2014 Tech study guide: RF hazards

In this section, the answer to T0C11 was changed, and T0C12 and T0C13 were added. All good additions, in my opinion…Dan

Finally, let’s consider the safety hazards of being exposed to radio waves. When using high power, you are required to perform an RF exposure evaluation, even though VHF and UHF radio signals are non-ionizing radiation. (T0C01) RF radiation differs from ionizing radiation (radioactivity) in that RF radiation does not have sufficient energy to cause genetic damage. (T0C12)

Even so small levels of RF energy can be unsafe. The maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required is 50 watts PEP at the antenna. (T0C03)

How do you perform an RF exposure evaluation? All of these choices are correct as acceptable methods to determine if your station complies with FCC RF exposure regulations (T0C06):

  • By calculation based on FCC OET Bulletin 65
  • By calculation based on computer modeling
  • By measurement of field strength using calibrated equipment

One of the factors to consider when performing an RF exposure evaluation is the duty cycle of your transmissions. The term “duty cycle” when referring to RF exposure is the percentage of time that a transmitter is transmitting. (T0C11) Duty cycle is one of the factors used to determine safe RF radiation exposure levels because it affects the average exposure of people to radiation. (T0C10) A transmission with a lower duty cycle would be less hazardous than a high duty cycle transmission.

Consider this example: If the averaging time for exposure is 6 minutes, 2 times as much power density is permitted if the signal is present for 3 minutes and absent for 3 minutes rather than being present for the entire 6 minutes. (T0C13)

Because of the way radio waves interact with the body, the exposure limits are different for each amateur radio band. Exposure limits vary with frequency because the human body absorbs more RF energy at some frequencies than at others. (T0C05) The 50 MHz band has the lowest Maximum Permissible Exposure limit. (T0C02) All of these choices are correct when talking about factors that affect the RF exposure of people near an amateur station antenna (T0C04):

  • Frequency and power level of the RF field
  • Distance from the antenna to a person
  • Radiation pattern of the antenna

So, what should you do if your RF exposure evaluation shows that people are being exposed to excessive RF? One action amateur operators might take to prevent exposure to RF radiation in excess of FCC-supplied limits is to relocate antennas. (T0C08) You could also lower the power or simply transmit less.

After the initial RF exposure evaluation, you make sure your station stays in compliance with RF safety regulations by re-evaluating the station whenever an item of equipment is changed. (T0C09)

2014 Tech study guide: antenna safety

There were no changes to this section that I could find…Dan

Antenna safety is also of primary concern. There are two aspects of antenna safety—being safe when installing an antenna and safely operating an antenna.

When putting up an antenna tower, an important safety precaution is to look for and stay clear of any overhead electrical wires. (T0B04) When installing an antenna, make sure that it is far enough from power lines, so that if the antenna falls unexpectedly, no part of it can come closer than 10 feet to the power wires. (T0B06) This is the reason you should avoid attaching an antenna to a utility pole. The antenna could contact high-voltage power wires. (T0B09)

You also should position the antenna so that no one can touch it while you are transmitting. If a person accidentally touched your antenna while you were transmitting, they might receive a painful RF burn. (T0C07)

Another safety tip is to use a gin pole designed for use with the tower that you’re installing. The purpose of a gin pole is to lift tower sections or antennas. (T0B05)

At all times when any work is being done on the tower, members of a tower work team should wear a hard hat and safety glasses. (T0B01) Before climbing an antenna tower, it is a good precaution to put on a climbing harness and safety glasses. (T0B02) It is never safe to climb a tower without a helper or observer. (T0B03) When using a crank-up tower, an important safety rule to remember is that this type of tower must never be climbed unless it is in the fully retracted position. (T0B07)

Grounding is very important when installing a tower because the tower is basically a big lightning rod. Local electrical codes establish grounding requirements for an amateur radio tower or antenna. (T0B11)

Separate eight-foot long ground rods for each tower leg, bonded to the tower and each other is considered to be a proper grounding method for a tower. (T0B08) When installing ground wires on a tower for lightning protection, it is good practice to ensure that connections are short and direct. (T0B12) Sharp bends must be avoided when installing grounding conductors used for lightning protection. (T0B10)

