Using Fuses Effectively

A Communicator Reprise: May 2012

In today’s electronics, fuses play a very important role that is frequently misunderstood, with the result that expensive equipment is often not fully protected, and this can result in expensive repair bills. The history of fuses is as old as the use of electricity and probably goes back to the time of the first short circuit! At first, fuses were simple open-wire affairs, but around 1890 Edison enclosed the wire into a lamp base to make the first enclosed fuse. By 1904 the Underwriters Laboratories had introduced specifications covering fuse size and ratings to meet the safety standards of the period. In 1927 Littelfuse started making the first of their range of low amperage fuses for the budding electronics industry. Since that time many new types of fuses have appeared on the market, some with very special characteristics for particular types of protection. Today the choice is extremely large and protection can be provided inexpensively and the risk of expensive repair bills reduced.

Purpose of a Fuse

A fuse is a device which is wired into an electrical circuit to prevent excessive current flowing when a fault occurs. On overload the wire forming the fuse element will heat up and melt (‘blow’) thereby breaking the circuit, interrupting current flow, preventing damage from excessive current to the remaining circuits. It is the electrical equivalent of a "safety valve".
The most important characteristic to the user is the current rating which, unfortunately, is often misunderstood. The current rating of a fuse is established by the manufacturer after a series of tests under controlled conditions. This enables the manufacturer to publish a set of specifications for its product which design engineers can use to decide which is the correct type of fuse for a particular circuit. In order to understand the current rating of a given fuse it is important to know the conditions under which this rating was achieved. There are three main groups of fuses:

1. slow-blow (or anti-surge),
2. normal quick acting, and
3. very fast acting

There is also a fourth type known as time delay fuses. 
Each of these types will protect a circuit from excessive continuous current, but act very differently under surge or short time conditions. The fitting of the wrong type could mean no protection at all is being provided, or fuses that keep "blowing" for no apparent reason.
Let’s take a detailed look at each type. The blowing time in seconds plotted against percentage overload for the three main types of fuse mentioned above is shown.


It can be seen that up to 100 per cent overload there is very little difference between the three types. But if we take a current overload of say 500 per cent we can see that the fast acting fuse blows in 0.001 seconds (a millisecond) and the slow-blow in about 2 seconds with the normal acting fuse at about 0.01 seconds. Quite a considerable difference between the three types. In fact the ratios (taking our normal acting fuse as the reference) work out at one tenth of the time for our fast acting fuse and 200 times longer for our slow-blow type! A very big difference indeed and more than enough to ‘release the smoke’ in expensive semiconductors under fault conditions.
Temperature also has an effect on the current rating. As the ambient temperature becomes lower the amount of current required to "blow" a fuse becomes higher and this can make a considerable difference to the blowing times under surge conditions.
Ah, you may be thinking, let's use a fast acting fuse all the time and be safe. Regretfully this is not practical as many circuits have a high surge current when first switching on, or switching to change operating conditions.

Construction

Let us now consider the construction of a typical cartridge fuse. First it has to have a body or barrel and this is normally made of glass or ceramic material. The barrel will have some form of termination at each end, usually brass or copper, which has been plated to prevent corrosion. The fuse element will be connected between the two end terminations and enclosed within the barrel. 
It will consist of a single wire in the case of a quick acting fuse, or may be one or more wires arranged in a specific way for delay and anti-surge types. Sometimes a filler is used to modify the action of the fuse and this may be sand or quartz powder. This filler will absorb the energy of the arc when the current is interrupted.
Fuses are of course marked in some way as to type and ratings, normally on one or both of the end caps and in addition there may be an indication of one of the many standards that the particular fuse complies with, e.g. BS, SEMKO, etc. The size of fuse may vary but there are a number of standard sizes and the most common are the  Standard metric which is 20mm long by 5mm diameter and the US standard of 1 x .25 inch diameter.  Many other sizes are available ranging from 5mm long to over 200mm. 
Particularly in automotive applications there are also ‘blade’ type fuses (see graphic below), and of course there are a number of other styles available for special purposes.

