Friday, February 17, 2012

DIY Magnetic Loop Antenna - Part 1

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 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. 
For example, my 30-10 Meter loop is 8.5 ft in circumference and 23% efficient at 10.1Mhz but 91% efficient at 28Mhz. This represents a difference of about 6dB or 1 S-Unit.

The diameter of the conductor determines its resistance and this becomes important due to the large current flowing through the loop. Large diameter copper pipe is better since the radio frequency current flows primarily on the outside of the conductor rather than the core. Typical 3/4 inch plumbing pipe is a good balance between low resistance and weight, Heliax coax is good for portable antennas where the loop has to be rolled up into a compact space.

Probably the most problematic part of the MLA is the tuning capacitor. While everything else can be found in your junk box or on a shelf at the local hardware store the capacitor takes a little more digging. Ideally it should be variable from a minimum to a fairly high capacitance (0-300 pF would be good) it should also be able to withstand high voltages. If you are planning to run 100W then you could expect at least 4000 volts across the capacitor. This is not as bad as it sounds as it takes 3300 volts to jump just 1mm or .04 of an inch.

Ideally you could purchase a vacuum variable capacitor, the Rolls Royce of high voltage variable capacitors. You would expect to pay anywhere from $100 up when buying a 10 - 500 pF unit on Ebay and a LOT more if you buy new. If that is the way you want to go then great, you're all set. If not then read on ...

There are several alternatives to the vacuum variable capacitor that will work almost as well. The primary concern now becomes resistance and plate separation. For QRP power levels you can use the tuning capacitors from old vacuum tube radios, the type that have a set of fixed metal plates and a set of moving plates that mesh into them.
For higher power there are two other types of capacitor that are suitable. 
One is the split-stator capacitor, so called because each terminal of the capacitor is connected to a stator that is electrically isolated from the other and the frame. The rotors and shaft form the rest of the circuit so there is very low resistive loss and no sliding contacts.
The other type is called a butterfly capacitor due to the shape of the rotor plates. In this type the stator plates are placed opposite each other with the butterfly rotor in between. When each of the butterfly "wings" are fully meshed between the stator plates the capacitor is providing maximum capacitance, when rotated 90 degrees the wings are completely un-meshed and the capacitor is providing minimum capacitance. Because of the construction of butterfly capacitor there is a higher minimum capacitance that should be noted when planning the frequencies your loop will cover.
If all else fails then you can construct your own variable capacitor. The idea is to have two conductors separated by an insulator, the larger the area of the conductors and the better the insulator the higher the capacitance. Sliding metal plates, trombones of copper tube and even Coke cans have been used as variable capacitors.

A fixed capacitor can be created out of coaxial cable if you intend to operate on one frequency and a table of approximate capacitances for different types of coax is included here, click to enlarge the table below.

You can cut a length of coax a bit longer than the length suggested by the table above and trim it to frequency once it is attached to the loop. Make sure there are no stray pieces of braid between the shield and the center conductor as the voltage rating is determined by the spacing of the two closest conductors.

Part two of this post will continue with calculating the antenna dimensions and performance.

One important thing to remember ... If you don't have access to the best materials then use what you have and improvise. If you don't have copper pipe then use coax or heavy wire. If you don't have a high end variable capacitor then use what you can find and keep the power levels QRP. The only antenna that is a complete failure is the antenna you never get around to building.