Mag Loop Overview |
Magnetic Loop Overview:
A magnetic loop is simply a small loop with a series capacitor between it's two ends. The loop inductance and variable capacitor form a resonant circuit. The value of the variable capacitor is chosen so the resulting resonant frequency is at the operating frequency. Loop size and capacitance value determine the efficiency of the antenna so are important considerations. The 50 ohm source can be coupled to the loop in several different ways. I chose a small loop with a circumference about 1/5th of the circumference of my main loop which is approximately ten feet. I chose this coupling method as it seemed simpler than any of the others I considered. Here are the current physical parameters of my loop. This website page chronicles my adventure that led up to the latest one. It is in reverse date order and is kind of like a lab notebook.
A magnetic loop is simply a small loop with a series capacitor between it's two ends. The loop inductance and variable capacitor form a resonant circuit. The value of the variable capacitor is chosen so the resulting resonant frequency is at the operating frequency. Loop size and capacitance value determine the efficiency of the antenna so are important considerations. The 50 ohm source can be coupled to the loop in several different ways. I chose a small loop with a circumference about 1/5th of the circumference of my main loop which is approximately ten feet. I chose this coupling method as it seemed simpler than any of the others I considered. Here are the current physical parameters of my loop. This website page chronicles my adventure that led up to the latest one. It is in reverse date order and is kind of like a lab notebook.
- Main loop is made with 1/2" copper tubing and is 10' circumference and about 40" in diameter.
- Driven loop is made with 1/4" copper tubing and is 24" circumference and about 8" in diameter. It is as close as possible to the main loop but not physically touching the main loop.
- The main loop is mounted to the plastic electrical box using SO239 and PL259 coax connectors soldered to the ends of the 1/2 inch copper pipe. They provide physical mounting only. Electrical connection is provided by links as described next.
- The variable capacitor is a two section broadcast type capacitor. One end of the main loop connects to the stater of one section and the other end of the mail loop connected to the stater of the other section. The connections between the capacitor sections and the main loop are made using three "strands" with 12 gauge electrical wire links which are soldered to both the capacitor and the copper tubing of the main loop. The rotor is floating, not connected electrically to anything. It serves to "connect" the two stater sections in series. The idea is that this avoids inherent losses in the connections to the rotor thus providing higher Q. The resulting capacitance range is about 5 to 200 pf.
- The feedline is a 15' section of RG-58 coax.
- The antenna provides a close to 1:1 SWR from the 40 meter band to the 10 meter band.
According to the loop calculator at 66pacific.com, its efficiency on
- 20m: Antenna efficiency: 50% (-3.0 dB below 100%).
- 40m: Antenna efficiency: 8% (-10.9 dB below 100%)