Transcript Chapter 10 - Electrical, Antenna and RF Safety

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
T0-1
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
T0-2
Normal, Open and Short Circuits
SW
SW
+
+
R
E
R
E
I=E/R
I=0
Normal Circuit
Open Circuit
SW
Short circuit is a very low resistance path
across voltage source.
+
R
E
I
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.
Short Circuit
T0-3
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.
T0-4
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.
T0-5
Electrical Safety Guidelines (Cont’d)
• Electrical codes require threewire 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.
T0-6
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.
T0-7
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.
T0-8
Grounding
T0-9
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 electromagnetic radiation (EMR) including both RF
energy and power frequency (50-60 Hz)
electromagnetic fields.
T0-10
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.
T0-11
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.
T0-12
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.
T0-13
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.
T0-14
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.
T0-15
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 MHz
2.
Power density is plane wave equivalent power density.
T0-16
MPE Power Density Limits
100
100
100
mW/cm2
10
1
1
5 Controlled
5
1
1
Uncontrolled
1
0.1
1
10
100
0.2
0.1
.3 to 3
1000
10000
100000
0.2
HF
VHF
3 to 30
30 to 300
UHF
300 to 1500
1500 to 100000
Mhz
T0-17
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.
T0-18
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.
T0-19
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.
T0-20
Power Thresholds for Routine Evaluations
97.13(c)
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.
T0-21
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.
T0-22
Station Evaluation Using Tables
T0-23
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
T0-24
Field Strengths Around Your Antenna
Dipole
Antenna
2L2
λ
Far Radiating Field
Reactive
Field
L
Near Radiating Field
T0-25
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.
T0-26
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.
T0-27