First, the Q value of this spherical tuning coil was measured with a Q meter. The capacitor of the Q meter is fixed at 55 pf, and the measurement result is: Maximum inductance: 550 KHz Q = 68 The capacitor of the Q meter is fixed at 80 pf, and the measurement result is: Maximum inductance: 478 KHz Q = 77 It can be seen that the Q value of this coil is not high, especially when the inner and outer coils are reversed and the inductance cancels each other, the Q value is lower. However, it can be used for ore machine tuning without variable capacitors, and the circuit is simpler. The following is the circuit diagram. The distributed capacitance of the antenna (generally dozens to more than one hundred pf) and the coil are connected in parallel to form an inductance tuning circuit, and even a fixed capacitor can be omitted. Results of preliminary audition: Connected to the heaven and earth wires of my usual test machine, the output wire of the high-frequency signal generator was wound around the antenna a few times to form a small cross-connect capacitor to input the high-frequency amplitude modulation signal. S1 is connected to the first gear (that is, the antenna is connected to the coil through C1). Check the resonance frequency of the variable inductance coil. When C1 is about 70 pf, the resonance frequency range is 530-2200 KHz, which can cover the entire mid-band. But for several stations with different frequencies, adjust S1 and S2 to the proper position to get a louder sound. The antenna is only suspended next to the ore machine without receiving the sky and earth wires, and can receive several radio stations.
HIGH QUALITY:Made of only the highest quality materials and built to last! Our chargers are gauranteed to match the original brand name chargers' quality.
EASY TO USE: Simply plug the charger into a wall outlet and then directly into your scooter.
The charger protected: Short circuit protection/ over current protection/ over voltage protection; red light -- charging, green light -- charge OK, it stop automatically when the battery is full. Warm Tips: To avoid the temperature of your charger too high, please do not charge too long.
Aviation Plug Scooter Charger,Power Scooter Charger,Li-Ion Battery Charger Shenzhen Waweis Technology Co., Ltd. , https://www.waweis.com
Intermediate inductance: 745 KHz Q = 60
Minimum inductance: 2770 KHz Q = 30
Intermediate inductance: 995 KHz Q = 47
Minimum inductance: 2300 KHz Q = 15 
C2: 250 pf fixed capacitor
D: detection diode
L1 – L4: Variable inductance coil, consisting of 4 hemispherical coils, with a maximum inductance of 1188 µH and a minimum inductance of 50 µH. Each loop is wrapped with 22-gauge yarn for 30 turns. (Outer ring diameter 4.0 inches, L1-outer left hemisphere, L2-outer right hemisphere. Inner ring diameter 3.9 inches, L3-inner left hemisphere, L4-inner right hemisphere)
S1: Single pole 10 throw small rotary switch (only 4 gears)
S2: Single pole 10 throw small rotary switch (only 3 gears)
Headphones: 4500Ω DCM headphones
The tuning coil is fixed on a 4.5-inch diameter high-stamped styrene drain connector. This material connector has a relatively low dielectric loss. The switches and terminals are distributed on both sides of the drain pipe connector. The semi-adjusting capacitor is installed at the back, and the capacitance can be adjusted with a screwdriver through the small hole to match the antenna. 
Only the ground wire, the sound is quite loud, and the selectivity is good.
Only connect the antenna, adjust S1 and S2, the selectivity is good, four or five stations can be clearly separated. The sound is louder.
Connect the sky and earth, adjust C1, S1 and S2, the selectivity is not ideal, the low end of the middle wave is acceptable. High-end radio stations are basically inaudible. The strong station is very loud.