4:1 Transmission Line transformers - Which core?

 Right. So I know have a rule of thumb and a growing pile of test cores and results. The rule of thumb I found, see earlier posts on this subject, is very helpful when dealing with unknown cores. But I went searching and I was surprised at the reasonable cost of buying brand new cores from Mouser. 

For 200:50 ohm bifilar wound transformers I am narrowing down on a "standard" part. I am confident that small cores will cope with the maximum power a 12V class A amplifier can deliver across 200ohms (12^2/400 ~ 0.4Watts) so size is unlikely to be a selection criteria.  Taking into account price and performance across HF I present some results for your consideration:

Small Binocular Core ~A$0.30 each

I could only get 3 bifilar turns through this tiny toroid with the relatively large wire I was using so those bifilar windings were jammed on top of each other. Despite this the results were surprising:

3 turns 200 ohm termination and Open circuit termination

With finer wire and 4t I expect this core will result in a useful transformer. Confirmation in due course.

The next physically larger size core is substantially larger than the toroids below with a similar cost. So if 4 turns doesn't cut it then I'd put these to one side for now in this application.

EPCOS / TDK Toroid ~A$0.60 each

After 7 bifilar turns suggested more turns were needed I guessed 11 turns. Very good transformer at 30MHz but still needs a few more turns. I'll try 13 turns next.

11 turns 200 ohm termination and Open circuit termination

 

Fair-rite Toroid 43 Mix ~A$0.75 each

Results for 7 bifilar turns were pleasing. 8 turns might be better. I only had one of these cores and have put it to one side for now.

7 turns 200 ohm termination and Open circuit termination

Next steps:

The Epcos/TDK toroid is, on results to date, the leading contender. I still have a few smaller (and cheaper) ferrite toroids to test and I am interested to see how they perform. At some point I will have picked my standard for 200:50 ohm transformers and I will write it up.  Then it will be time to get back to my amplifier testing which is how I ended up down this rabbit hole.

 

73's