Extra Class question of the day: resonance

Yesterday, the National Committee of Volunteer Examiner Coordinators (NCVEC) question pool committee released the latest version of the Extra Class question pool, and I’m pleased to announce that I will be writing an Extra Class study guide. I’m planning on having it ready to release it at the Dayton Hamvention.

In conjunction with that, I’m also planning to do an “Extra Class Question of the Day” here on my blog. In reality, the “question of the day” will cover more than one question, as so many of them are related. As in my study guides, the correct answer will be highlighted, and the question will be marked with the pool question number.

Today’s question of the day is about resonance. Resonance is one of the coolest things in electronics. Resonant circuits are actually what makes radio, as we know it, possible.

What is resonance? Well, a circuit is said to be resonant when the inductive reactance and capacitive reactance are equal to one another. That is to say, when

2πfL = 1/2πfC

where L is the inductance in henries and C is the capacitance in farads.

For a given L and a given C, this happens at only one frequency:

f = 1/2π√(LC)

This frequency is called the resonant frequency. Resonance in an electrical circuit is the frequency at which the capacitive reactance equals the inductive reactance.(E5A02)

Let’s calculate a few resonant frequencies, using questions from the Extra question pool as examples:

The resonant frequency of a series RLC circuit if R is 22 ohms, L is 50 microhenrys and C is 40 picofarads is 3.56 MHz. (E5A14)

f = 1/2π√(LC) = 1/6.28x√(50×10-6 x 40×10-12) = 1/2.8×10-7 = 3.56 MHz

Notice that it really doesn’t matter what the value of the resistance is. The resonant frequency would be the same is R = 220 ? or 2.2 M?.

The resonant frequency of a series RLC circuit if R is 56 ohms, L is 40 microhenrys and C is 200 picofarads is 1.78 MHz. (E5A15)

f = 1/2π√(LC) = 1/6.28x√(40×10-6 x 200×10-12) = 1/5.6×10-7 = 1.78 MHz

The resonant frequency of a parallel RLC circuit if R is 33 ohms, L is 50 microhenrys and C is 10 picofarads is 7.12 MHz. (E5A16)

f = 1/2π√(LC) = 1/6.28x√(50×10-6 x 10×10-12) = 1/1.4×10-7 = 7.12 MHz

The resonant frequency of a parallel RLC circuit if R is 47 ohms, L is 25 microhenrys and C is 10 picofarads is 10.1 MHz. (E5A17)

f = 1/2π√(LC) = 1/6.28x√(25×10-6 x 10×10-12) = 1/9.9×10-7 = 10.1 MHz

Is it easier now to be a hacker / experimenter / DIYer?

In a recent blog post, EE Times editor Bill Schweber notes the passing of Norman Edmund, the founder of Edmund Scientific, and speculates on whether or not it’s easier now to be an experimenter/hacker/DIYer than it was years ago.

Those who say it’s not point out that years ago we had magazines, such as Popular Electronics and Electronics Illustrated, companies like Heathkit. They also point out that it was possible to disassemble TVs and radios for the parts and use them for your own projects.

Schweber, however, thinks that it is easier today for hackers and experimenters. He writes that  those magazines may be out of business, but we now have access to “countless user groups, informal forums, and blogs” on the Internet.

One thing he failed to mention was the hacker/maker groups that have sprouted up around the country. Here in Ann Arbor, for example, we have a group called Go Tech that provides support for hackers and makers of all stripes. You’ll find groups like this all around the U.S.

I tend to agree with Schweber that while the environment has certainly changed for experimenters, it is definitely better. What do you think?

Digital QST coming soon

In his last missive to ARRL members in the Great Lakes Division, Jim Weaver, K8JE, reports:

ONE TOPIC MANY members will appreciate is a free, digital version of QST via the Internet.  This version offers the ability to read our membership journal online in a manner quite similar to that in which we read the hard-copy version.  It also offers the ability to include full schematics and other information that sometimes can merely be referenced in the print edition, as well as direct links to items contained in the ads.

