KB6NU's Ham Radio Blog

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2016 Extra Class study guide: E0 - Safety

Posted: 25 Mar 2016 08:10 AM PDT
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E0A Safety: amateur radio safety practices; RF radiation hazards; hazardous
materials; grounding

No matter what amateur radio activities you engage in, I hope that you will
engage in them safely. Every year, we lose amateur radio operators because
of injuries they sustained while putting up antennas or doing things that
could be dangerous. We dont want to lose you.

Perhaps the most common danger is from lightning strikes. To mitigate the
danger of a lightning strike, you should use properly grounded lighting
arrestors. The primary function of an external earth connection or ground
rod is lightning protection. (E0A01)

RF exposure is another hazard. One of the ways that RF exposure can be
hazardous is by causing human tissue to heat up. The amount of heating is
proportional to the specific absorption rate (SAR). SAR measures the rate
at which RF energy is absorbed by the body. (E0A08)

In general, the SAR increases as the frequency increases. Think microwave
ovens. They heat food because water in the food absorbs microwave
radiation. The maximum permissible exposure (MPE) is the level at which
harmful biological effects can occur. Several organizations, including the
National Council on Radiation Protection and Measurements (NCRP) and the
Institute of Electrical and Electronics Engineers (IEEE) have provided the
data used by the FCC to set MPEs.

Localized heating of the body from RF exposure in excess of the MPE limits
is an injury that can result from using high-power UHF or microwave
transmitters. (E0A11) One of the potential hazards of using microwaves in
the amateur radio bands is that the high gain antennas commonly used can
result in high exposure levels. (E0A05)

The FCC, as you might expect, has a lot to say about RF exposure. They have
set limits on the field strengths that humans may be exposed to. These
limits are called maximum permissible exposure, or MPE.

The MPEs for the electric field and magnetic field of an electromagnetic
wave differ. All of these choices are correct as to why there are separate
electric (E) and magnetic (H) field MPE limits (E0A06):

The body reacts to electromagnetic radiation from both the E and H fields
Ground reflections and scattering make the field impedance vary with
location
E field and H field radiation intensity peaks can occur at different
locations


To make sure that your transmissions do not expose you or others to field
strengths above the MPE limits is to measure the absolute field strengths.
Unfortunately, this is not easy to do. The equipment used to measure field
strength is very expensive and difficult to use. An alternative is to use
software that calculates field strength. Using an antenna modeling program
to calculate field strength at accessible locations would be a practical
way to estimate whether the RF fields produced by an amateur radio station
are within permissible MPE limits. (E0A03)

Remember to include your neighbors when evaluating RF exposure levels. In
some cases, your antennas may actually be closer to your neighbors’ houses
than they are to your house. When evaluating RF exposure levels from your
station at a neighbor’s home, you must make sure signals from your station
are less than the uncontrolled MPE limits. (E0A02)

Typically, amateur repeater stations are located in places where there are
transmitters for other radio services, such as cell phone and pager
services. These sites should be regularly evaluated so that RF field
strengths don’t exceed the MPE limits. When evaluating a site with multiple
transmitters operating at the same time, the operators and licensees of
each transmitter that produces 5 percent or more of its MPE exposure limit
at accessible locations are responsible for mitigating over-exposure
situations. (E0A04)

Lightning and RF exposure are not the only dangers posed by an amateur
radio station. For example, in emergency situations, you may want to use a
gasoline-powered generator. One of the dangers posed by a gas-powered
generator is that its exhaust contains carbon monoxide. Dangerous levels of
carbon monoxide from an emergency generator can be detected only with a
carbon monoxide detector. (E0A07)

Some of the materials used in electronics pose a danger to amateur radio
operators. They are used because they have some desirable electrical
property, but may be dangerous if used improperly. For example, beryllium
oxide is an insulating material commonly used as a thermal conductor for
some types of electronic devices that is extremely toxic if broken or
crushed and the particles are accidentally inhaled. (E0A09) Polychlorinated
biphenyls, or PCBs, is a material found in some electronic components, such
as high-voltage capacitors and transformers, that is considered toxic.
(E0A10)

The post 2016 Extra Class study guide: E0 Safety appeared first on KB6NUs
Ham Radio Blog.


