The Ham’s Toolbox- Part 1: Digital Multimeters- The Hidden Difference


In this multi-part series, we’re going to discuss what tools and gadgets we think are essential for the ham’s toolbox.  For my part, I’ve been a licensed ham and an electrical engineer for enough decades to have seen my share of the excellent, the decent, and the truly ugly when it comes to tools and equipment.  One of the very foundational items every ham needs is a multimeter.

This article will discuss Digital Multimeters (DMMs).   All DMMs measure voltage, resistance, and usually current but some have additional features like measurements of capacitance, frequency, temperature, and other parameters.  You can buy a DMM for less than $10.  You can buy one for $400.  They both fit in one hand, have test leads, and display some numbers.  Sometimes the cheap meters have more features than the expensive ones.  So, what’s the difference?

Let’s ignore the features for the moment and look at basic survivability, both the meter’s and your own.  Most any meter can measure a flashlight battery or your 13.8 volt power supply.  Even a cheesy meter can be used for resistance measurements and general puttering around the test bench.  That’s usually easy and fairly safe.  Things get more serious when you’re measuring electrical utility power sources such as standard 120 volt wall outlets, 240 volt lines in breaker panels, or even the main feeders before the big breaker in your home panel.

Properly-rated DMMs have Category numbers which are displayed as Roman numerals, I through IV (1 through 4).  There is an excellent write-up on meter safety including the rating system on the Fluke website (http://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf).  In short, it’s all about the energy that’s available at a given point in an electrical system.

Consider these word pictures:

  1. Here you are, sitting in your parked car, and along comes a bicyclist traveling at 15 mph. He’s headed straight for your passenger door.  Wham!  He piles into your door and leaves a big dent.  He’s not looking too good, either.  But, all in all, it could have been worse.
  2. Now let’s make it worse. Let’s change the picture to have a small car aiming straight at your door, still going 15 mph.  This time you’re really paying attention.  The car whacks yours.  Everybody’s air bags detonate.  Your car is seriously damaged and you might be suffering from whiplash and you’re wondering which lawyer to call.
  3. Let’s really make it worse this time. Your car is stalled on a railroad track (!) and here comes a freight train with a pair of lumbering locomotives leading 50 loaded coal cars.  It, too, is running 15 mph.  Sadly, you and your car are totally obliterated so you won’t be calling anybody.

What changes in each case?  The speeds are the same.  The available impact energies are vastly different.  On the end of an extension cord you can get spikes up to 6kv or 200 amps during a short. At the mains it can be 10,000 volts or 10,000 amps!

DMM category numbers are based on where you are in the electrical system and the voltage you’re measuring.  A cheap meter, if it’s rated at all, might be rated Category I, 250 volts.  Category 1 means you’re hiding behind your main circuit breaker and your branch breaker and many feet of small wire so they can’t convey huge amounts of current.  If a high-energy line spike strikes at the wrong moment when measuring your AC line, your wall outlets will arc over at 6000 volts which will limit the damage to your meter.  In that position you’re probably OK when measuring up to 250 volts unless the meter is total junk.  If your meter arcs internally, it probably won’t explode because the electrical system is energy-limited.  It may smoke and make bad smells.  Then you’ll go buy a better meter.  On the other extreme is the full unprotected mains to your breaker panel.  It is fed from a huge transformer through fat wires.  If a line spike causes your meter to arc and short with that combination there is little to prevent a giant blast of energy, thousands of amps, from instantly incinerating your meter and possibly blowing up in your face.  If your cheap meter has crummy test leads, they may act like fuses and burn off first.  The mains are where Category IV meters are required.  They’re equipped with big energy-limiting fuses and other safeguards to limit the destruction.  Unless you’re an electrician you probably have no business measuring unprotected mains anyway.

What’s a ham to do?

  • Avoid cheap no-name meters from the far side of the world. If it costs less than a large pizza, leave it alone!  You can have cheap or you can have good.  Not both.
  • Look for a name-branded meter that is clearly marked as CAT III (or CAT IV) 600V and has been independently tested by a recognized test lab such as UL, ETL, TUV, SGS, CSA, or others on the NRTL list. ( https://www.osha.gov/dts/otpca/nrtl/nrtllist.html )  Anybody can print Category numbers on their meter but a recognized laboratory makes it mean something.
  • Another good “mark” is the CE mark. It designates that the meter has been tested to European standards and is legal to sell there IF it was tested in a real lab.  BUT, beware: anybody can apply a CE sticker to anything they like.  You can buy sheets of them for pennies in Hong Kong.
  • What are “name brands”?
    1. Fluke: If you can afford one, it will likely outlive you if you’re not a complete savage. It is the standard of industry and the one everybody tries to imitate, including flocks of dangerous fakes from Asia.  I’ve had a pair of them for 35 years.  They’ve been around the world and on factory floors during service calls.  I have a 10-digit high precision HP benchtop meter but the Flukes are still my “go-to” meters for most needs.
    2. For great value in a “rated” meter that is much less expensive than Fluke, try Amprobe, Digi-Sense (from Cole-Parmer), B&K Precision, or Greenlee. They’re rated and they’re lab certified.  There are probably others.  Just look for the rating and the NRTL lab.

