This ATU, I call my design FRI-Match, has been devised in 1972 as an unbalanced tuner for improving the SWR at the transmitter end of coaxial feeders to resonant antennas (vertical dipole, trapped dipole, G5RV, Yagi loop quad etc.).

The coil is made with: 15 turns of 2.5 mm diameter enamelled copper wire or 6 mm² tinned copper wire, 4.5 cm long with 5 cm ID or 5.5 cm long with 4 cm ID. Taps on 3, 6, and 9 turns from earthy end.

It is essentially a "kiss" approach cheaper and almost faster than an automatic ATU provided that the calibrated settings on each band for minimum SWR are known so that the capacitors can be quickly reset.

In practice the ATU has proved more flexible than expected and in many cases permits matching to non-resonant antennas.

With an extra 3 – 6-turn bifilar winding over the earthy end of the coil (fig») a "balanced" output for 50/75 Ω twin wire or 300 Ω ribbon feeder is feasible.

However, it should be noted that this design couldn't satisfy all possible conditions such as random length wires and antennas. This can sometimes be overcome by increasing or decreasing the length of the coaxial feeder or ladder line and/or reversing the input/output thermals of the ATU. To meet all possible matching conditions a more complex arrangement should be necessary. 

With this FRI-Match I do not recommend the use of a coupling coil but have the preference for a suitable balun at the output of the ATU.

A toroid, self-shielding because oh it's low external field, facilitates compact construction. For ± 400 W power, a T200-2 toroid can be used with 25 turns on 75% of the circumference. Taps 5, 10 and 15 turns from earthy end. A 5 to 8 µH coil seems to be the best as the result of experiments.

Before winding, several layers of Teflon plumbing tape must be applied to the core, to insulate it from the coil-windings. Another method of insulation is to cement two flat isolating washers (e.g. made from bare glass fibre board, see figure) on each side of the bare core. Apply a small quantity of super glue, possibly only a few drops, around the sides of the core. Work swiftly; the glue hardens quickly. The glue prevents the washers from moving out of alignment while the core is being prepared for winding. For a T200-2 core, the inner diameter should be 28 mm and the outer diameter 55 mm. With this last construction it might be even possible to use bare copper wire for the windings.