Before I start experimenting, it was time to collect some prototype magloop baseline data. My antenna analyzer come in handy.
One of the first things I learned about wideband magnetic loops is that loop area and inductance are the key variables for signal pickup. According to LZ1AQ, the loop figure of merit is simply Area ÷ Inductance. Bigger, lower inductance loops generate more current. More on that later.
You want a large loop area (πr²) that keeps the loop circumference (πd) less than roughly one third of a wavelength. My one meter loop has a circumference of 3.14 meters which is one-third of a wavelength at 27 MHz. So, this size of a loop will perform as an inductive loop through to upper HF. At the same time, it will optimize signal current generated.
On the other hand, you want your wideband magnetic loop to have as low inductance as possible. Inductance generates reactance, which is a form of AC resistance which reduces signal current available to your amplifier. Adding multiple turns of wire, or using thin wire rather than a fat tube, increases inductance quickly. The formulas say my one meter loop should have an inductance of around 2.3 μH.
After constructing the prototype, I used my Rig Expert AA-30 antenna analyzer to measure loop inductance. My result was 2.9 μH, pretty close to the theory. The difference might be accounted for additional wire inductance in the conductor.
Next, I took detailed spot impedance measurements of the loop across the range of 100 kHz to 30 MHz. You can see the results in the above graphic. Generally, resistance was quite low, around 0.5 Ω, except for a 400 Ω peak around 9 MHz. On the reactance side of things, my loop demonstrated rising inductive reactance up to around 9 MHz and thereafter switched – rather dramatically – to capacitive reactance.
So, what’s the story? The answer is stray capacitance. Every inductor also creates some capacitance. At some point, the L and C combine to create a self resonant frequency. In my case, my one meter loop is self-resonant around 9 MHz.
Prototype Magloop Baseline Data – Now What?
My next step is to create a SPICE simulation model for a magnetic loop antenna. How will I know if the model is accurate? In part, the theory should guide the construction of the model. But equally, the model should produce results that match my actual loop. That’s what the prototype magloop baseline data is for.
In particular, the resistance and reactance demonstrated by the prototype should also be evident in the SPICE model. And that will be demonstrated and described shortly.
Spoiler alert! The SPICE model works. If you review my previous article on wideband versus narrowband loops, you will see the self-resonance at 9 Mhz when this loop is operated in open-circuit mode.