What is a magnetic loop antenna?


Franz Fiedler (1885-1956)
Child and Radio 1925

My name is Alexandre Grimberg, PY1AHD, and I was born in October, 1949, in Rio de Janeiro, Brazil.

I started in electronics when I was 10 years old, when I got hooked on crystal radios.

I received my first ham radio license when I was 22 years old

After a long period being QRT, I returned to ham radio in the year 2000, at a new QTH as an antenna restricted ham. This was not a problem at all, considering my passion for portable QRP operations.

One year later, I put my hands on a Yaesu FT-817, an incredible piece of ham technology—a portable jewel that I have dreamed of all my life.

My challenge was to create a truly portable and effective HF antenna as a companion for the FT-817 during my outdoor mini-expeditions.

A friend of mine, Flavio Pohlmann, PY1ZFP, who is living in Montreal , showed me a homebrew VHF magnetic loop with the diameter of a DVD and told me how impressed he was with this tiny antenna.

After Flavio explained the basic parameters of the magnetic loop antenna, two hours later I had scaled the antenna to cover 6-12 meters. I used a 45 cms diameter RG-213 coaxial cable loop connected to a standard 3 Pf to 30 Pf. variable capacitor.

The very day I put the antenna and the FT817 in my car to go to the shopping mall where I was working at that time.

As soon as I parked the car around 11:00 am, I connected this small loop to the FT-817 and answered a Chile station calling CQ on 10 meters and he gives me a 59 report!

I returned around 6:30pm to the parking lot to check the propagation, and worked a Texas station who gave me an excellent report.

From this moment I decided to invest all my effort and time in designing and field testing my own small magnetic loop antennas.

It was a great challenge because there was very little information back then about small magnetic loops available on the Internet.

Today there are calculation programs that make the life of a magnetic-loop home-brewer a breeze, but when I get started in 2001 / 2002 I had to spend a lot of time using the trial and error method--cutting cables a quarter of an inch at a time, and writing down the results.

On the top of this page, you will see dozens of examples on how to build your own small magnetic loop.

You can make your first small magnetic loop spending almost nothing.

The most important part is the variable capacitor. That capacitor can be a butterfly or a split-stator type, but if you do not have one you can get started with whatever you have in hand. Just use a piece of RG-213 coaxial cable for the loop, and have fun.

For those who are looking for a ready-to-use solution, I offer the AlexLoop Walkham.

The AlexLoop Walkham was the result of more than 12 years of hard work to develop a really portable HF antenna that can be used anytime and anywhere.

All the antennas are carefully handmade by myself, and are fully tested before shipment.

Notebook showing Small Magnetic Loop antenna results by cutting and testing method.

I will try to explain in few words how the small magnetic loop antenna works.

1) The magnetic loop antenna is an RF transformer. The primary is the small inner loop that is 1/5 of the size of the main loop. The secondary is the main loop, and is the actual antenna. This outer conductor is closed by a variable tuning capacitor. The two loops are isolated, with no electrical contact between the inner loop and the outer loop.

2) The magnetic loop antenna is a high “Q” device, so the bandwidth is very narrow. Even minor frequency changes will require you to retune the antenna, but in exchange the antenna provides superior rejection of unwanted signal reception—giving the best receive gain and the lowest SWR on transmit.

3) The magnetic loop antenna can be used for either vertical or horizontal polarization, and the small size makes it easy to alternate between the two.

4) In most radio activities we use the loop in the vertical position supported by a small pole. The antenna can be turned freely in both directions to minimize or even completely eliminate man made noise coming from other directions. This position provides a "figure-8" radiation pattern similar to larger dipole antennas.

For condo windows, hotel balconies, and other space-limited locations, we use the loop in the horizontal position, which (of course) gives vertical polarization and an omni directional radiating pattern.

5) The small magnetic loop works perfectly at low heights. A one meter diameter loop works perfectly one meter above the soil.

6) The small magnetic loop doesn’t need counterpoises, making it ideal for temporary operating locations, or for situations in which there is no room available for counterpoise wires.

Historical references in portable magnetic loop antennas

          Balkan’s War in 1942                     Alder-shot England, November 1937

AlexLoop Walkham Portable Small Magnetic Loop Antenna

The AlexLoop Walkham Small Magnetic Loop antenna can be quickly mounted and disassembled for easy carrying.

The field tests confirmed more than 800 QRP DXs and has even set a distance world record which is registered in the Hall of Fame as recorded by the HF Pack group: HF Pack Hall Of Fame

AlexLoop Walkham Small Magnetic Loop Antenna Features

  • Easily Tune in 5 seconds any frequency between 6.9 MHz and 30MHz
  • You simply need to adjust the built in tuner to peak the signal in receive mode and then fine tune in transmit mode for the lowest SWR
  • No taps on coils for band changing and no whip adjustments needed to fine tune.
  • All those features above with the incredible freedom of not requiring a counterpoise

Window Mounted AlexLoop Walkham Magnetic Loop Antenna

The Ultimate Solution for Hams with Antenna Restrictions

  • The unique low profile design offers a solution for antenna restricted hams and short wave listeners.
  • You can use the AlexLoop Walkham antenna supported on windows, balconies or even inside your radio shack.
  • A discrete antenna that can be used and finally removed from the window or balcony once radio communications have been completed
  • The perfect companion for the traveling ham.

AlexLoop Walkham Portable Operation

AlexLoop Walkham carrying bag