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# chapter 10 electrical, antenna structure and rf safety practices - PowerPoint PPT Presentation

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. Normal, Open and Short Circuits.

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Presentation Transcript

• Open and Short Circuits

• Electrical Safety

• Grounding

• RF Environmental Safety Practices

• RF Awareness Guidelines

• Maximum Permissible Exposure (MPE) Limits

• Limiting RF Exposure

Normal, Open and Short Circuits Practices

• 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

SW Practices

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

Fuses Practices

• 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.

Electrical Safety Guidelines Practices

• 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.

• 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.

• 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.

Antenna and Tower Installation Practices

• Antenna Installation

• Make sure no one can come in contact with the antenna

• Make sure that the antenna can never touch a power line, especially if it breaks or falls.

• Antenna height considerations are more restrictive near airports

• Stainless steel hardware is often used in antenna construction as it resists corrosion.

• Tower Installation

• Guy wires for towers must be installed per manufacture's recommendation

• Crank up towers must never be climbed except when it is in it’s fully lowered condition

Grounding Practices

• 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.

Grounding Practices

RF Environmental Safety Practices 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.

RF Environmental Safety Practices 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.

RF Awareness Guidelines Practices

• 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.

RF Awareness Guidelines (Cont’d) Practices

• 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.

RF Awareness Guidelines (Cont’d) Practices

• 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.

FCC RF Exposure Regulations Practices

• 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.

• Notes:

• f = frequency in MHz

• Power density is plane wave equivalent power density.

MPE Power Density Limits Practices

Controlled

Uncontrolled

HF

VHF

UHF

.3 to 3

3 to 30

30 to 300

300 to 1500

1500 to 100000

FCC RF Exposure Regulations Practices

• 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.

FCC RF Exposure Regulations Practices

• 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.

FCC RF Exposure Regulations Practices

• 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.

97.13(c)

* Transmitter power = Peak-envelope power input to antenna.

Routine Station Evaluations Practices

• 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.

Field Strengths Around Your Antenna Practices

• 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

• Field strength varies as inverse square of distance

Field Strengths Around Your Antenna Practices

Dipole

Antenna

Reactive

Field

L

Limiting RF Exposure Practices

• 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.

Limiting RF Exposure (Cont’d) Practices

• 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.