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Well, I finally have had time to sit down and put together part three of the DIY Magnetic Loop Antenna, sorry it has taken so long!

This post will cover building and coupling the loop to your transceiver. After reading through posts one and two you should have a good idea of the parts you’ll use and the physical dimensions of the main loop.

DIY Magnetic Loop Antenna – Part 1
DIY Magnetic Loop Antenna – Part 2

Most magnetic loops have the capacitor at the top of the main loop and the gamma match or matching loop at the bottom, this arrangement avoids running the feed-line through the center of the antenna.

You can assemble the main loop from continuous copper tube or from eight straight sections and 45 degree joiners. Make sure you have a blow torch or propane torch to solder the joints as you’ll need more heat than a soldering iron can supply. Whichever way you decide to build the main loop make sure that all joints are soldered or clamped as securely as possible, you want the lowest resistance possible to avoid your output power turning into heat. Other materials can be used for the main loop such as aluminium or low loss coax but copper pipe is easy to work, has low resistivity and available from just about every hardware store.

To construct the frame of the antenna you can use PVC pipe. It is a cheap and relatively sturdy building material and is available in a range of thicknesses, just about any hardware store will stock a wide selection of fittings. It insulates well and can be glued once you are sure your project is in its final form.

Once the main loop is constructed you’ll need to connect your capacitor to the two ends of the pipe at the top of the loop. Depending on the capacitor you may want to solder tags to the ends of the loop so they will be easier to attach. Copper pipe is a great conductor of heat and takes a lot to heat up and solder while it is not advisable to apply the same amount of heat to your capacitor.
It is also a good idea to attach the capacitor to a solid support so that the connections are not under strain.
The main loop and the capacitor forms the resonant circuit of the magnetic loop antenna.

To couple the main loop to your transceiver and match the expected 50 Ohms impedance you can use one of two methods. Probably the easiest is to use is a loop of insulated wire 1/5 the circumference of the main loop. The smaller loop is placed at the bottom of the main loop and can be shifted around to provide the best match. If you have an antenna analyzer you’ll be able to set it to the desired frequency, tune the variable capacitor for resonance and then move the small matching loop around till you have achieved close to 1:1 SWR. If you don’t have an antenna analyzer you can tune the capacitor for the greatest received noise and then on low power tweak the capacitor and move the coupling loop around for best SWR. Do NOT touch the loop while it is transmitting, use a wood or plastic rod to make adjustments as there are high voltages and intense RF fields near the loop.
An alternative to the coupling loop is the gamma match. The shield of the coax feed cable is connected to the base of the main loop while the inner conductor is connected to a point approximately 1/5 of the circumference around the loop. Its a good idea to use stiff wire (large gauge) for the gamma match as it can be critical of the position and orientation and once you have it in the right position you won’t want to move it again.

It would be preferable to have the ability to remotely tune the loop. A motor with a reduction gear could be used to move the variable capacitor but because the point of resonance is very narrow there should be a way of slowing the motor down. A simple control circuit using variable pulse width modulation could be used to slow the motor down while still retaining enough torque to move the capacitor. Whatever method is used to move the capacitor it should be well insulated from the other components of the antenna. Several thousand volts are generated on the MLA and care should be taken to ensure they don’t find their way onto control leads and back into the shack. Control leads should also be wrapped around toriod inductors as they leave the near field of the antenna to reduce the possibility of RF travelling along them.

With a SWR bridge and microcontroller you could build a fully automatic tuner that swept through the range of the tuning capacitor when the SWR rose above a defined limit indicating that the transmit frequency had changed.

With a little creativity and knowledge you could have an impressive MLA the equal of multi-thousand dollar military style units.

Hopefully this has given you some ideas for constructing your own loop antenna. Regardless of if you go top-of-the-line and buy a vacuum variable or build for economy and QRP you’ll have a compact, useful and unique antenna.

How many of you ham radio operators are also classical musicians? (I’m using “classical” loosely here, the way NPR does — not merely referring to the classical period per se, but broadly referring to all music of higher artistic expression.) I’ve always been intrigued by this kind of simultaneous development of artistic ability and scientific/technical ability. I’ve long heard that the two complement one another nicely, e.g. I’ve heard that musicians make better programmers.

