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Magnetic Loop Antenna – Next Project

magnetic loop

My next project is to build a magnetic loop antenna.

These antennas are very small, typically a loop of copper around one meter in diameter. Also called a small transmitting loop (STL), these are typically only a fraction of a wavelength in size. Typically, an STL circumference is five times smaller than the length of the popular half-wave dipole. They work almost as well as full size antennas, with the added benefit of performance at ground level, versus the typically 50’ or greater heights of regular antennas.

A magnetic loop antenna is basically a coil and a capacitor, which is tuned to a specific frequency. Radio signals are called electro-magnetic waves. Every radio signal contains an electronic and magnetic component. Most antennas couple with the electric field. The STL or magnetic loop antenna gets its name from its ability to couple magnetically.

There are two challenges to building a magnetic loop antenna. The first is efficiency. This requires low losses, particularly across the electro-mechanical connections in the loop. The second is voltages. Because of the physics, very high voltage will exist across the capacitor, say 3-5 kilovolts. This makes the design of the capacitor critical to ensure low losses and handle high voltages. These capacitors are normally quite expensive. One of the reasons I built a CNC machine is so that I can make my own capacitor from aluminum sheet.

The main impediment to using STL is the requirement to tune the antenna each time you change frequency. This antenna is narrow-band. In my project, I will use a stepper motor and Arduino microcontroller to tune the capacity remotely. If possible, I will also link this antenna to my home WIFI network so that I can control the antenna wirelessly. We wills see if that works when I get a bit further along.

Magnetic Loop Antennas and RFID

Every time you go shopping, you actually see these antennas in action. They are usually positioned around the entrance and exit doors to most large stores. Many RFID systems operate at 13.56 MHz, which is a shortwave frequency very close to the 20 meter ham band.

These antennas near the retail store doors emit a low power signal. If your packages contain an RFID chip that has not been deactivated, they respond to this signal and an alarm sounds.

Look at those vertical panels by the doors at a store or library. They contain a hidden magnetic loop antenna, just like the one I am going to build for ham radio. (RFID systems use low, high and ultra-high frequencies. Many systems are moving to UHF so the antennas will get smaller.)

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