A Switchable L-tuner that Matches Almost Everything

 

The L-Tuner is the simplest antenna tuner (transmatch) circuit known.  One capacitor, one coil.  Period.  Yet, with two components and appropriate switching, the L-tuner is capable of matching almost any antenna to any load, including the 50-ohm coaxial antenna output of modern HF transmitters and transceivers, at almost any HF frequency.

The trick to matching widely varying impedances to the transmitter is to employ switching that will reconfigure the relationship between the components.

An L-tuner can be configured as a low-pass tuner (coil in series with the load, shunt capacitor) or as a high-pass tuner (capacitor in series with the load, shunt coil). The circuit also can be set up with the shunt on the input (transmitter) or output (antenna) side of the circuit.

Under various combinations of antennas and frequencies, all four of these configurations may be needed. At my station, my HF antenna is a 40-meter horizontal loop, which comes close to resonance at 40, 20 and 15 meters. However, to be able to use use this (or most types of) antenna on all HF ham bands, you need a transmatch, or antenna tuner. On various bands, I employ all of the four possible configurations with this antenna. I won’t go into details of tuner construction, since numerous good articles are available in the QST archives and online. With respect to components, the rotary switches are four pole, five position Russian military surplus ceramic switches, available on eBay from a seller in Estonia; the variable capacitor is a two-section Cardwell 30-330 µFd each section unit, with both sections in parallel at S2 positions 2 and higher; the variable inductor is a an E.F. Johnson; the counter dial for the inductor is from Surplus Sales of Nebraska. Extra capacitance, needed for 80 meters with my antenna, is provided by home-brew fixed capacitors made from alternating layers of copper sheet and glass or acrylic plastic). Switch 1 selects the tuner configuration (Low Pass Input, Low Pass Output, High Pass Input, High Pass Output, and Bypass Tuner, in sequence for switch positions 1 to 5). SW2  selects the total capacitance available, with ranges (rounding) of 30-300, 60-600, 600 to 1200, and 1200-1800 µFd; a stop is set on the five-pole switch to make it a four-position switch.

An SWR meter could be added if needed (see ARRL Antenna Handbook), as well as switchable outputs to additional antenna jacks, an internal balun for a balanced antenna output, etc. This is the basic tuner only; it does a great job for me.



 

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