Fuse Characteristics

There are two main characteristics which will concern the Amateur and these are maximum continuous current rating and the surge rating of slow-blow types. The rupturing capacity of a fuse may also be important and for completeness is mentioned here. A high rupturing capacity fuse is capable of interrupting currents in the order of thousands of amperes. It would have a ceramic body and also contain an arc-quenching medium. Non-HRC fuses (more common in Amateur Radio equipment) do not have an arc-quenching medium and are only suitable for surge currents up to about 50 amps. With higher currents than this they would be very likely to explode when they blow.

Fusing Speed

As we have seen, a quick-acting fuse is designed to react both to short and long term overload conditions. They are very robust in construction and will withstand shocks and vibration. But they do tend to have a higher resistance and the voltage drop caused by this may be a problem in some applications. This higher resistance also means that more heat is produced and this must be effectively dissipated.
Time delay fuses will react to long term overload currents but will withstand transient surges without harm; several types are available. For example, one type has what looks like a spring inside the barrel and these will stand up to surges of around ten times the normal rating for 75 milliseconds. Another type has a "blob" in the middle of the fuse element and this type has a reduced surge capacity, typically ten times rated current but only for 25 milliseconds. Time delay types have a very low resistance and can be used in enclosed places as there is little self-generated heat but they are only available in the lower current ratings. Both the "spring" and the "blob" type are time delay fuses.
So far we have mentioned only the current rating of fuses, but they also have a maximum voltage rating. This voltage rating has no effect on the current rating but is important nevertheless. When a fuse "blows" an arc is developed between the two ends of the broken fuse element and, if the voltage across these ends is high enough, the arc will be maintained and the current will not be interrupted. This condition could result in considerable damage to the equipment. Arcs are readily produced in high voltage circuits or where inductive loads are being used and, in these conditions, the voltage rating of a fuse must not be exceeded. Fuses can be used at their current rating at all voltage levels up to their maximum. When it is known for certain that, although the circuit has a high voltage present, the power available is limited, it is possible to use a fuse at a higher voltage than that for which it is rated. This is common practice in domestic electronic equipment and quite safe. But. if in doubt, keep within the voltage ratings given by the manufacturers.

Mobile Fuse Applications

In one sentence: Place the fuse as close to the battery as possible in both the positive and negative line.

That’s good advice because if for any reason the positive wire’s insulation is damaged and the wire touches the chassis or engine (a hot manifold is a frequent cause of this problem) then it will blow the fuse if it is close to the battery but not if it is between the radio and the short.

It’s also good practice to place a fuse in the negative line.  Many hams think that is unnecessary because you will not cause a short if the negative wire touches the metal of the vehicle.  So why place a fuse in this line? In many installations the negative wire goes straight back to the negative terminal of the battery.  If the battery cable develops resistance between the cable and the body of the vehicle by rust or corrosion or the wire itself corrodes to the point that it is not a good conductor this type of installation can cause problems. When the engine is being started a lot of current is being drawn from the battery and the wiring to the mobile radio is not designed to handle any where that much current.  Simple Ohm’s law will tell you that the maximum current will flow through the path of least resistance and if that path happens to be through the negative wire of the radio to the negative terminal of the battery then that is the where the most current will flow. Frequently the unit is not grounded well at the mounting bracket but the shield side of the coax makes a good ground by the antenna mount.  In that case the current for the starter will attempt to flow through the coax shield to the coax connector on the radio then on to the negative wire to the battery. If that wire is not fused the coax shield will smoke. If the radio is grounded at the mount, the negative wire to the battery is not big enough to handle the load and it will smoke.  Either way there is a fire danger.

This type of hookup is not recommended.  If you do this and the fuse blows you may not know it is blown because the radio finds sufficient contact between the mounting bracket and/or the antenna ground to continue to operate.  The resistance between the battery and the vehicle chassis does not have to be high enough that the vehicle will not start to cause this phenomenon. The antenna ground and the mounting bracket are not designed to be the negative source for the DC power of the radio and it will cause more problems than you can imagine. Tracing the source of these problems can drive you crazy. The suggestion is therefore to run the ground wire to the chassis of the vehicle. Use an eye terminal with one outer locking lock washer between the head of the screw and the lug and another between the lug and the chassis.  Scrape the paint off the place where the lug will come in contact with the metal. Run the screw down tight but do not strip it out.

