The Smith chart, a thing of beauty… and great utility #2

A recent online posting offered measurement of an ATU balun in a Heathkit SA-2060A and sought advice on the ferrite core type, losses etc.

Above is a diagram from the manual. It is a pair of cores stacked, each core is wrapped in self adhesive glass fibre tape.

Straight away, the glass fibre tape is an important fact. Hams often use glass fibre tape on any and everything, but for a high power application like this, a conductive core (eg powdered iron) requires additional insulation, the paint is not really sufficient. Powdered iron cores usually have radiused corners to reduce the risk of punching through insulation, whereas ferrite cores are more commonly chamfered with relatively sharp edges.

Above, the schematic shows the balun to be a Ruthroff 4:1 balun, a voltage balun.

Pics of the balun in the ATU show quite long connecting wires, so any attempt to measure the component will be frustrated by impedance transformation due to the transmission line effects, more so at higher frequencies.

The poster had made measurements with a Rixexpert analyser though a quite short coax lead and pigtails, perhaps 80mm in all. the test equipment was connected between J4 and ground, and a 200Ω resistor of unknown quality connected from J4 to J5. Again, there will impedance transformation due to the transmission line effects, more so at higher frequencies.

The good thing is his measurements were from 25kHz to 30MHz.

Above, the InsertionVSWR looks pretty awful, but remember that this is for use with an ATU, and non-ideal impedance transformation is corrected by the ATU adjustment.

Above, the same information presented as Return Loss.

Above is a plot of admittance. Note that over much of the plot, G is almost constant, and at the very low frequency end, B varies almost inversely proportional to frequency. The curves hint that at the lowest frequencies, the admittance is dominated by B, B is -ve, and it varies almost inversely proportional to frequency… so we are looking at an admittance of some fairly constant G and (in parallel with) some inductive susceptance inversely proportional to frequency… the latter hinting a constant inductance.

Above is the Smith chart presentation of the same data. Note that the curve almost exactly follows a circle of constant G from 25kHz (LHS) to the marker at 400kHz. The combination looks like about 51Ω resistance in parallel with 4.7µH of inductance… the magnetising inductance looking into one winding of the transformer.

One contribution to the non-ideal impedance transformation is inductance, worse with lower permeability cores, and worse with long windings, both are features of this transformer.

The conductance figure is attributable to the transformed 200Ω load, so there is not strong evidence of substantial loss, further hint of a powdered iron core.

Let’s look at the inductance of the common T200-2 core with 14t as an example.

A stack of two cores will simply double this at the frequency being discussed, so a pair of T200-2 cores with 14t should have an inductance @ 400kHz of around 4.8µH. It is quite likely that the transformer uses these cores or ones with similar characteristic.

So, it is a 4:1 voltage balun integral to a high power ATU, it has quite poor InsertionVSWR but that is relatively unimportant in an ATU, it appears to use a powedered iron core which has relatively low losses. The matter of whether a voltage balun is a good choice for HF wire antennas with two wire line feed is another question.

The Smith chart is so informative!