9:1 Transmission Line Transformers 50:5.55Ω - More Results

Ruthroff Type 9:1 on Two Ferrite Beads

As an alternative to using trifilar windings I tried two of these cores, each core with 6 bifilar turns. These are connected in such a way that a 9:1 transformer is achieved. My expectation was this was just not enough ferrite or wire to work. I was surprised then to see a useful return loss from 160m to 20m. 

Ruthroff 9:1 Transformer Two lots of 6 turns on a single core

Ruthroff Type 9:1 on Small Binocular Ferrite

Switching to a regular binocular core I wound 6 bifilar turns on each half and repeated the measurements.

Binocular core, 2 x 6t bifilar, each winding on separate half of core

Comparing these two cores with the 4 bead 4 trifilar turns blogged about last time shows the 2c beads can be just as good as this binocular core:

Upper: 6 bifilar turns on each of two beads
Middle: 6 bifilar turns on each half of binocular core
Lower: 4 trifilar turns on 4 beads

 

Surprised again I looked at the Smith Chart of S11 for the binocular based 9:1 transformer. Since parallel capacitors are often seen compensating RF transformers I captured a few tests. Unfortunately I lost track of the compensating values used but the impact is clear. As the capacitor value increases the S11 curve rotates clockwise on the Smith Chart:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

 The resulting plot of the magnitude of S11 shows the outcome:

2 x 6t bifilar on Binocular Core
Upper no compensation, Middle some compensation and Lower More Compensation

Compensation can improve the performance of this style of transformer. It struck that a series capacitor could be used to "slide" the uncompensated curve around the Smith Chart. A 1nF capacitor was about right at 14MHz to cancel the series inductance present:

So yes the trace slid around the Smith Chart, but the transformer became narrower in bandwidth:

So if you need compensation to achieve the desired performance perhaps a conventional style transformer with compensation can be used? I'll look into that next.

73's

Richard