File:Large antenna loading coil.jpg

Large copper antenna loading coil used at the Tuckerton, New Jersey, USA transatlantic wireless telegraphy station built by the German Telefunken Co. in 1912. This station transmitted with a power of about 200 kW at 60 kHz with a Goldschmidt alternator, and communicated 4000 mi. to Eilvese, Germany, one of the most powerful radio stations in America at the time. It fed an umbrella antenna consisting of a vertical radiating tower 825 ft (250 m) high with radial wires extending from the top to 40 ft. poles in a circle at a distance of 1500 ft from the tower. These types of VLF antennas were very electrically short (.05 wavelength) and the radial wires served as a "capacity hat" to increase the efficiency of the antenna. The above loading coil was mounted at the antenna's base and served to tune out the high capacitance, making the antenna resonant so it could be fed energy efficiently.

Because of the low radiation resistance, the antenna and coil acted as a huge high Q tuned circuit and was very finicky to adjust. The coil had to have very low resistance at radio frequencies, and to reduce proximity effect losses it was made of very large size, of large diameter tubing to reduce skin effect losses. The antenna had a very narrow bandwidth and also had a variometer coil to adjust it to resonance. If the variometer was misadjusted, the antenna power would be reflected back into the rotating Goldschmidt generator, overheating it and causing the rotor to expand and grind to a halt against the stator.