A Short Course On Baluns

A Communicator Reprise: April 2012

Before getting into this, it is necessary to describe a somewhat puzzling phenomenon (called "skin effect") associated with radio frequency currents.

SKIN EFFECT

High frequency AC currents in a conductor flow only in that portion of the conductor which is very close to the surface.  Very close being 0.1 mm (the "skin depth") or less.  While a wire has only one surface, the shield on a coax cable, being a cylinder, has two surfaces - an inner surface and an outer surface.  Consequently, you can have a high frequency AC current flowing on the inner surface and an entirely different one flowing on the outer surface, with neither of them affecting each other. It's like there's an insulator keeping them separated.
Weird but true.

BALUNS

Unless precautions are taken to minimize it, all coax fed antennas will have significant current flowing on the outside of the coax shield while transmitting.
If the antenna is a horizontal dipole with the coax hanging straight down then the coax acts as a vertical antenna.  This is because this  current flowing on the outside of the shield will radiate energy exactly as a vertical antenna would.  
For casual ham HF operation, this can be a good thing as radiation will take place over a wider range of elevation angles which in turn makes it possible to make contacts over a larger range of distances.  However, for a communications antenna system designed to have a specific radiation pattern, this unwanted radiation from the coax changes the pattern and reduces the effectiveness of the system.
When receiving, the coax still acts as a vertical antenna and couples whatever it picks up into the actual antenna and from there into the receiver.
An example of these effects on a Near Vertical Incidence Skywave (NVIS) antenna system follows:

TRANSMITTING

For NVIS the desired radiation elevation angles (the range in angles with respect to the horizon over which the antenna radiates most of the transmitted energy) is about 60 to 90 degrees, 90 degrees being straight up.  A horizontal dipole 1/4 wavelength or less above ground will provide this.

If this antenna is fed with coax and no precautions are taken to minimize current flowing on the outside of the shield, the shield will act as a vertical antenna with a range of radiation elevation angles between about 5 to 40 degrees.  The amount of power radiated at these low angles can be 50% of the power radiated at the desired elevation angles.  This means that less power is available to be transmitted at the desired high angles.  In addition, this power radiated at low angles can cause interference to other stations thousands of miles away.

RECEIVING

This is where the coax shield acting as an antenna really sucks.
The shield, acting as a vertical antenna, will pick up those previously mentioned stations several thousand miles away and feed them into the dipole which will then happily send those signals to the receiver where they will interfere with the signals you want to hear.  
In addition, most noise on the lower HF bands is caused by lightning in the tropics.  If your antenna responds only to high angle signals (NVIS) it won't respond to these lightning induced static crashes because they arrive at low angles.  Consequently, you won't hear them.  However, if the coax shield is acting as an antenna you will hear them and they may make it impossible for you to copy the signals you want to hear.
Because the coax comes right into your station, it runs close to all kinds of noise generating equipment such as computer monitors, plasma TVs, switch mode power supplies, etc.  Because the coax is acting as an antenna, it will pick up all kinds of garbage that the dipole won't hear because it's much further away from these noise sources.

BALUNS TO THE RESCUE

Well, not just any old balun. "Balun" is a contraction of the phrase "balanced to unbalanced". An ideal horizontal dipole is said to be "balanced" with respect to ground.  What this means is that:
The voltage between ground and the point where one wire of the feedline is connected to the antenna is exactly equal (but of opposite polarity) to the voltage between ground and the point where the other wire of the feedline is connected to the antenna.
The current flowing out of one wire of the feedline into the antenna is exactly equal to the current flowing into the other wire of the feedline out of the antenna.

This is an ideal which is never achieved in practice.  Things like one end of the antenna being lower than the other, closer to a tree or building, etc., will all cause the currents in the two legs to be different.  Consequently, no antenna is truly balanced.  Some come close, though.
Feedlines such as the old TV twinlead, window line and ladder ("open wire") line are all considered to be balanced as they are symmetrical with respect to ground (as long as one side of the feedline isn't closer than the other to ground, metallic objects, etc).  
Our transmitter outputs are "unbalanced" with respect to ground.  This is because the outer shell of the output coax connector is connected directly to ground through the metal case of the radio. Similarly, the coax cable we connect to the transmitter is unbalanced as the shield is connected directly to ground via the coax connector on the radio.
So now we have a balanced antenna to which we want to connect an unbalanced coaxial cable. Physically, this is easy to do, we just connect them.  But, as soon as we do, we destroy the balance of the antenna and we get current flowing on the outside of the coax shield with all the resulting bad effects.

