9:1 Conventional RF Transformers 50:5.5Ω

I started with one of these binocular cores simply because it was already on the bench. It's also enjoyable to get something that has no data to work!

Using the braid from a piece of miniature coax cable as a 1 turn winding, I teased two holes in the closed end so I could wind 3 turns through the inside of the coax braid.

1 turn of braid, 3 turns inside braid

Good for 80m to 10m. What struck me some time later is the improved frequency response of this conventional style transformer compared to the 9:1 transmission line transformers I tested on the same core. This was contrary to my expectations that transmission line transformers had wider bandwidths. If I have learned anything in recent months it's that measurement beats folklore.

 I know that compensating caps are used in conventional transformers so I added 18, 22 and 27pF caps in turn across the 50Ω side.

Same transformer with different caps shunting 50Ω winding

The improvement in return loss is clear. I then repeated this on the 5.55Ω side of the transformer. The capacitors ranged from 220pF to 470pF. But these values appeared too large and I was getting inconclusive results. 

It appears a convectional transformer can be wound on these binocular cores. However, I need to do some reading on how to select the compensating capacitor values should they be needed. 

73's

Richard

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

9:1 Transformers 50:5.5 - Initial Results

With the success testing 4:1 transformers I examined some 9:1 transformers. A trifilar winding, being a bit thicker than a bifilar winding using the same gauge wire, proved difficult with the small cores previously used. I wondered how a single winding through 10 of these cores would perform. I passed a single trifilar winding through 10 turns, arranged them into a U or long binocular shape, and secured them with piece of heatshrink.

1 trifilar turn through 10 cores

Note the Smith chart response is curving away from the 50Ω origin upwards and to the right. This is the same with a bifilar winding.

Returning to the test results here is the sweep of S11: 

1 trifilar turn through 10 cores

 

Very useful from 160m to 30m applications and so easy to wind. Just one turn. But it looked a bit cumbersome and not as broad as I had hoped. So I tried two turns through 6 cores. This was better but still didn't improve the upper frequency. So I tried 4 turns through 4 cores. 

Comparing 1t 10cores v 2t 6cores v 4t 4 cores

Clearly 4 turns through 4 cores is a better transformer, and would handle the 1W I intend to push through it at 80m. However, not as broadband as I had hoped given the success I had with 50:12.5Ω transfomres using these core.

At present these transformers are destined to be used in the 10W Class AB amplifier for 80m I am planing to drive with the 1W Class A amp I have developed. It will be interesting to see if they handle the power.

Overall, these tiny cores are a useful part. I certainly consider them useful for 4:1 transformers but trifilar windings are more time consuming to wind and solder.

73's

Richard