What is a Symmetric Antenna Tuner?

My previous post on multi-band delta loop antenna had a mention of symmetric tuner as the recommendation from one manufacturer. Another previous post on basic types of antenna tuners had a mention of symmetrical Pi network which uses a couple of synchronized variable inductors, meant for balanced antenna systems. There is a detailed article with schematic diagram and photographs on Symmetrical Antenna Tuner by LY1O. According to the article, balanced L network is the way to build a balanced antenna tuner. It has very low loss. In that schematic, it can be seen that two roller inductors can be tuned simultaneously. Variable capacitor can be switched from the input side to the output side using a switch.

In the earlier post I had mentioned that L network antenna tuner could be implemented in two ways. One with variable capacitor at the antenna end, for matching an antenna with higher impedance than the radio and another with capacitor at the radio end to match an antenna with a lower impedance than the radio. In both, the variable inductor is placed in series and variable capacitor in parallel, as shown in the diagram. A rotary switch can be used to switch the conncection of the shunt capacitor from one side to other, to change from one type to another. This arrangement may be useful when you wish to change from an antenna with higher impedance like a folded dipole to one with a lower impedance like a half-wave dipole.

As mentioned in that discussion, symmetrical antenna tuner by LY1O also uses additional fixed capacitors switched in parallel to the variable capcitor. It uses a 1:1 current balun wound on a couple of stacked FT-240-43 toroids on the transmitter side of the antenna tuner to convert the 50 Ohms balanced impedance to unbalanced. Stepper motors and vacuum relays were used to control the tuning process. You may be aware that stepper motors move in steps unlike the continous motion of usual motors. With each pulse given to the stepper motor, the shaft rotates at a fixed angle. LY1O used a DC-to-DC converter to get 24 volts from 12 volts, needed for the vacuum relays.