4:1 Transmission Line Transformers - Experimental results

Progress on my testing of general purpose transistors for RF amplifiers had stalled because I ran out of 22uH chokes when populating the test boards. Using a 4:1 transformer on the collector of the first stage would work but I wanted to understand, if I could not eliminate, variation between the test boards.

I had 5 different 4:1 transformers already wound in the parts bin. The pic shows these and acts as a legend for the charts to follow. There is also a 1206 sized part for scale.

 

Now to determine just how well these transformers worked. I swept each transformer over the range 1 - 100 MHZ with the nanoVNA. Firstly with the transformer un-terminated, then terminated in 200 ohms. A perfect transformer would have transformed the 200 ohm load to 50 ohms across all frequencies, showing as a dot in the center of the Smith Chart.

Core 1, the red lines below, appeared to be a ferrite core and I had expected it to work well as the frequency increased. It failed. While it presented roughly a 45ohm resistive load to the VNA across these frequencies, the reactance was significant until the frequency approached 100MHz. Expanding the swept range showed 100MHz was about as good as it got. Given this came from commercial equipment, perhaps it works better in a 75:300 ohm application.

Core 2, the yellow lines,  was a tiny twin hole balun core. I know this came from a commercial TV distribution amplifier. The open circuit trace suggested it might work up towards 100MHz, but when terminated in 200 ohms it also presented 45 ohm resistive at the top end of the swept frequencies. At 2MHz it was 34+j17 ohms.

Core 3, the green lines, was something I had wound on a ferrite bead, say 15mm x 10mm,  removed from a power cord for EMI suppression. I had no expectations. The open circuit trace showed it might have enough reactance at the low end, but it quickly presented a capacitive reactance to the VNA above marker 4, 10MHz. While the 200ohm load transformed well in the 2-4MHz range, but above 10MHz it was unsuitable.

Core 4, the light blue lines, appeared to have potential above 30MHz when inspected in the open circuit configuration. Terminated in 200 ohms it was a pass above 30Mhz. I suspect a few more turns would probably make this a suitable toroid for a HF transformer.

Core 5, the purple lines, appeared unsuitable from the open circuit trace. This proved to be the case when terminated.

Tentative Findings:

There is still more investigation to take place but some themes were noted from the work to date.

If you have a core with unknown characteristics choose one that tracks the outside of the Smith Chart when a representative number of turns is swept with a VNA. There are frequencies for which the blue trace tracks the outside of the Smith Chart. More turns would rotate the curve clockwise. Perhaps core 3 would benefit from less turns.

The useful frequency of the core will be in the range of frequencies where the open circuit response of the bifilar winding crosses the axis eg 2-4Mhz for the green trace. Getting the right number of turns could be the key to having a useful transformer for your applications. 

Being on the edge of the Smith Chart means the core losses are low and do not impress themself onto the response as a series resistance. The blue line above (LHS) is an example of this though it starts to spiral inwards above 30MHz which suggests it is not ideal above 30MHz.

The EMI suppression core would work well at 80m and perhaps 40m with further testing of turns. Above that the core losses, which a suppression core would be expected to have, appear limit it's usefulness.