Using a Multimeter

 

DMMEvery ham should have a multimeter and know how to use it.  Multimeter use is a practical skill not only for ham radio but also around the house for general power and wiring work.  As with many topics on this site, details of the subject are too extensive to cover in a simple post so we will give you just basic info along with some resources for further study on your own.  Even with minimal detail this is still a long, involved post.

By definition multimeters measure more than one thing.  In electrical work a multimeter typically measures voltage, current, and resistance.  Sometimes it is called a volt-ohm-milliammeter or simply a volt ohm meter (VOM).  Note that these are the three fundamental electrical parameters as described by Ohm’s law.

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In addition to measuring the three core electrical parameters, multimeters may also read other things such as temperature, frequency, capacitance, plus provide quick checks of diodes and continuity.   At minimum they will measure voltage and resistance, since these are the two most commonly read values.

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To measure voltage  you connect the meter leads between (across) two points. This parallel connection allows real-time undisturbed readings in live circuits or power sources.

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Note that resistance measurements are also made with test lead parallel connections but never on live circuits; more on that below.

Measuring current with a common multimeter is more disruptive because it requires the circuit to be broken somewhere and have the meter leads inserted in-line to read amperage, and that is not often very convenient. 

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This series connection requirement for measuring current is the main reason it is less commonly used, and why clamp-on (non-contact) current meters are sometimes favored at the expense of accuracy and greater cost.

So what would the ham or handy homeowner need a multimeter for?  The possibilities are endless but common scenarios are testing batteries, locating blown fuses, verifying DC and AC power supply voltages, and checking cables for undesired opens or shorts.  For kit or DIY circuit builds it is also useful to verify resistor values and test active circuit voltages.  Here is a link to a list of Ten uses for a multimeter from Ham Radio School.


There are a few safety considerations to note when using a multimeter.  Two involve the test leads which connect the meter terminals to the circuit being measured.  First, the leads (probes, wires, and connectors) should be rated for at least the voltage being measured. The meter itself must also be rated to exceed this voltage.

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Good meter leads will have shrouded plugs, high quality flexible insulation, and finger flanges at the probe tips:

DMM LeadsThey will also be rated at least 600V as marked on the probe for general measurement below that voltage.  Special Continue reading

Generator Use & Safety

Spring is [hopefully] coming soon with expected turbulent weather.  A new hurricane season is also upon us.  Both are liable to cause disruptions to utility electrical power.  Annual ARRL Field Day is also approaching.  Time to think about auxiliary power generators.

Small gasoline-powered generators are relatively common and widely available for emergency or portable electrical power.  It’s a good idea for the prepared homeowner to have one, mainly to keep the fridge/freezer cold during times of sustained power outage.  The savvy radio amateur also recognizes the importance of communications capability in a blackout scenario as well.  Power is needed to make our radios work beyond what limited battery capacity we have, particularly in emergency situations.  Most EmComm groups include generators in their plan and have them on hand.  Not every ham does.

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Consider having a small generator for essential power when the lights are out for hours.  Having a generator is good; knowing how to use it safely is the focus of this topic.  We will look at four related safety considerations:

  1. Carbon Monoxide hazards— CO
  2. Fuel handling and storage (fire)
  3. Generator grounding
  4. Shock hazard

Carbon Monoxide (CO) is the primary hazard with generators, since  CO is produced in the exhaust of all gasoline and natural gas combustion engines. 

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NEVER, ever run a generator inside a dwelling or garage where exhaust can seep into the occupied space.  CO is a colorless and odorless gas which can kill or injure humans and animals.   It can be detected only with chemical or electronic CO detectors; it’s wise to have detectors in every home.

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Always operate a power generator outdoors in a well-ventilated area. Continue reading

Indirect RF Hazards

Part 3 on Safety

Safety is an important topic in ham radio.  There are 11 questions on electrical hazards in the USA Technician class license exam pool, 13 questions on tower safety and associated grounding, and 13 questions on radio frequency (RF) hazards.

Part 1 on general electrical hazards and Part 2 on contact RF hazards were posted previously.  This post will address indirect RF hazards.  In case you are not familiar with the specifics of RF energy, refer to our post on the subject.

Here we are concerned about non-contact RF energy.  A long and involved topic (sorry about that) but full of useful detail.

While it involves radiation, RF energy radiates at lower wavelengths where it is least hazardous.

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From the electromagnetic spectrum diagram above we see that radio waves are on the low end of energy levels.  As the frequency increases (wavelengths decrease) the energy in electron volts increases exponentially.  Energy above 250eV (or so) is ionizing, which in addition to radiation burns can cause cell damage and mutations, leading to cancer and other maladies, as would radioactive material.

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Fortunately for hams, all radio frequencies are well below the ionizing radiation energy levels.

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Ham radio operators are radio  active, not radioactive. 🙂Amateur radio activeNow just because RF radiation is non-ionizing doesn’t mean it is completely safe.  Besides the direct contact hazard, exposure to radio frequency energy may cause localized tissue heating, particularly in the eyes and male reproductive area (here’s where a lady ham has an advantage, hihi).  Non-thermal effects of RF radiation are being studied constantly because, while compelling, they are somewhat ambiguous and unproven.

Because RF energy has this radiated exposure risk, rules and regulations have arisen to protect people from such hazards.  In the USA this is done at the federal level by both the FCC (radio communications) and OSHA (occupational).  There are also guidelines for RF radiation published by the ARRL and the IEEE.  Internationally, most countries apart from the US have similar guidelines, as does the World Health Organization (WHO).  References to some of these are given at the end of this presentation.