Becoming accustomed to the online version may require a little familiarization, but I believe it will become a welcome enhancement to membership.

It is inevitable that two questions regarding digi-QST will arise.  The first is if a member can opt-out of receiving the hard-copy version.  The answer:  yes.  The second question is:  Is there a reduction in dues for members who opt-out of the print version.  The answer to this is no.  The hard-copy version is financed largely by advertisement, not by membership dues.

Digi-QST should be unveiled yet this winter or early spring.

I think this is great, but it sounds to me like we’ll have to read the magazine on-line rather than being able to download a PDF file.  I’ve never really liked these online magazines. Reading them is a real pain, and you often can’t download and archive them. Let’s hope the ARRL thinks about this when they set this up.

Rig on the fritz? Crack it open and have a look.

There’s no doubt that today’s rigs are pretty complicated. Even so, there’s no reason not to crack them open should they quit working. Often, the repair is something simple. Two recent experiences back me up on this.

New fuse makes Heathkit happy
A couple of weeks ago, a ham friend expressed an interest in one of the Heathkit IP-2717 HV Power Supplies that I rescued from the University of Michigan’s Property Disposition center a couple of years ago. I purchased three of them at the time, one working, two not working. I told him that I’d let him have one of the non-working units for $25, exactly what I paid for them.

He agreed to the price, and we agreed to meet for lunch about a week later and make the transaction. A couple of days beforehand, though, I decided to pull one off the shelf and just have a look inside.

Well, as soon as I popped the top off, the problem was obvious. It had a blown fuse. I just happened to have that value and when I stuck a new one in, the power supply fired right up and worked like a charm. My friend not only gave me a few more bucks for fixing it, but also bought lunch!

A two-minute fix for 2m rig
This evening, as I was working a guy in ME on 30m CW, I spotted on my shelf, an old 2m FM rig that someone had given to me. It had quit working on him, and since he wanted to get something newer, he didn’t feel like fooling around with it. He told me that if I could fix it, that I could sell it or give it to someone who attended one  of my classes and was looking for a starter radio.

That radio has sat on the shelf for at least two years. When I caught sight of it this evening, though, I was inspired to open it up and see if there was anything obviously wrong with it that I could fix easily.

Sure enough, all I had to do was remove the two screws holding the top of the case to the radio and peer inside. The problem was readily apparent. Somehow, one of the connectors connecting a cable to the main board had worked itself loose. I plugged it back in, hooked it up to my power supply and 2m antenna, and worked N1RIT on the club repeater.

The point of these stories is that even though a piece of equipment might be quite complicated, often what goes wrong is something simple to fix. In the case of the power supply, all I had to do was change a fuse. All the 2m rig needed was for me to reconnect a disconnected cable.

So, next time one of your radios goes on the fritz, don’t just send it off to a repair center. Crack open the case and have a look inside. It may be a simple fix, and if it is, you’ll not only save yourself a lot of money, you’ll have the satisfaction of having fixed it yourself.

ARRL News – January 25, 2012

We’ve criticized the ARRL website here on KB6NU.Com from time to time, but there’s a lot of good stuff there, too. In particular, they regularly post timely news items. Here are three items I found particularly interesting:

  • FCC denies Anchorage VEC’s waiver request. In July 2011, the Anchorage VEC asked the FCC to permit individuals who have previously held an Amateur Radio license grant issued by the Commission — but which has expired and is beyond the two year grace period for renewal — to receive credit for elements previously passed, and thus a new license grant, without additional examination.
    I’m not altogether sure why this VEC would request this waiver. In fact, I’ve suggested that maybe it would be a good idea to regularly re-test ham radio operators.
  • Minutes of ARRL Board’s winter meeting posted. The ARRL board held their annual winter meeting the weekend of January 13-14, 2012. The  minutes from that meeting have now been posted. These are always interesting and worth perusing.
    One item of interest is that the board has approved electronic voting for division elections. I wonder how that would have affected, if at all, the two elections that I participated in.
  • LOTW to support CQ awards. The ARRL and CQ magazine have agreed that QSLs recorded in Logbook of the World can now be used to apply for CQ-sponsored operating awards. These are the first non-ARRL awards to be supported by LoTW. The first CQ award that you will be able to apply for via LOTW is the WPX award.