So, what we need is a box between the antenna and the coax to make the transition between the balanced antenna and the unbalanced coax feed line, and, yes, these boxes exist and they are called baluns.

There are two types of baluns:

Voltage Baluns

These force the voltage between ground and the point where one wire of the feedline is connected to the antenna to be exactly equal (but of opposite polarity) to the voltage between ground and the point where the other wire of the feedline is connected to the antenna.  The idea here is that, if the voltages are equal, the currents will be too.

However, as most supposedly balanced antennas are at least somewhat unbalanced, the currents in the two legs will not be the same.  This causes a current equal to the difference between them to flow on the outside of the coax shield.  i.e. the coax shield now acts as an antenna.  Just what we DON'T WANT.

Current Baluns (also known as "Current Chokes")

These force the currents in the two legs (halves) of the dipole to be equal.  As there is now no difference in these current values, no current flows on the outside of the coax shield.  This is just what we want, so these are the ones to use.

SUMMARY

Every coax fed antenna system designed to have specific characteristics needs a current balun located at the transition between balanced and unbalanced.  These are often built into VHF and UHF antennas (skirts, sleeves, etc.) but are generally separate items in HF systems.

Without a current balun the antenna is strongly coupled to the outside of the coax shield. 

This means that energy transmitted by the antenna will be coupled into the coax shield which will reradiate it and thereby affect the radiation pattern of the antenna system.

It also means that signals picked up by the coax shield acting as an antenna will be coupled into the antenna and passed to the receiver.

It is important to note that, like any other conductor, the coax shield will pick up signals and re-radiate them.  The current balun simply ensures that these re-radiated signals don't get coupled into the antenna and so don't reach the receiver.

APPENDIX

SKIN EFFECT DETAILS

If you pass a DC current down a wire, the current flows uniformly down the wire.  i.e. the amount of current flowing down the centre of the wire is the same as the amount of current flowing near the outer edges of the wire.

This is not true for AC currents.  For a current with a frequency of 1 MHz in a copper wire, the current will be concentrated in a region with a depth of less than about 0.1 mm from the perimeter of the wire.  This 0.1 mm is called the "skin depth".  The skin depth gets smaller the higher the frequency.  It doesn't matter if the wire is 1 cm or even 1 metre in diameter, only this 0.1 mm thick "skin" has significant current flowing in it.

See http://en.wikipedia.org/wiki/Skin_depth but skip the eye-glazing math and check out the chart at the end.

As mentioned earlier, the shield of a coax cable isn't a wire with one surface - it is a cylinder with two surfaces, an inner and an outer.  This means that, provided its thickness is more than a few skin depths at the frequency of interest, you can have a current flowing on the inside of the shield and an entirely different current flowing on the outside!  This is because neither current penetrates far enough into the shield material to affect the other current.  It's just as if there were an insulator keeping them separated.

The point to take away from all this is that, counter-intuitive as it may be, you can have two distinct AC currents flowing in a coax shield, one on the inner surface of the shield and one on the outer.

~Jim Smith VE7FO

Using Multi-Meters

A Communicator Reprise: March 2012

Using Multimeters—Part 2

Last month we looked at multi-meter types. Multi-meters have the ability to measure voltage, current and resistance and more expensive models may add other functions such as temperature. A basic addition to your household tool kit, there are low cost multi-meters available, frequently on sale for less than $10. I’d suggest purchasing a digital rather than analog model as a digital meter is easier to use and will suffice for basic measurements.

Digital models are generally "auto-ranging", a useful feature because you don't need to change the dial to measure different levels. If you think you might be using it in low light, consider getting one with a "back-light."
The first rule for getting the most out of your multi-meter is to read the manual. The manual will have instructions for basic operation of the instrument and safety information about potential dangers. Once you have read the manual, added the batteries, and attached the probes (the wires, which are usually red and black), try some of the example measurements below.

Basic Multi-meter Tests

Resistance Test

Set the multi-meter to read "resistance." Check that the two probes are inserted in the right holes.

