy between 3 kHz and 300 GHz. • RF and 60 Hz fields are nonionizing radiation. X-rays, gamma rays and some ultraviolet radiation are classified as ionizing radiation. • Thermal Effects • RF exposure limits for the human body is frequency dependent. • Amateur RF exposure for SSB and CW operations is reduced due to low transmission duty cycles.

12. RF Awareness Guidelines • Confine antenna radiation to the radiating elements. Provide a single, good station ground, and eliminate radiation from transmission lines. Use good coaxial cable, not open-wire lines or end-fed antennas that come directly into the transmitter area. • No person should near any transmitting antenna while it is in use. This is especially true for mobile or ground mounted vertical antennas. Avoid transmitting with more than 25 watts in a VHF mobile installation unless it is possible to first measure the RF fields inside the vehicle. At the 1 KW level, both HF and VHF directional antennas should be at least 35 ft above inhabited areas. Avoid using indoor and attic-mounted antennas if at all possible.

13. RF Awareness Guidelines (Cont’d) • Don’t operate high-power amplifiers with the covers removed, especially at VHF/UHF frequencies. • Never look into the open end of an activated UHF/SHF length of microwave waveguide or point it toward anyone. Never point a high-gain, narrow-bandwidth antenna toward people. Use caution if aiming an EME array toward the horizon. • When using hand-held transceivers, keep the antenna away from your head and use the lowest power necessary to maintain communications. Use a separate microphone and hold the rig as far away as possible.

14. RF Awareness Guidelines (Cont’d) • Don’t work on antennas that have RF energy applied. • Don’t stand or sit close to a power supply or linear amplifier when the ac power is turned on. Stay at least 24 inches away from power transformers, electrical fans and of other sources of high-level 60 Hz magnetic fields.

15. FCC RF Exposure Regulations • Maximum Permissible Exposure (MPE) • Regulations control exposure to RF fields, not the strength of RF fields. • All radio stations must comply with the MPE requirements. • MPE limits are specified in: • Maximum electric field (Volts/meter) • Maximum magnetic field (Amperes/meter) • Power density (mWatt/cm2) • If multiple MPE limits are specified for a given frequency and a station exceeds a single limit, then the station is not in compliance.

16. Limits to Maximum Permissible Exposure (MPE) • Notes: • f = frequency in MHz • Power density is plane wave equivalent power density.

17. MPE Power Density Limits Controlled Uncontrolled HF VHF UHF .3 to 3 3 to 30 30 to 300 300 to 1500 1500 to 100000

18. FCC RF Exposure Regulations • Environments • A controlled environment is one in which the people who are being exposed are aware of the exposure and can take steps to minimize that exposure. • FCC has determined that amateur operators and members of their families fall into this environment category. • An uncontrolled environment is one in which the people being exposed are not normally aware of the exposure. • The uncontrolled environment limits are more stringent than the controlled environment limits.

19. FCC RF Exposure Regulations • Station Evaluations • FCC requires that certain amateur stations be evaluated for MPE compliance. • FCC Office of Engineering and Technology (OET) Bulletin 65 and the Amateur Supplement to that Bulletin contains tables to assist in MPE compliance evaluations. • Power density levels in the FCC tables can be adjusted for the duty cycle of the operating environment being used. • Averaged over 6 minutes for a controlled environment. • Averaged over 30 minutes for an uncontrolled environment.

20. FCC RF Exposure Regulations • Categorical Exemptions • FCC has exempted certain stations from the MPE evaluation requirement: • If the transmitter output PEP is less than or equal to limits specified in Section 97.13(c). • Exemption includes a VHF transceiver of 50 watts or less. • Certain repeater stations. • Hand-held radios and mobile radios using a push-to-talk button.

21. Power Thresholds for Routine Evaluations 97.13(c) * Transmitter power = Peak-envelope power input to antenna.

22. Routine Station Evaluations • An amateur can determine that his station complies with RF exposure regulations by using a variety of methods: • By measuring the field strength using calibrated instruments. • By calculation, based on FCC OET Bulletin No. 65 • By calculation, using computer modeling. • Evaluation records should be retained by the amateur licensee.

23. Station Evaluation Using Tables

24. Field Strengths Around Your Antenna • Field strengths around an antenna can be determined by direct measurement, using calibrated instruments, or by calculations using either tables or computer software. • For analysis purposes, the area around an antenna is divided into the following regions: • Reactive near field • Considered to be within a half wavelength of antenna • Radiating near field • Field strength varies as inverse square of distance • Radiating far field

25. Field Strengths Around Your Antenna Dipole Antenna Far Radiating Field Reactive Field L Near Radiating Field

26. Limiting RF Exposure • Reduce transmitter power • Raise your antenna higher in the air and farther away from your neighbor’s property line • Half-wavelength dipole antennas generally generate a stronger RF field directly under the antenna that other types of antennas. • Do not aim your antenna in a direction where people are likely to be located. • Select an operating frequency with a higher MPE limit.

27. Limiting RF Exposure (Cont’d) • Use an emission with a lower duty cycle. • Single side-band (SSB) generally produces the lowest duty cycle. • Reduce your actual transmitting time • In a controlled environment, the RF exposure is averaged over any 6 minute period. • In a non-controlled environment, the RF exposure is average over any 30 minute period.