Over many years, the Jaycar LO1238 has appeared in some of my projects. I recommended them for a range of applications, particularly applications optimised for low HF.
Above, the core is 35x21x13mm, a mid sized core, two used in my redesign of a commercial balun and implemented by VK4MQ . The mid size limits dissipation, but compactness can be an advantage. The cores sell for less than $4.00 per core and are readily available in Australia.
The core is almost certainly made in China, and Jaycar does not publish complex permeability curves for the material, but above are my measured characteristics over HF. The Chinese factor does raise questions about continued supply of consistent quality product.
Above is a plot of Fair-rite’s complex permeability for #31 ferrite material.
If you compare the two, they are not identical, but are very similar and you could conclude that applications where #31 is a good material selection would be well served by L15. Notably #31 is a MnZn ferrite and the L15 appears to be a NiZn ferrite based on its very high resistivity.
It is interesting to observe the fashions in online discussions of the
best balun material that the current fashion amongst online experts is #31. #31 is certainly a good candidate for applications with emphasis on lower HF, but its suitability for a specific applications needs also to consider other factors like its loss.
For the same reason, Jaycar’s LO1238 using L15 material may be quite suitable to those type of applications.