What does the readout say when the probes are not touching anything? When the two probes are separated, there is an infinite resistance separating them, since air does not conduct electricity. Make a mental note of your multi-meter's readout for infinite resistance, because it varies with the manufacturer.
Touch the two probes together. Now what does the readout say? When you touch the two probes together, the resistance is close to zero. The metal tips are excellent conductors and the wires offer little resistance to current flow.

Try this…  I first did this as a science fair project. Set the knob to the highest Resistance scale on the meter Dampen two fingers and press one probe to each fingertip. Do you get a reading? With dry fingers you probably won’t get a reading. Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.
Try it with different liquids including salt water. Did the resistance change? What you are seeing is a Polygraph (lie detector) in its simplest form. As the subject is stressed from telling an untruth, the body produces perspiration which changes the skin’s resistance [scientifically known as Galvanic Skin Response]. Also measured in a professional instrument are blood pressure, pulse and respiration. Once calibrated, a polygraph and trained operator can record and interpret the readings to determine when the subject is truthful or not. For more experimentation check Google or have a look here for a basic kit.

Measure the resistance of some resistors that are not attached to a circuit. For example, test resistors of 100 Ω (ohms), 10,000 Ω, and 1 MΩ (mega-ohm, or 1 million ohms). You can buy these online or at a local supplier. Touch the probes to the wires on either side of the central cylinder. Watch the units: a "k" means kilo-ohms (thousands of ohms), and an "M" means mega-ohms. 
Never measure resistance in a circuit when power is applied. You must also discharge capacitors in a circuit before measuring resistance, because if there is any source of current other than the multi-meter itself, you will get erroneous readings.

Voltage Test

Touch the probes to the terminal ends of a 9-Volt battery [see photo]. You should get a reading of approximately 9 Volts. This one reads 7.57 volts so is obviously spent. The battery has a positive [red] and a negative [black] pole. Note that your multi-meter also has a positive and a negative probe. If you attach the positive probe to the negative side of the battery, it will still read voltage, but it will have a negative sign in front of it.

Current Test

This is probably the trickiest reading to make and one that can damage the meter should you pass too much current through it. Set the multimeter to read "direct current (DC)." Important: Check your multimeter to see where the probe should be plugged in so it reads "current."

Don’t be shy to experiment with your multi-meter.
You will likely find a multitude of uses for it around the house.

In order to measure current, you have to open up the circuit and attach the leads from the multimeter so that the current flows through the multimeter in series. To do this, use jumper wires and wires with alligator clips to add the multimeter to the circuit, as shown.
The current now flows from the 12 volt source through the switch, resistor and the multi-meter because it is part of the circuit.  Because it is connected in series, the meter can be inserted at any point in the circuit shown and show the same reading.

Fall Basic Licensing Course

Morse Lives in Hollywood

Near Hollywood and Vine

Located in the center of Hollywood, near the intersection of Hollywood and Vine, stands a very recognizable landmark, a circular office building, the home of Capitol Records, 
It was built in 1956 and has the distinction of being the world’s first circular office building. The shape was chosen as it was more earthquake resistant and more efficient for heating and cooling.

Known as the ‘Sound Capitol of the World’, the building includes a sound-proofed chamber located 20 feet below ground designed by famed guitarist Les Paul. The first recordings made in the building were a series of instrumentals by Frank Sinatra titled “Frank Sinatra Conducts Tone Poems of Color”. Since then artists such as The Beatles, Beach Boys, Tina Turner, Steve Miller, Bonnie Raitt and others have recorded there.

Of interest to hams is the spire atop the building. It has a red light at the peak to comply with federal airline safety regulations. It blinks the word ‘Hollywood’ in Morse Code. In fact, the light’s initial activation on April 6,1956 was ceremoniously conducted by Leila Morse, granddaughter of Samuel Morse.

Only once in the building’s 50 year history was the message changed.  This occurred in 1992 when Capitol records celebrated 50 years in business. For the occasion the red light blinked ‘Capitol 50’ in Morse.

And while you're in that neighbourhood...

Disneyland - New Orleans Square: The telegraph office attached to the New Orleans Square Railroad Station replays a part of Walt Disney's Disneyland Opening Day Dedication in Morse code.