Specific to US radio amateurs, the FCC instituted RF field exposure limits called Maximum Permissible Exposure (MPE). Continue reading

Contact RF Hazards

Part 2 on Safety

Safety is an important topic in ham radio.  There are 11 questions on electrical hazards in the USA Technician class license exam pool, 13 questions on tower safety and associated grounding, and 13 questions on radio frequency (RF) hazards.

Part 1 on general electrical hazards was posted previously.  This post will address contact RF hazards.  In case you are not familiar with the specifics of RF energy, refer to our post on the subject.  A future post will cover the broad context of non-contact or indirect RF (radiation) safety.  Both direct and indirect RF exposure will heat living tissue.

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Here we are concerned about direct contact with a RF signal of significant energy.  This might happen if a person or animal touches a conductor carrying RF energy.  This most likely happens when someone touches an antenna element while the radio is transmitting.  It’s painful…..as in, full of pain.

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Another risk of RF contact is while working on live transmitter equipment or an antenna connector.  It’s not hard to accidentally key the mic with your hands inside a transmitter enclosure or while touching an un-mated RF connector.

Human skin contact with live RF conductors is a painful experience above very low power levels. What makes it painful is that RF energy heats and damages tissue beneath the outer layer of skin, resulting in 2nd and 3rd degree burns.  Not normally superficial, RF burns heal slowly.

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Without going into physiological details, we will simply quote one person’s testimony: “For the first half-hour or so, all I could see was a tiny dot on my fingertip, and I didn’t think much of it. As the day went on, it hurt more and more, and by the end of the day there was a big, deep, dark blister that covered my entire fingertip and hurt like hell. It took weeks to heal.”  There are some good references to RF burns at the end of this post, including a number of personal experiences.

There are a several factors involved in RF burns:  Power level (available energy), contact surface area (more is better) , grounding/return contact (less is better) and radio frequency (the body absorbs more energy at certain frequencies).

How much power is needed to create a RF burn?  Again, it varies.  There are reports of people getting fingertip burns by touching the top of the antenna connector on a relatively low-power handheld VHF transceiver (5W) and keying the PTT button with no antenna screwed in.  Obviously, a 100W HF transmitter can do much more damage than this.  You don’t need to touch a 50kW AM broadcast antenna to get a nasty RF burn.

Treatment of RF burns are just like any other type– run cold water over it and/or ice it and seek medical attention.  Avoidance is the best defense.  Of course, most RF burns are unintentional so the best we can do is make you aware.  Stay safe!

Direct RF burns cause more immediate tissue damage than indirect radiation but there is a hazard with both;  be looking for a future posting in indirect RF hazards.


RF burn references

Ham radio school

Forum with several detailed experiences reported

eHam My Very First RF Burn! interesting with experiences of many hams

EEVblog forum how does RF feel/taste? how about rf injury?

Compilation of various research studies

Biological effects of RF energy

Reddit

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Electrical Hazards

Safety is an important topic in ham radio.  There are 11 questions on electrical hazards in the USA Technician class license exam pool, 13 questions on tower safety and associated grounding, and 13 questions on radio frequency (RF) hazards.

Several of these have been used by us previously but in retrospect we should have given the safety topic more airtime, pun intended.  New hams are unlikely to have antenna towers so we don’t plan to discuss tower safety much.  That leaves electrical and RF hazards to cover.

This post will address general electrical hazards and related safety; a future post will focus on RF hazards.

Radios and accessories are electrical devices so let’s start with the most obvious hazard: electric shock, which is caused by current flowing through a human body.  Current is useful in electronics but harmful when flowing through a person.  Current can disrupt heart and lung function at even low levels.  It can also cause unwanted muscle movement, or prevent it (can’t let go).  At higher levels, electric current will damage skin and internal organs.

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There are many factors in electric shock and there are other electrical hazards.  But this is a big one and you should avoid touching live circuits.

Fire is another electrical hazard.  When too much current flows in conductors, the wires can get very hot and ignite combustible material.  In fact, the US National Electrical Code is actually a document of the National Fire Protection Association (NFPA), not a government agency.

To limit the risk of fire and other damage, every power circuit needs some form of  protection.  Fuses are quite common; their internal metal melts at a pre-determined current to disconnect power.

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Also, a smaller (amp rating) fuse can safely be inserted in a protective circuit but one should never put in a larger one.  A fuse is sized to the circuit requirements and wiring  is sized to the fuse.  So a higher-ampacity fuse will not properly protect the wires or the circuit and serious overheating may occur in both AC and DC power circuits.

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In addition to one-time use fuses, circuit breakers are another popular form of circuit protection; these may be reset and are often used as an on/off switch.

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While fuses and circuit breakers do not directly provide shock protection, they may do so Continue reading

Flat Ground Strap

Ever wonder why RF grounds should be flat straps and not regular wires?

This is because ordinary wires are not good conductors at frequencies higher than 50-60Hz. This complicates wiring and bonding requirements.

Impedance (effectively, AC resistance) of a conductor increases with frequency and length due to inductive reactance.  The higher the frequency, the greater the impedance.

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All conductors have some measurable inductance, and it doesn’t take much to yield significant impedance.  At KHz or MHz frequencies, long round wires might present hundreds or even thousands of Ohms impedance; not suitable for grounding.

A good ground has less than one ohm impedance.  This is a genuine safety issue.

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Since inductive reactance increases with frequency and length, safety grounds and module bonds need to be something other than long round wires when radio frequencies are involved.

When high frequency grounding is required, use short, wide, and flat conductive straps.  The high aspect ratio minimizes electrical inductance vs. a round wire, as does a short conductor.  This lowers the ground wire’s impedance at higher frequencies.

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So now you know.  Keep it flat and short (KIFS is a lousy acronym).

It’s not just a suggestion; this one might just bite you if you don’t heed the guideline!