My 40m Magnetic Loop Antenna

  1. Input Loop
  2. Some Calculations

Related Pages:
  1. /Antennas/Magnetic-Loops/Magnetic-Loops/Magnetic-Loops.html

My 40m MagLoop antenna has a 5 meter circumference is made from 25mm Gaspipe with a 20mm copper trombone capacitor. It was inspired by the Plastic Fantastic Magnetic Loop published by Jim Tregellas VK5JST,


Input Loop

Input-loop-v2-2.jpg Input-loop-v2-3.jpg


Some Calculations

I am using the Plastic Fantastic by Jim Tregellas VK5JST, as a source.

The article states the following:
The transmitting loop has an estimated effective circumference of 5200 which is very close to mine. Capacitance of the copper trombone is 624 pF per meter or 1.603mm per pF. To resonate at 7MHz the transmitting loop requires 117 pF in series The inherent capacitance of the transmitting loop is 14 pF The two sections of the trombone capacitor are separate capacitors and are in series.

So the calculations are:

To tune across the 300 kHz bandwidth of the 40 meter band requires 30 mm. So the calculations are: 300 kHz / 30 mm = 10 kHz per mm

Now my questions are:
  1. What is an acceptable increment in turning? If I can control the trombone to +- 1 mm is 10 kHz accurate enough to get a good tuning? This will depend on the Q-factor. If this is not accurate enough I could miss the tuned bandwidth all together. More analysis is required to resolve this.
  2. 2. If 1 mm is sufficient can I return to that position with accuracy so I can calibrate the antenna?
  3. Can I design a cost effective motorised control mechanism with 1 mm accuracy for my loop layout?

Glenn Lyons VK4PK
Ver:gnl20230228 - pre published v0.9