"TO ALL WHO COME TO DISNEYLAND, WELCOME. HERE AGE RELIVES FOND MEMORIES OF THE PAST, AND HERE YOUTH MAY SAVOR THE CHALLENGE AND PROMISE OF THE FUTURE."

Amateur radio operator George Eldridge helped restore the message in 1997. After taping the message, Eldridge discovered that it had been cut short. The telegram was improperly edited when Imagineers transferred the message from a continuously looping tape, to a digital recording. Thanks to Eldridge, the recording now plays the correct message.

Multi-meters

A Communicator Reprise: February 2012

I’m sometimes surprised when a fellow ham asks a troubleshooting question and they have no knowledge of simple voltage, current or resistance measurement. When instructing the Basic course I used to spend a fair bit of time on series and parallel circuits and the means to make basic measurements, and there are several questions in the question bank that test these skills. I use my multi-meter several times a week, to check for a short, open circuit or even whether a dry cell battery requires replacement. This month we’ll look at the meters themselves… next month the basics of how to use them.
Multi-meters or multi-testers, also known as a VOM (Volt Ohm Meter) is an electronic measuring instrument that combines several measurement functions in one unit. They are inexpensive and very handy tools for measuring what is going on in a circuit and will offer Voltage, Current and Resistance ranges adequate for home use. Most new multi-meters are digital. Until recently, digital multi-meters were expensive, and some lab quality instruments still are, as much as $5,000. For as little as $10 you can purchase one on-line or on sale at Canadian Tire. The average home user can get by with a basic model. 

History

The first moving-pointer current-detecting device was the galvanometer in 1820. These were used to measure resistance and voltage by using a resistor bridge, and comparing the unknown quantity to a reference voltage or resistance. While useful in the lab, the devices were very slow and impractical in the field. These galvanometers were bulky and delicate. By adding a series or shunt resistor, more than one range of voltage or current could be measured with one movement.

Multi-meters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. The invention of the first multi-meter is attributed to British Post Office engineer, Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunications circuits. Macadie invented an instrument which could measure amperes, volts and ohms, so the multi-functional meter was then  named Avometer. The meter comprised a moving coil meter, voltage and precision resistors, and switches and sockets to select the range.
Any meter will load the circuit under test to some extent. For example, a microammeter with full-scale current of 50 microamps, the highest sensitivity commonly available, must draw at least 50 microamps from the circuit under test to deflect fully. This may load a high-impedance circuit so much as to affect the circuit, and thereby give a false low reading.

To eliminate loading, Vacuum Tube Voltmeters (VTVM) were used for voltage measurements in electronic circuits. The VTVM had a fixed input impedance of typically 1 megohm or more, usually through use of a vacuum tube input circuit, and thus did not significantly load the circuit being tested. Modern digital meters and some modern analog meters use electronic input circuitry to achieve high-input impedance—their voltage ranges are functionally equivalent to VTVMs. Before the introduction of digital electronic high-impedance analog transistor and field effect transistor (FETs), vacuum tubes were commonly used. 

How Does It Work?

An un-amplified analog multi-meter combines a meter movement, range resistors and switches. For an analog meter movement, DC voltage is measured with an internal series resistor connected between the meter movement and the circuit under test. If no resistors were used, the excessive voltage or current would quickly burn out the small wires that make up the meter coil. A set of switches allows greater resistance to be inserted for higher voltage ranges. As an example, a meter movement that required 1 milliamp for full scale deflection, with an internal resistance of 500 ohms, would, on a 10-volt range of the multi-meter, require 9,500 ohms of series resistance. Why? Remember Ohms Law, R = E / I or 10 volts divided by .001 amp which equals 10,000 ohms. The meter has an internal resistance of 500 ohms so we must add series resistance of 9,500 ohms to obtain a full scale reading. Now any voltage between 0 and 10 volts will produce some proportional deflection of the meter and this value can be read from the scale.  

For analog current ranges, low-resistance shunts are connected in parallel with the meter movement to divert most of the current around the coil. Again for the case of a hypothetical 1 mA, 500 ohm movement on a 1 Ampere range, the shunt resistance would be just over 0.5 ohms.

Moving coil instruments respond only to the average value of the current through them. To measure alternating current, a rectifier diode is inserted in the circuit so that the average value of current is non-zero. 