Rehearsal of Exultate Festival Choir and Orchestra (Kristi Brackett, Photographer)

Right now I’m working hard with the Exultate Festival Choir and Orchestra to get ready for an upcoming performance of The Messiah by George Frideric Handel. Ham Radio has taken a back seat in my life during this seven-week project (as well it should, if we have our priorities straight). Yesterday’s rehearsal in the Twin Cities was exhilarating, and once again proved that the 2 1/2 hour drive to get there is definitely worth it. Dr. Tom Rossin is an outstanding conductor, and the choir is so good I have to pinch myself sometimes to see if my place there in the bass section isn’t just a dream. (If any of you happen to be in the Twin Cities on the weekend of March 9-11 and would like to hear The Messiah in its entirety, send me an email and I’ll email you a coupon that will get you two tickets for the price of one.)

Music was part of my life as a boy before I became a ham, but it didn’t blossom until 13 years ago at the age of 31. That was when my brother Tom (NØBSY) got me involved in a cappella shape-note singing from The Sacred Harp. This taught me how to sing parts; without it I could never have gotten into choral singing the way I have. My first choral work was with Exultate, singing Bach’s Mass in B-Minor, followed by Brahms’ German Requiem (in Rutter’s English translation). Since then I’ve been involved in a small choir here in the church, too. All of this has been a huge surprise to me. Up until I was 31 years old I was afraid to sing in front of other people, and I couldn’t sing parts if my life depended on it! So if any of you think you can’t sing, think twice — you might be surprised at what has been lying dormant in those vocal chords of yours, just waiting for the proper nudge to burst forth into beautiful song.

I see Tyler Pattison, N7TFP, is not only an accomplished ham radio operator but an accomplished musician. Along with his excellent tutorials for ham radio operators, Tyler has also posted a video of his performance of Charles-Marie Widor’s Toccata in F from Symphony No. 5. In this video you can see the organ from Tyler’s perspective, not only as a musician but as an electrical engineer. He is bringing both sides of his brain to bear upon the magnificent task of rebuilding and upgrading this organ — and then making beautiful music on it.

How many others like Tyler and myself are out there? If you are a musician and an amateur radio operator, what do you think? Has one influenced the other in your life? How?

Anyone looking at my blog could be forgiven for thinking that I had dropped off the face of the earth for a while, has nothing been happening in my world?

Well, the answer is that a LOT has been happening and all at once. My daytime job has become busier and there have been several non-radio projects at home that needed to be completed. All this has kept me away from Amateur Radio blogging even though I have spent more time on the computer than usual.

In between projects I did manage to stumble across this video of Rear Admiral Grace Hopper explaining just how “long” a nano-second is and what it looks like. This has relevance to radio as we’re usually well aware of frequency and wavelength but don’t usually spend too long thinking about speed.

My family sure was nice to me on my birthday this past Wednesday! One of the gifts I received was something I’ve wanted, but I couldn’t bring myself to cough up the cash for it: the 2012 edition of The ARRL Handbook for Radio Communications, graciously given to me by my daughter. Since the most recent copy on my shelves was several decades old, I was glad to get my hands on it.

I can see why this is a required textbook for the classes my friend Scott has been taking. He is a Signalman with BNSF Railway and periodically travels to Kansas City for training (no pun intended). The ARRL Handbook is a great resource for more than just ham radio operators. It is well-written and comprehensive — so comprehensive that I felt like an ignoramus as I paged through it on my birthday! I may have my Amateur Extra Class, but now I see more clearly than ever how little I really know.

But don’t let that scare you away from this book, Technicians. There’s still plenty in this book that you will find accessible, and anyhow we’re never going to learn if we don’t push ourselves.

One thing that isn’t covered in this book is the topic of operating procedures. But I can see why this topic has been relegated to a book of its own. The ARRL Handbook is quite a hefty tome as it is.

As with most of the ARRL library, you’ll find this book cheaper over at Amazon.

The time has come when I can’t put off learning Morse code any longer, With an interest in vintage amateur radio and the impending restoration of a Heathkit AT-1 I’m going to need to use CW sooner or later.

So I have been checking out resources for learning Morse code and stumbled across two that really intrigue me.

The first is LCWO.net, a web browser based Morse code learning tool that is usable on any internet connected computer. It is available free of charge and there is no software to install. LCWO.net keeps track of where you are in your lessons and where you need to concentrate your effort. The Koch method is the primary tool available but they also offer code group practice, callsign and plain text training modes along with a service to convert text to Morse MP3s for download and use offline.

