Symmetric AntennaTuners

The ST-1500 is a symmetric Pi-network tuner. This is a manual tuner that contains 2 air variable capacitors and 2 roller inductors. The unit also has a set of switched fixed capacitors for matching lower frequencies to electrically short antennas. In this design, the entire circuit floats above ground and the chassis. The tuner comes with a 1:1 choke balun located on the input side of the tuner for controlling common mode currents.  All of the components of this tuner were designed in house and built by Blue Ridge Amateur Radio.  The ST-1500 is an excellent tuner for multi-band antennas. The tuner interfaces with coaxial cable for the input and open wire line for the output. Typical open wire lines for this tuner include 450 ohm window line and 600 ohm open wire line. This tuner was designed to work with balanced antennas and provides excellent return loss values (50+db) on all bands 160-10 meters on an 80 meter dipole antenna with a 1.0:1 SWR. As usual, your mileage may vary as all antenna installations are different and feed line considerations also play a significant role in antenna system impedance. 

These are various pictures of symmetric tuner construction and the finished product.

Note that in our tuner the rollers and caps are mounted as pairs of devices. This greatly simplifies mounting the components to the mid-plate, but also allows for more rigidity between the devices. This also allows us to color coordinate the mounting hardware with the outside case color. (yes this is trivial, and not required, but we're building a quality product here. The quality doesn't stop at the front cover).

Included in these photos are pictures of the roller inductor drive system (drive belt), air variable capacitor drive system with shaft RF isolators and vernier reduction drives. You can also see the capacitor dial skirts on the picture of the unit's front cover. The dial skirts are driven by the reduction drive, which provides for highly accurate tuning, especially when the point of resonance is extremely sharp.

Use of this tuner in the shack has proven that this  design is both efficient and a solid performer. We can tune 1.0:1 on 160 - 10 meters on a dipole that is "short" for 160 meters. The addition of switched fixed capacitance allows for tuning highly inductive loads on bands like 160 meters. No additional fixed capacitance is required to tune any antenna that is 1/2 wavelength or better on any band.

See this section for information on Symmetric Tuner efficiency: Tuner Efficiency Proof