That’s enough for this episode.  Get a good meter. You’ll be glad you did.

73,

Chuck- NA3CW

 

 

So What’s Happening Inside My Coax? Part 2


Please read Part 1 if you haven’t read it yet.  This article picks up where Part 1 left off.

A couple of principles:

  1. Light travels slower through solids than it does through vacuum or air. Sometimes much slower It’s how optical lenses work.  And remember, light and RF are the same thing: electromagnetic waves of different frequencies.  Different colors, if you like.
  1. A wavelength is the physical distance (feet, meters, centimeters, etc.) between one wave peak and the next in an electromagnetic wave. Frequency is the number of waves that occur during a unit of time.  The speed of light (RF) is what relates wavelength and frequency.

In Part 1 we talked about electrical things happening in coax such as voltage, current, resistance, and dielectric loss.  Now we’re going to talk about the “light” in your coax.  The RF that’s flowing through your coax isn’t just in, or on, the copper surfaces.  It’s a beam of light that’s shining up through the dielectric layer.  (If you’re into extra reading, do a google search on “TEM mode.”)

Our Principle 1 says that light travels slower through solids.  That includes your RF and it includes the polyethylene dielectric layer in your coax:

  • If your coax has normal non-foamed polyethylene dielectric, the speed of RF through the poly layer is about 66% of the speed of RF (light) in a vacuum or dry air.
  • If your coax, like RG8X, has “foam” poly with lots of air bubbles, the RF speed is about 82% of the speed of light.
  • If you get the really fancy coax (Heliax) that’s mostly filled with air, RF runs about 95% of light speed.
  • Also note that there is a relationship between speed of RF and efficiency (loss). Generally speaking, the lower the speed, the lower the efficiency.  (There is more material for the RF to push through.)

In other words, it takes more time and effort for RF to get through an RG8 cable than it does in Heliax, or in free air.  The percentage of light speed in coax is called “Velocity Factor” or VF.  Every kind of coax or open wire line comes with a VF rating.  Mind you, that rating is not exactly accurate for every inch of coax that’s manufactured but it’s close enough for most uses.  So now the big question everyone wants to know:  What uses?

In your ham career you may want to connect multiple antennas together with a “phasing line” or need to use a quarter wave coax impedance matching section or want to avoid an unhappy length of feedline on a given band.  For all of these things you need to know the “electrical length” of the coax to achieve your goal.

As an example, let’s say you need a quarter wave of RG8 for some purpose on 40 meters centered on 7.2 MHz.  Remember the formula for a halfwave dipole?  (468/ f MHz). It’s 468 because a dipole has capacitance which makes it shorter than a free space half wave.  A free space half wave is 492/f MHz.  (Memorize that.)  So let’s calculate: 492/ 7.2 Mhz= 68.33 feet.  But remember, that’s a half wave and we want a quarter wave so: 68.23/2= 34.16 feet.  Now let’s apply our RG8 VF: 34.16 x .66= 22.5 feet.  That’s an electrical quarter wave of RG8 on 7.2 MHz.   If you used RG8X with its foam dielectric, it would be 34.16 x .82 = 28.01 feet.  See?  The higher the VF, the longer the electrical quarter wave length.  True open wire line with few spacers has a VF of about 99% so a quarter wave of open wire line would be nearly the same as free space.

An observation: If “hollow” coax is better than coax with solid dielectrics, why do we use the “inefficient” stuff?  One good reason: the solid stuff is very bendable.  You can bend it or flex it around your antenna rotator and the solid dielectric will maintain the position of center conductor in the center of the cable.  Foam coax, especially types with stiff, non-stranded center conductors, should only be bent with gentle radii because under stress the center conductor will slowly migrate through the foam dielectric and eventually short out to the shield.  The “hollow” coax like Heliax is really a corrugated copper pipe with either foam or helical/air dielectric.  It doesn’t take much imagination to figure what would happen if you flexed it back and forth.  It’s only used for fixed installations, like the W3GMS repeater antennas.

Conclusions:

  1. All other things being equal, the less solid stuff that is present in a feed line, the more efficient it is and the faster RF travels through it.
  2. The Velocity Factor (VF) is different for different dielectrics and configurations. It’s always a published specification and, for extra credit, there are ways of directly measuring it for a given piece of cable.
  3. When you need a specific electrical length of feedline, you need to use VF to calculate it.

Remember the beginning of Part 1 when I said there was a lot going on inside your coax?  Trust me, there still more.  But we’ll leave your coax alone for now.  😉

73,

Chuck NA3CW

So What’s Happening Inside My Coax?


