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I performed an experiment to try to improve the Q of the close-wound loading coil wound with Litz wire. I used a twisted pair of Litz wires connected together at the ends. The idea was to reduce the proximity effect by reducing the magnetic coupling between adjacent turns. Unfortunately, there would still be a proximity effect between the two wires of the twisted pair itself. I did not know what would happen, because I have not seen this configuration described before. I am not able to calculate a Q because Butterworth does not have formulas for a winding made up of a twisted pair of Litz wires. More recently, formulas have been developed for calculating the Q of “bundled” insulated solid wires, which are several insulated wires twisted together. Bundled wires are more uniform than Litz wires, and, therefore, different formulas apply. I have not been able to find formulas related to bundled Litz wires.
For the twisted pair of Litz wire coil, the-low frequency inductance is 273 uH. The diameter of the 49 turn winding on a styrofoam cylinder is 4.5 inches and the length is 4.12 inches. The inductance at 1610 kHz is 325 uH, and the Q is 436. This is slightly higher than the Q of 382 obtained with the Litz wire coil without the twisted pair winding. The effective series resistance of the coil is 7.6 ohms.
The measured self-capacitance is 5.7 pF. The length/diameter ratio is .916. The self-capacitance obtained with the Medhurst method is 5.1 pF. As I already mentioned, the Butterworth method cannot be used to calculate the Q of this type of coil.
Although the Q improvement is only slight, it might be worth using a twisted pair of Litz wire, because the highest obtainable Q for the loading coil should be used.