To measure resistance, a small dry cell within the instrument passes a current through the device under test and the meter coil. Since the current available depends on the state of charge of the dry cell, an analog multi-meter usually has an adjustment for the ohms scale to zero it, to compensate for the varying voltage of the meter battery. In the usual circuit found in analog multi-meters, the meter deflection is inversely proportional to the resistance; so full-scale is 0 ohms, and high resistance corresponds to smaller deflections. The ohms scale is compressed, so resolution is better at lower resistance values. Inexpensive analog meters may have only a single resistance scale, seriously restricting the range of precise measurements. 

Resolution of analog multi-meters is limited by the width of the scale pointer, parallax, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display. Accuracy of readings obtained is also often compromised by miscounting division markings, errors in mental arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual and adequate for the limited precision needed for most measurements.

Analog meter movements are inherently much more fragile physically and electrically than digital meters. Many analog meters have been instantly broken by connecting to the wrong point in a circuit, or while on the wrong range, or by dropping onto the floor.
On the favourable side, Analog meters are able to display a changing reading in real time, whereas digital meters present such data in a manner that's either hard to follow or more often incomprehensible. Also a digital display can follow changes far more slowly than an analog movement, so often fails to show what's going on clearly. 
Analog meters are also useful in situations where its necessary to pay attention to something other than the meter, and the swing of the pointer can be seen without looking at it. This can happen when accessing awkward locations, or when working on cramped live circuitry.

Analog displays are also used to very roughly read currents well above the maximum rated current of the meter. For this, the probes are just touched to the circuit momentarily, and how fast the pointer speeds towards full-scale deflection is noted. This is often done when testing state of charge of dry batteries.
The ARRL handbook also says that analog multimeters, with no electronic circuitry, are less susceptible to radio frequency interference, important if working on radio gear.

Digital Meters

The first digital multi-meter was manufactured in 1955 by Non Linear Systems. Modern multi-meters are often digital due to their accuracy, durability and extra features. In a digital multi-meter the signal under test is converted to a voltage and an amplifier with electronically controlled gain preconditions the signal. A digital multi-meter displays the quantity measured as a number, which eliminates mechanical errors. Measurement enhancements available include:
Auto-ranging, which selects the correct range for the quantity under test so that the most significant digits are shown. For example, a four-digit multi-meter would automatically select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters may include a facility to 'freeze' the meter to a particular range, because a measurement that causes frequent range changes is distracting to the user. Other factors being equal, an auto-ranging meter will have more circuitry than an equivalent, non-auto-ranging meter, and so will be more costly, but will be more convenient to use. An other reason to 'freeze' the range is that this somewhat avoids 'hunting' which is a situation where the meter continuously switches between two neighbouring ranges as when the instrument is in the low range, the value is too large but too small in the larger range.
Auto-polarity for direct-current readings, shows if the applied voltage is positive (agrees with meter lead labels) or negative (opposite polarity to meter leads).
Sample and hold, which will latch the most recent reading for examination after the instrument is removed from the circuit under test.

Current-limited tests for voltage drop across semiconductor junctions. While not a replacement for a transistor tester, this facilitates testing diodes and a variety of transistor types.

As you can see, not all meters are created equally and the choice depends upon your needs. For general home use however, a $10 digital multi-meter will accomplish most tasks with the least possibility of damage to the circuit or the meter.

Community Involvement

Can we do more to preserve our hobby?

Amateur Radio has spectrum, a lot of it. We have frequencies from the low bands into the gigaHertz. The demands for commercial use are very great... and persistent. Frequency auctions often generate millions of dollars for government. 

I can't help but wonder if the current low use of some of our bands, particularly on VHF and 70cm will yield to pressure from commercial interests and be reduced to a smaller range of frequencies. There have been a number of articles written that wonder about the future of the hobby and our ability to draw in young Amateurs.

I believe that every Amateur has a responsibility to 'give back' to the community for the privilege of access to our spectrum. I'm also a strong believer in exposing our hobby to the public - particularly the work Amateurs do in emergency response. Public demonstrations, science fairs, library and school visits have been well attended and receive favourable response locally.   'Spreading the word" will serve us well to gain public support when commercial interests come knocking.

~ John VE7TI