This article is rather long but it may be worth your time.  If you’re in a hurry, jump to Conclusions.

A coax cable is like a steel beam in a bridge: You don’t see anything moving but there is a lot going on inside.  (There is outside, too, but that’s another article.)

Coaxial cable or “coax” is a special kind of shielded cable that is optimized for use as an RF transmission line.  It has an inside wire (the center conductor) and metallic shield “tubing” (copper braid, foil, or even pipe.).  The insulation that separates the center conductor from the shield is called the dielectric. Dielectrics can range from solid or foamed polyethylene, to Teflon, to nothing at all but air except for occasional spacers.  The electrical characteristics of the cable are primarily established by the diameter of the center conductor, the thickness and type of the dielectric, and the inside diameter of the shield “tube.”  We’ll save the finer points of that for another chat, too.

Coax comes in several major flavors: flexible, semi-rigid, and rigid.  Most of us are using flexible coax which is handy because, well, it’s flexible, can be routed almost anywhere, it’s reasonably priced, and can be effective when used the right way.  It can also be a long skinny dummy load when used the wrong way and, yes, you can actually melt it.

All transmission lines, be they coax, twisted pairs, or open wire feeders, have a characteristic impedance in “ohms”.  It’s not their resistance.  It’s the ratio of voltage to current (volts per amp) that the cable tries to establish and at which it passes RF power with the least loss.  If you feed a signal into one end of the cable and connect a resistor equal in value to the characteristic impedance across the other end as a load (like a 50 ohm resistor for a 50 ohm line), the wave from the source will pass cleanly through the line to the load and will disappear as heat in the load.  If you replace the load resistor (the dummy load) with some other load with the same voltage to current ratio, like a resonant antenna or a well-adjusted tuner, you get the same effect.  The wave passes cleanly from source to load with minimum loss.  This is a “matched” load.

All coax lines have loss.  It turns your RF into heat.  Coax lines have resistive heat loss in their copper conductors just like any other wire. The more current, the more heat.  But unlike 60 Hz house wiring, coax at radio frequencies has dielectric loss, too.  The dielectric plastic heats up.  The more voltage, the more heat.  Keep this in mind for the next concept.

If you connect a load that is other than a matched load, not all of the signal will be absorbed by the load on the first pass.  Some of the wave will be reflected back toward the source.  The outbound wave (forward power) “collides” with the reflected wave coming back.  There will be a place, or places, in the line where the forward voltage and the reflected voltage add to each other.  In that zone will be a voltage that’s higher than the original signal.  Elsewhere in the line the current waves will add to each other and will create a zone, or zones, of higher current than the original signal.  The zones of higher voltage will have higher dielectric heating than you would get with a matched load.  The zones of higher current will have more resistive heating than you would get with a matched load.  The greater the mismatch (higher SWR), the greater the heating and the less transmitted and received signal you’ll have.  With a high SWR when running high power, the coax could actually melt or arc.  Or when passing tiny received signals, loss means they never make it to your receiver.

And it gets worse.  Because of a magnetic phenomenon called “skin effect”, the higher the frequency, the more the current crowds to the surface of the conductors.  It’s like cramming four lanes of traffic into one.  This increases effective resistance, hence more heat.  The dielectric also heats more at higher frequencies.  The RF voltage causes the molecules in the plastic to twist back and forth.  The higher the frequency, the more twisting and the more heat.  A cable that works fine on the low bands might be a long black dummy load at 2 meters depending how well matched it is.  In coax, and especially at high frequencies, bigger is better even when running a matched load.  Because of dielectric loss, UHF TV stations running real power use coax made of concentric pipes with nothing by dry nitrogen and a few ceramic spacers to keep the center conductor centered.  No plastic, no loss.  Big pipes, lots of surface, less loss.  When you’re running 300kw, it matters.

Conclusions:

  1. When using coax you want low SWR in the line. Having a tuner that makes your transmitter happy with a crummy load doesn’t change the SWR in the line.  The RF that turns into heat is gone.
  1. The higher you go in frequency, the bigger the coax needs to be. A hundred feet of RG58 might be fine on 75 meters but will waste a lot of your 2 meter signal, even more if the SWR is greater than 1.5:1 or so.
  1. If you plan to use a non-resonant antenna, or the same antenna on several HF bands (like I do), you’ll want to seriously consider using open wire line which has a small fraction of the loss of coax. If a feedline has zero loss, high SWR means nothing.  Prior to the flood of WWII surplus coax hitting the market, hams used open wire line.  They rarely if ever measured SWR even if they knew how or cared.  It just didn’t matter.  The loss of open wire line isn’t zero.  But, to put it in perspective, while coax is rated in db loss per hundred feet, with open wire it’s more like db loss per mile.  At one time I multi-banded an 80 meter dipole, fed with coax.  Pretty dull results.  I converted to open wire.  My receiver LIT UP.

Thanks for staying awake.  See you on the air!

73,

Chuck, NA3CW