A 1:4 RF transformer for measurements – based on Noelec 1:9 balun assembly

The Noelec 1:9 balun (or perhaps Chinese knock off) is available quite cheaply on eBay (<$5) and provides a good hardware base for a 1:4 version.

Above is a modified device with the original transformer replaced with a Mini-circuits TC4-1TG2+ 1:4 transformer. The replacement is not exactly the same pads, but it is sufficiently compatible to install easily.

The loose parts are OSL calibration parts using 2.54mm pitch header pins with the middle pin removed. The resistor comprises two 100Ω 1% 1210 SMD resistors soldered back to back. (The 1% resistor code 1000 is for 100 * 10^0.)

The most notable departure from ideal of these small transformers is leakage inductance of 30nH give or take.

The existing (ridiculously large) compensation capacitor was removed from the original and a sweep done from 1-300MHz. The Noelec schematic shows this part as CR1 and they talk of AnntennaGaurd varistors (A Kyocera trademark), but no part number. There is no doubt this Chinese knockoff had a capacitor in that position.

Above is the sweep with the transformer terminated in a SC presented as R,X. Focus on the X (blue) line, it is almost a straight line from 1 to 30MHz, \(X \propto f\), and the slope is about 29nH.

Above is the InsertionVSWR with 200+j0Ω load, it is very good at the high frequency end, and degrades at the low end as magnetising admittance becomes higher. This is specified as a 0.5-300MHz transformer in the datasheet.

We can make a first estimate of the compensation capacitance \(C=\frac{L}{Z_0^2}=\frac{29e{-9}}{50^2}=11e{-12}\). We could undercompensate a little so as not to too adversely affect performance above the primary range, perhaps 4.7pF, but it is hardly worth doing… unless you wanted to improve its insertion VSWR above 30MHz..

Above is a plot or R,X with the transformer terminated in 200Ω. It is really quite good.

So, the result has low InsertionVSWR from 1-30MHz, and can conveniently OSL calibrated using the small calibration parts shown.