4:1 (200:50 ohm) Transformers - Two Suggested Cores

 Still more testing but I have narrowed it down to a choice of two:

Small Binocular Core ~A$0.30 each

My initial test with this binocular core was only 3 bifilar turns due to the size wire being used. I  suggested if 4 turns didn't perform better it should be put to one side. I twisted up some fine wire and had little difficulty passing 7 turns through the core. It appears useful across HF:

 

 

 Small Toroid Core ~A$0.20

Now this core was a real winner. It's like two FT-24-43 cores stacked. I had tried some smaller cores which appeared to be too small for HF. I wound 10 turns and measured in my fixture and thought this could work. I rewound with some twisted wire and increased the winding to 13 turns. It just felt right looking at the curve for 10t.

The result was outstanding and far exceeded my expectation being useful beyond 150MHz as the sweep below from 1MHz to 150MHz shows. 

I am very happy that a small amount of time with the nanaVNA, some systematic investigation and thought have resulted in a 4:1 transformer I can use from 160m to 2m.

73's

Richard