What’s your favorite test instrument?

Simpson Model 60 VOM

The venerable Simpson Model 60 VOM has been in production for more than 60 years.

In the Test&Measurement World group on LinkedIn, editor Martin Rowe asks, “What’s your favorite test instrument”? He gets some interesting answers, but the two top vote-getters are the Bird Model 43 Wattmeter and the Simpson Electric Model 60 VOM (see right).

About the Bird wattmeter, one engineer said, “That’s easy – The Bird Model 43 Wattmeter. No other instrument has been in production, in its original design, for so long. Since 1952, which makes next year 2012 its 60th anniversary. Also, it must be the simplest, most rugged instrument ever produced. We regularly see Model 43 Wattmeters returned to us for calibration that are 30-40 years old, still working and, when they leave our facility, as accurate as the day they first came of the production line in Solon, Ohio.”

About the Simpson VOM, another replied, “My favorite test instrument of all time is the Simpson 260. I used it back in the 1970s while serving in the US Navy. It is rugged, easy to use, and does the job!”

Those are really two classics, and either is a great choice. But, what do  you think? What’s your favorite test instrument?

Last weekend a busy one for KB6NU

KB6NU teaching the Jan. 14, 2012 One-Day Tech Class

Me making a point (apparently about SWR) at Saturday's One-Day Tech Class

Last weekend was pretty busy for me, ham radio-wise. It started bright and early Saturday morning with the latest One-Day Tech Class. There were twelve students in the class, and all twelve passed!

This class was a bit odd in that most of them signed up during the week before the class. So few had signed up by Saturday, January 7th, that I was even thinking that I might cancel the class.

On Sunday, January 8, I sent out a reminder to my mailing list, and after that, the class quickly filled up. Many of them were engineering students from the University of Michigan, who are part of a project that sends up weather balloons. They use amateur radio to track the balloons and to find the payloads once they’ve returned to Earth.

Hamfest not so festive
Sunday morning, I was up even earlier to attend the Hazel Park ARC hamfest. Since I was selling some junk, errrrr good stuff, I wanted to get there by 8 am. So, I was up and out of the house by 6:45 am. I needn’t have rushed, though.

Attendance was way down, and while I did sell a little over $100 worth of books and other stuff, I was hoping that I’d sell more. In particular, I thought I’d sell a few of my new “Hams Obey Ohm’s Law” stickers. I didn’t sell a single one, however. One friend of mine offered to purchase one, but I just gave him one.

I think clubs have to think twice about when they schedule these things. Holding them so early on Sunday mornings almost guarantee that only the old farts will show up for these things. And the old farts are a dwindling audience.

No propagation, no glory
Sunday night, I tried to participate in the monthly Run for the Bacon (RTFB). This low-pressure QRP contest, run by the Flying Pigs is usually a fun event. Sunday night, however, the band had gone way long by 9 pm EST, when the contest started. I managed one contact with a station in Idaho, but no one else could hear me.

To be honest, I don’t really know how much power I was running. Instead of hooking up the KX-1, I just cranked down the power on the IC-746PRO until the power out meter was showing just a single bar. I really gotta get that wattmeter kit finished.

TAPR Conference Videos Online

From the ARRL Letter, 1/12/12:

Amateur Radio Video News (ARVN) has released high-definition videos of all the talks presented at the 2011 ARRL/TAPR Digital Communications Conference (DCC), held September 16-18. The programs are now available online on the ARVN website.

