Gregorio, 4:1 HF unun

Gregorio is a unun with a 4: 1 transformation ratio, useful to couple an unbalanced transmission line (such as a coaxial cable, Zl = 50 Ω) to a 12.5 Ω unbalanced load. It is the first unun that I built myself, on the hypothesis that the sloper had an impedance equal to a quarter of what it would have had if it had been a quarter wave long. My assumption turned out to be wrong.

Schematic diagram

Gregorio is a transmission line transformer, made by winding a transmission line with a characteristic impedance equal to

on a core of ferromagnetic material. The one used for Gregorio is a transmission line composed of 3 parallel and side-by-side conductors: each conductor is an AWG14 copper wire coated with Thermaleze, an insulating material capable of withstanding overheating of the underlying copper up to 200 ° C without degrading or damaging itself.

A transmission line made with 2 conductors like these would have a 40 Ω characteristic impedance, approx.: the introduction of a third conductor, having one of its ends isolated from the rest of the transformer circuit, causes an increase in the capacitive component of the line impedance, which results in it decreasing from 40 to 25 Ω, as required.

This line was wound on an Amidon FT140-61 ferrite toroidal core, with 8 turns around it. This way, Gregorio has a constant transformation ratio over the HF range and can withstand a 150 W CW maximum RF power.

Making of

The starting point in Gregorio's homebrewing is the construction of the three-wire transmission line that will be wound on the toroidal ferrite core. 3 AWG14 Thermaleze-coated copper wires are required: the 3 conductors must be placed side by side and kept in this position throughout their length with some insulating tape strips wrapped around the three-wire line and applied on it at regular intervals. Obviously, the more and tighter the strips are, the more constant the characteristic impedance of the line will be, but without exaggerating.

Once the line is ready, it is possible to start winding it 8 times around the toroidal core, taking care to cover its entire circumference and to keep the distance between one turn, the next and the previous one as much as possible. At this point, the insulating Thermaleze coating must be removed at the ends of the transmission line, to solder the wires to the SO239 input coaxial connector and to the load, respecting Gregorio's schematic diagram.

Performance

The test that allows to verify Gregorio’s correct functioning consists in applying a 12.5 Ω resistive load to its output. I used 4 resistors, 12 Ω/2 W each, connected in series-parallel to each other and soldered to the output terminals of the transformer. CW transmission tests performed with my Kenwood TS-140S allowed me to detect a SWR lower than 1.1: 1 from 1 to 30 MHz, confirming Gregory's excellent stability on resistive load.

Test 160m

Test 80m

Test 40m

Test 20m

Test 10m