Having built the servo pan-tilt base and the plastic mounting jigs, it was time to wind the receiving magnetic loop.
A receiving magnetic loop is basically a tuned circuit comprised of a coil and a capacitor. By turning the variable capacitor, the loop system will become resonant on the radio frequency you want to listen to, and convert radio signals into a voltage for your receiver to amplify.
You design a receiving magnetic loop by selecting the capacitance and coil inductance to give you a resonant point on the frequencies of interest. The formula for resonance is really quite simple. The harder part is winding the coil of wire to provide the correct inductance. The formula for inductance is a bit more complicated and results will depend on:
- the size and shape of the loop, both effective diameter and total length of wire. For a receiving magnetic loop, both the circumference and length of multi-turn wire have to be quite small relative to the wavelengths to be received. The formula works differently according to shape – most are circular, square or octagonal.
- the diameter of the wire and the distance between windings. The size of the wire effects various things, including mutual inductance of the wires within the coil and stray capacitance. In my case, I am using #26 AWG salvaged magnet wire with a diameter of around half a millimeter.
- the relative permeability of the core upon which the loop is wound. Permeability the is ability of the core to to support the construction of a magnetic field within itself. Fortunately, this is an air core loop and the relative permeability of air is “1”. Other materials such as ferrites and iron have relative permeability orders of magnitude higher.
The range of frequencies I want to tune are from 0.5 MHz to 10.0 MHz. This is Medium Wave and lower Short Wave. The capacitance range of the voltage variable capacitors I have chosen is approximately 20 pF to 500 pF. Working backwards from the resonance formula, I calculated the coil inductance required for resonance. As I had suspected, I would need two different coils. A smaller coil for short wave, and a larger coil for medium wave.
Winding the Receiving Magnetic Loop
Based on the four foot lengths of mounting dowels, this loop will be square and each side of the loop will be 34”. The first coil will be two turns, to provide smaller inductance for the higher frequencies. The second coil will be seven turns to add more inductance for the lower frequencies.
I 3D printed a small bracket to hold M2 machine screws as mounting posts for the wire. Each dowel end has a 3D printed spacer bracket with notches to hold the thin wire in place. Spacing between the wires is 3 millimeters, which is around six times wire diameter. (If I was using larger gauge wire, I would need much more spacing.) The two coils of 2 and 7 turns respectively will be joined together to provide a 9 turn coil for the lower frequencies. This will be done with a remote controlled relay. A two turn pickup coil is used as a transformer to couple the receiving coils.
Fine grit sandpaper was used to remove the enamel insulation where the wires attached to the mounting posts. Otherwise, all of the wires in the coils remain insulated.
A quick check with an ohm meter proved that all coils remained isolated from each other at DC. A second check with my Rig Expert Antenna Analyzer and a variable capacitor proved that I could tune the loop to resonance.