Once you are on the way to CW proficiency and want to communicate with others you can always fire up a rig and get on the air … What if you don’t have a rig or need a confidence boost before ‘going live’?

Well, you could always send Morsemail using the Morsemail client from http://brasspounder.com:8873/.

Morsemail is, “A simple text format that encodes mark and space times to make it possible to send Morse coded messages via email” but a recently added feature allows for QSOs using a internet repeater hosted on brasspounder.com. You can use a mouse or actual key wired to the mouse or joystick buttons to send CW which can be emailed or sent through the repeater live.

Now I just have to carve out the time to sit down and use these resources!

Part 1 of the DIY Magnetic Loop Antenna covered mostly theory and materials so now its time to move on to designing the magnetic loop antenna (MLA).

If you have priced a commercially made MLA you’ll see prices start at $400 and keep going up, and up. If they cost so much you would think they must be difficult to build or use expensive parts, right? Well, it is certainly possible to spend more and get a higher quality MLA but a low cost MLA will still work very well.

For the purposes of this article we’ll assume that you want to build a loop to cover the 20-10M bands. I’ll run through the calculations required to build the MLA.

The required information for the MLA calculator is:

1. Length of the loop
2. The conductor diameter
3. Frequency/s of operation
4. Input power to the antenna

1. We don’t really know the best length of the loop at the moment so I’ll pick 9 feet circumference as a starting point (It’ll still fit in the trunk of my car)
2. Since we seem to be having better luck with sunspots now I’d like to try 10M so we’ll start with 29 Mhz as the highest frequency we’ll use.
3. I have some copper pipe left over from an ice-maker install, it is 1/4 (0.25) inch in diameter.
4. Input power to the loop will be 100W.

A peak voltage of 5181V will require a minimum spacing of 1.7 mm (peak voltage / breakdown voltage per mm) between the closest conductors in the capacitor. That would rule out an old air spaced variable capacitor from a vacuum tube radio but you could still use a wide spaced variable capacitor from an antenna matching unit or transmitter. A vacuum variable capacitor would be great (watch the minimum capacitance) or a home-made capacitor would also be fine provided you checked the breakdown voltage of the insulating material.

Do you live in a neighborhood with a restrictive antenna policy and despair of having a useful HF antenna?

Can you solder or know someone who can?

A magnetic loop antenna may be the answer and they are not as difficult to build as you might think. Like getting on the air for the first time or taking your license exam there is a certain amount of uncertainty when you first approach magnetic loop antennas, there are a few new ideas to grasp. However, thanks to other hams like Steve AA5TB there are tried and tested designs, calculators & building methods that are known to work and that you can follow.

At the heart of every radio and MLA (Magnetic Loop Antenna) is the resonant circuit. The combination of an inductor (a wire has inductance, but a coil of wire has more) and a capacitor (two conductors separated by an insulator) in a circuit will resonate or ‘ring’ at a certain frequency. Sound vibrations at a certain frequency can cause a piano string to vibrate in sympathy and a vibration of the correct radio frequency will cause a resonant circuit to electrically vibrate in sympathy.
Since there is no such thing as a free lunch, the sacrifice you make with a MLA is that it needs to be re-tuned whenever you change frequency on your transceiver. The frequency range over which it is resonant is very small, typically only a few hundred kilohertz at the most.

The materials you can get your hands on is going to decide the capabilities of your MLA. Ideally you’ll have a loop made from a conductor with very low resistance (usually copper) and a capacitor that can handle high voltages. A variable capacitor is required if you want to use your antenna on multiple frequencies but you can use or make a fixed capacitor if you operate on one frequency, for Eg PSK31.

A MLA calculator like the Excel spreadsheet from Steve AA5TB or this web page from 66pacific.com will help you to decide what size components you’ll need to make your antenna.

The four pieces of information required are:

1. What frequency or frequencies do you wish to transmit on?
2. How large do you want the loop to be (It should have a circumference less than 10% of the design frequency wavelength, both calculators help you figure this out)
3. The diameter of your conductor (Three quarter inch (0.75 inch) copper pipe is a good start)
4. How much power you want to use (The voltage across the capacitor is proportional to the input power to the MLA)

A MLA of a certain circumference will be more or less efficient based on the frequency you transmit at. It is worth changing the loop size in the calculator to get the best efficiency possible in your favorite band.