T0-1

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

T0-2

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

T0-3

Normal, Open and Short Circuits

RE

+

I

SW

Short Circuit

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.

RE

+

I=E/R

SW

Normal Circuit

RE

+

I=0

SW

Open Circuit

T0-4

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.

T0-5

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.

T0-6

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.

T0-7

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.

T0-8

Antenna and Tower Installation

• 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

T0-9

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.

T0-10

Grounding

T0-11

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.

T0-12

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.

T0-13

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.

T0-14

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.

T0-15

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.

T0-16

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.

T0-17

Limits to Maximum Permissible Exposure (MPE)

Frequency Range (MHz)

Electric Field Strength (V/m)

Magnetic Field Strength (A/m)

Power Density (mW/cm2)

Averaging Time (minutes)

(A) Limits for Occupational/Controlled Exposure

0.3 to 3.0 614 1.63 100 (Note 2) 6

3.0 to 30 1842/f 4.89/f 900/f2 (Note 2) 6

30 to 300 61.4 0.163 1.0 6

300 to 1500 - - f/300 6

1500 to 100,000 - - 5 6

(B) Limits for General Population/Uncontrolled Exposure

0.3 to 1.34 614 1.63 100 (Note 2) 30

1.34 to 30 824/f 2.19/f 180/f2 (Note 2) 30

30 to 300 27.5 0.073 0.2 30

300 to 1500 - - f/1500 30

1500 to 100,000 - - 1.0 30

Notes:1. f = frequency in MHz2. Power density is plane wave equivalent power density.

T0-18

MPE Power Density Limits

100 100

1 1

5 5

100 100

0.2 0.2

1 1

0.1

1

10

100

0.1 1 10 100 1000 10000 100000

Mhz

mW

/cm

2 Controlled

Uncontrolled

.3 to 3 3 to 30 30 to 300 300 to 1500 1500 to 100000

HF VHF UHF

T0-19

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.

T0-20

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.

T0-21

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.

T0-22

Power Thresholds for Routine Evaluations

Wavelength

Band

Evaluation Required if

Power* (watts) Exceeds

160m 500

80 m 500

75 m 500

40 m 500

30 m 425

20 m 225

17 m 125

15 m 100

12 m 75

10 m 50

VHF (all bands) 50

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

97.13(c)

T0-23

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.

T0-24

Station Evaluation Using Tables

T0-25

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

T0-26

Field Strengths Around Your Antenna

Far Radiating Field

L

λ

2L2

DipoleAntenna

Near Radiating Field

ReactiveField

T0-27

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.

T0-28

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.