For the past five years I have experimented with magnetic loop antennas. I thought it was time to publish a magnetic loop compendium of my work, complete with links to all articles.
My experience with design, construction and use of magnetic loop antennas really started in the fall of 2014. I was searching for the sources of RFI in my neighborhood. To support these efforts, I built a quick and dirty tuned loop covering 2-20 MHz, shown above as #1. It contained switchable frequency range and resistive attenuation. You can see this DF loop in detail in my video Adventures with Radio Frequency Interference, at the 13 minute mark.
Next up in the magnetic loop compendium was my 40-10 meter small transmitting loop, shown above as #2. This was a five foot diameter copper tube, with a home made butterfly capacitor. I motorized the tuning with a stepper and plastic gears. My local WIFI enabled remote tuning.
Built in the spring of 2015, my STL was really an internet-of-things project leveraging my home made 3D printer for gears and mounting and CNC machine to mill the capacitor plates.
My final tuned loop, shown as #3, covered medium wave and lower short wave bands. This was a multi-turn loop made of magnet wire mounted on a 2′ square frame made from dowels and plastic. You will see innovations in this summer 2016 loop included wired remote control, azimuth and elevation rotation using servo motors. Also, I used relays to switch tuning range and a digital-to-analog converter to provide tuning voltage for the varicaps.
All of these projects produced working products and were a lot of fun to build. But then I decided to take the leap into active wideband receiving loops, which work on very different principles.
Magnetic Loop Compendium – Going Wideband
Last year, I bought an Afedri dual channel coherent receiver. I thought it might be fun to build some spaced active loops to experiment with spatial filtering, including noise reduction and diversity reception.
Over the past six months, I have written several series of articles describing:
- Wideband magnetic loop theory, including a working LTSpice model you can download and use. I tried to describe the theory with only most basic math.
- Simulations to compare the performance of various published loop antenna amp designs.
- An original design of a wideband loop amplifier using a high speed op amp transimpedance amplifier. Unfortunately, I could not get enough stability out of my design.
- My own PCB design for the proven LZ1AQ amplifier. If you wish, you can order the PCB from OSH Park and build a surface mount version of this amplifier for a few dollars. Details are in the articles.
At present, I have a Mostly Plastic Wideband Receiving Loop up and running (see picture #4), using the LZ1AQ amplifier and constructed of aluminum core PEX. Just starting out is a DIY receiving loop rotator which should be finished soon.
If you are interested, take the time to enjoy these articles in my Magnetic Loop Compendium.