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Teaching is hard work

Posted: 24 Mar 2016 12:20 PM PDT
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Ive started teaching some basic electronics classes at the Ann Arbor
District Library. They recently setup a makerspace theyre calling The
Secret LabÂ*and hiredÂ*a guy namedÂ*Steve TeeriÂ*to run it. I got hooked up
with Steve after I inquired about the possibility of teaching some basic
electronics classes. As it turned out, they had recently gotten to the
point where they could start doing things like electronics classes inÂ*the
Secret Lab so my inquiry had come at a fortuitous moment. Ive since become
the de-facto electronics/ham radio consultant to the library.

The first class we held was on how to use a digital multimeter. Five people
showed up, and it turned into a class on circuits as much as a class on how
to use a DMM. Overall, it went pretty well, and we followed that up with a
class on how to use an oscilloscope.

This evening, we held a class on basic transistor circuits. This evening,
we had nine people show up. There were a couple of Â*older guys, three
younger guys, two women, one who brought her two kids. The kids were eight
and ten.

Initially, I had planned to have the students breadboard two circuits a
simple switch circuit and a common-emitter amplifier circuit. Both of these
circuits can be found on the sheet below.



I figured that wed be able to blow through the switch circuit, then dig
into the amplifier circuit. WRONG! It took us nearly the entire class for
everyone to get the switch circuit to work.

Some of it was my fault. First of all my schematic was lacking. It was a
schematic that I got off the Internet, and while it was correct, it wasnt
detailed enough. As you can see from the red ink above, there were several
omissions and errors:

On the schematic, I didnt explicitly show which pins on the transistor were
the emitter (E), base (B), and collector (C).
I didnt include a diagram showing which pins on the transistor package were
the emitter (E), base (B), and the collector (C).
I didnt explicitly show how to connect the battery.
I included a switch that we did not use in class.


An added complication was that the resistors that the library had purchased
had blue bodies and very thin color bands. The result was that it was
really difficult to really read the color code. At first, I thought it was
just me and my failing eyesight, but I was relieved when one of the
students had the same complaint. We actually had to dig out the multimeters
and measure the resistors to make sure that we had the right ones.

Another reason that it took us so long is thatÂ*I had to teach the students
some really basic stuff, even before we got to the point where they could
put the circuits together. This included the resistor color code and how to
use the proto boards that we were using. This was certainly OK, but I hadnt
anticipated having to do that.

After about 45 minutes, all of the circuits were built, and the LEDs were
lit. I asked them to disconnect the 2.2 k resistor to demonstrate how
removing the base current turns off the transistor, and I think they all
got that idea. I also explained how in practice that 2.2 k resistor wouldnt
be connected directly to a power supply but to perhaps an Arduinos digital
output. I also mentioned that instead of just turning an LED on and off, we
might use the transistor to do some real work like switch a relay onÂ*and
off. I think they got thoseÂ*ideas, too.

One guy asked how much current that the 2N2222 could switch. I had brought
along with me a 2N2222 data sheet, and we looked up the maximum collector
current for a 2N2222 (1.0 A). We then discussed how running the transistor
at its maximum current rating might not be a good idea.

There was just enough time to go over the amplifier circuit quickly.
Fortunately, I had the foresight to bring my own protoboard with the
amplifier circuit already assembled on it. I quickly hooked up the scope
probes, the signal generator, and the 9 V battery, and demonstrated how the
circuit turned a 100 mV signal into a 2 V signal. For those who were
interested, I was also able to talk a little bit about biasing.

So, the first thing that IÂ*take away from this experience is that I really
need to gear down the level of the presentations. Second, I need to be a
little more explicit with my instructions to students.

Im hoping to do a lot more with the Ann Arbor District Library. I think
that perhaps the next class will be an Arduino Basics class. At some point,
too, Ill want to reprise my DMM and oscilloscope class. I really love it
that the library is giving me the opportunity to do this.

The post Teaching is hard work appeared first on KB6NUs Ham Radio Blog.