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I've had this suggested to match 50 ohm coax to a lower feedpoint impedance, like a vertical, especially mobile shortened verticals. What's it called?

schematic

simulate this circuit – Schematic created using CircuitLab

(CircuitLab has no symbol for an inductor with a tap, but you get the idea)

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This is commonly called a "Tapped Loading Coil", and combines the functions of loading coil and RESONANT transformer. Typically used with a shortened vertical antenna, the impedance of the coil is selected to cause the antenna to be resonant on the desired frequency. The position of the tap is then adjusted to match the impedance of the transmitter to the resonant antenna circuit.

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  • $\begingroup$ If it were like a transformer, wouldn't it serve to make the feedpoint impedance lower, which is the opposite of what's necessary with a shortened vertical? Also, searching "tapped loading coil" doesn't turn up much of anything useful -- can you provide any references? $\endgroup$ – Phil Frost - W8II Jun 9 '14 at 21:12
  • $\begingroup$ @Phil The best I've found is strobbe.eu/on7yd/136ant/#TappedCoil then search "Resonance transformation". The key is that you're coupling to a resonant circuit, so the impedance is NOT that of the antenna only. (I have used this for decades without understanding it!) $\endgroup$ – HarveyB Jun 10 '14 at 16:46

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