The DCC is a three-day conference on Amateur Radio digital technology. Among the video presentations are 18 seminars, the Saturday Banquet and the welcome introduction by TAPR Chairman Steve Bible, N7HPR. According to ARVN Producer Gary Pearce, KN4AQ, most of the talks are fairly technical, although there are four separate sessions that cover “Intro to” topics. “All of the talks — except the ‘Intro to’ talks — were shot in three-camera high-definition, with wireless mics for ‘close-up’ audio of the presenter, as well as the question-and-answer period,” he explained. “The ‘Intro to’ talks were shot with a single, standard-definition camera.

This year, Pearce decided to release the programs on the web instead of the DVDs that have been produced in previous years (although the DVDs will be available later). “I wanted to make the programs available more quickly and easily to a worldwide audience,” he said. “The web has become an easy, high-quality distribution medium.”


International Museums Weekend, June 16-17 and 23-24, 2012

Since I’m the station manager for WA2HOM, I’m always interested in ham radio events that take place at museums. I am, therefore, excited to note that the operating event, International Museums Weekend, will take place on both June 16-17, 2012 and June 23-24, 2012.

This has been primarily a European operating event, but I’m hoping that we can also get something going here in the U.S.  I know that I plan to participate in this event at WA2HOM.

Here’s how the IMW’s website describes the event:

The intention of the event is to set up amateur radio special event stations at as many of the museums as possible throughout the whole of the world. I would hope for an HF, VHF, and if at all possible, a Ui-View (APRS) packet station to be set up at each museum site, but the scope of your station is entirely up to you. The choice of museum is also left very much up to you, however aim for the largest and/or most unusual you can find.

The museums taking part over the years have included ships, castles, air museums, Napoleonic forts, pumping stations, wireless museums, racing museums and many others. For the purposes of the event, the word ‘museum’ is loosely interpreted. There really is no shortage of venues in which such an event can be staged, no matter where in the world you might live.

Related video
Just coincidentally, I received in an e-mail a link to a video that  documents two QSOs from a portable station to two museum ships.

Next VOIP Conference, Friday, May 12, 2012 in Reno, NV

From Kent, W7AOR:

The next VOIP Topical Conference is Friday, May 4, 2012 at the Grand Sierra Resort in Reno.

Since 2001 Nevada Amateur Radio Repeaters, Inc. (NARRI) sponsored the Annual VoIP IRLP Conference that is held in the Spring. Prior to 2012 the conference was held in Las Vegas as a separate event. Now it is part of EMCOMMWEST event. Each year the meeting has broadened its scope and now includes all the major VoIP systems in use by the amateur radio community, i.e., IRLP, Echo link, EchoIRLP, All Star, D-Star and DV Dongle. Report on success of the Pacific Division D-Star Emcom Net held on Western Reflector REF014B. See the program for topical speakers.

Here is a summary of details:

  •  Place: Grand Sierra Resort Conference Area
  • The conference area is in the lower level – take elevator down from main floor.
  • Date: Friday May 4, 2012
  • Time: 9 am to Noon and 1 to 4:30 pm
  • Program: elaborates on the various VoIP systems with presentations and demonstrations. See past VoIP conference session at http://www.narri.org/voip_conference.html
  • Venue: 10th Annual Topical VoIP Conference
  • Cost: $25 per person. No one allowed without payment, no exceptions.
  • RSVP: Pre-Registration is Required by April 16, 2012.
  • Registration: Not available at this time. Will begin on February 1st.
  • Hotel Rooms: www.EmcommWest.org/accommodations.html

Please direct your questions to Kent W7AOR,w7aor@narri.org or phone 702-452-4412.

It is the hopes of NV Section leadership that this combo will alternate yearly between Reno and Las Vegas. Look forward to seeing you in Reno for the 10th annual VoIP Conference and EmcommWest.