Looking back, I have had a lot of fun with magnetic loop magic. Next up, I will try my hand at wideband active loops for diversity reception.
I have been fascinated by magnetic loop antennas since I was a kid. Back then, when I opened up an old BCB receiver, I discovered the ferrite loopstick. Lots of wire wrapped around a ferrite rod, and tuned with a capacitor. And, very directional for the AM stations nearby.
My next magnetic loop magic project will be two broadband loops for diversity reception. I will begin documenting this project shortly, together with some of the theory about how these work.
Magnetic loops are electrically small antennas which mainly interact with the magnetic portion of radio waves. Radio waves are comprised electromagnetic fields. From this name, you should be able to guess that these two fields – electric and magnetic – are intertwined and travel together through space.
Normal receiving antennas, like dipoles and verticals, work by collecting voltage from the electric part of EM waves. On the other hand, receiving loops work by collecting current from the magnetic component through induction.
Tuned loops are resonant single-frequency antennas which combine an inductance (the loop) and a capacitor. You will find they have a very narrow bandwidth, so you have to keep tuning the loop whenever you change frequency. This is a real pain for SDR.
On the other hand, un-tuned loops can be made broadband, e.g. covering all frequencies from 100 kHz to 30 MHz. That’s what we will be trying to achieve soon.
Magnetic Loop Magic – Looking Back
To date, I have built two magnetic loops, both tuned. The first was a five foot diameter circular copper loop for HF use, 7 to 30 MHz. It used a homebrew butterfly capacitor and worked just fine. As it was a small transmitting loop, I could use it for transmitting and receiving anywhere it its range.
My second project was a receive only tuned loop for medium wave broadcast and lower HF. I made this 2 foot square loop using multiple turns of magnet wire. Tuning was achieved electronically using a voltage-controlled varicap and a relay to vary the inductance. Also, I added servo motors to rotate the antenna.
Both of my loop projects so far also achieved remote control using NodeMCU over my home network.
Looking forwards, my next loop will be something completely different – broadband with no tuning. Spacing two wideband loops 100 feet apart should enable some great experiments with diversity reception and spatial filtering.