Tuned loops magnify voltages like magic. We need to understand how tuned loops work before moving on to un-tuned or wideband loop antennas.
In our last article, we described the magnetic loop equivalent circuit as a starting point for analysis and design of a wideband loop antenna. But while both narrow and wideband loop antennas apply the same equivalent circuit, their principles of operation are entirely different.
A magnetic loop antenna is electrically small. You will discover that this means a small area or circumference relative to wavelength. As described earlier, loop antennas react to the magnetic component of radio waves. Also, because of stray or distributed capacitance, all inductive loops are also self-resonant at some particular frequency. My one-meter diameter aluminum loop produces about 12μH of inductance and around 30pF of stray capacitance, with a self-resonant frequency of 8.4 MHz. I get these results from both actual measurements and SPICE simulation.
We can turn this base loop into a tuned loop by simply attaching a tuning capacitor CT across the output terminals as shown above.
Engineering magic means that tuned loops magnify voltages at the resonant frequency. If I adjust my tuning capacitor to 10 pF, the resonant frequency moves to 7.5 MHz. Turning the tuning capacitor to add 100 pF lowers the resonant frequency down to 4.0 MHz.
As you can see from the frequency responses above, tuned loops are very narrowband, with a big signal at a single frequency. Tuning will provide 30-50 dB of voltage gain on-channel relative to other frequencies. And this is over and above any directional loop effects. Now you can see why DX listeners on lower frequencies love loops!
Tuned Loops Magnify Voltages – Quality Factor or Q
Q or Quality Factor is the ratio of reactive power stored to real power dissipated in a tuned circuit. Voltage across the tuned circuit is multiplied by Q. Voltage magnification takes place at resonance. Thus a very small current can create a very large voltage.
But the bandwidth of the loop becomes quite narrow with a passband near 50 kHz or less. This means the antenna has to be tuned whenever you change frequency.
Here is the takeaway from all this. When you work with a small loop in Open Circuit Voltage Mode, the antenna must be frequency dependent. This is true because of either a tuning capacitor or stray capacitance. Furthermore, the antenna must be narrowband, with a relatively high Q.