Lightning can also be conducted down a feedline and into your shack. To prevent this, several manufacturers make devices designed to shunt this current to ground before it gets into the shack. When installing devices for lightning protection in a coaxial cable feedline, ground all of the protectors to a common plate which is in turn connected to an external ground. (T0A07)

2014 Tech study guide: electrical safety

Several changes were made to this section that I don’t like. For example, the question pool committee removed the question about 30V being the commonly accepted value for the lowest voltage that can cause a dangerous electric shock. The question about 100 mA being the lowest amount of electrical current flowing through the body that is likely to cause death was removed from the 2010 question pool, so now there are no questions at all about these values. I’m leaving them in the study guide, though, because I think they’re important.

The committee also removed the question about charging a 12 V battery by connecting it to your car battery and running the engine. I’m all for removing that question. It was replaced by a question about what might happen if you short the terminals of a 12V battery…Dan

BE SAFE!

When operating or working on amateur radio equipment, it’s possible to come into contact with dangerous voltages and currents. Because it would be a shame to lose a single person, it’s important to know how to be safe when working with electricity. Having said that, 30 volts is the commonly accepted value for the lowest voltage that can cause a dangerous electric shock, and 100 mA is the lowest amount of electrical current flowing through the body that is likely to cause death. These are not very large values.

All of these choices are correct when considering how current flowing through the body can cause a health hazard (T0A02):

  • By heating tissue
  • It disrupts the electrical functions of cells
  • It causes involuntary muscle contractions

Three-wire electrical outlets and plugs are safer than two-wire outlets and plugs, and you should use three-wire plugs for all of your amateur radio equipment. The third wire provides an independent, or safety ground. Safety ground is connected to the green wire in a three- wire electrical AC plug. (T0A03)

All of these choices are correct when choosing a good way to guard against electrical shock at your station (T0A06):

  • Use three-wire cords and plugs for all AC powered equipment
  • Connect all AC powered station equipment to a common safety ground
  • Use a circuit protected by a ground-fault interrupter

Individual pieces of equipment may have their own fuses to protect that piece of equipment should something happen that causes that equipment to draw excessive current. The purpose of a fuse in an electrical circuit is to interrupt power in case of overload. (T0A04) When replacing a fuse, always replace the blown fuse with a fuse of the same type and value. It is, for example, unwise to install a 20-ampere fuse in the place of a 5-ampere fuse because excessive current could cause a fire. (T0A05)

If you plan to build your own equipment, be sure to include fuses in your designs. A fuse or circuit breaker in series with the AC “hot” conductor should always be included in home-built equipment that is powered from 120V AC power circuits. (T0A08)

Whenever you’re working on equipment, be sure to disconnect it from the power lines, and even then be careful working around a power supply’s capacitors. If a power supply is turned off and disconnected, you might receive an electric shock from stored charge in large capacitors. (T0A11)

Finally, it’s necessary to take precautions when using batteries to power your amateur radio station. Conventional 12-volt storage batteries present several safety hazards. Shorting the terminals can cause burns, fire, or an explosion (T0A01), explosive gas can collect if not properly vented (T0A09), and, if a lead-acid storage battery is charged or discharged too quickly, the battery could overheat and give off flammable gas or explode. (T0A10)

From my Twitter feed: lighting safety, 40m Moxon, diy lead bender

KI4OZG's avatarTracy A Stephens@KI4OZG
Lightning Safety Week: June 23-29, 2013 “When Thunder Roars, Go Indoors!” lightningsafety.noaa.gov/index.htm #hamradio #fieldday

This is a little late, but better late than never…….Dan

 

stahlbrandt's avatarBo G. Stahlbrandt @stahlbrandt
This looks interesting, a 40m Mini-MOXON Beam Antenna by W7XA via @dxzone bit.ly/10EI1T6 #hamr #hamradio

 

DIYEngineering's avatarDIY Engineering @DIYEngineering<
Electronic Component Lead Bending Tool – When assembling a circuit it is pretty common to hook up a resistor or ot… ow.ly/2xBzM7

I still have a lead bender I got from making a Heathkit many moons ago…..Dan