Topband: flag matching transformer ferrite material

[Roger Graves]

I have a flag rx antenna that is working well on 160 mtrs. I would like to try to use it on 630 mtrs.
The current matching transformer is 3:12 turns on a BN-73-302 core. I assume more impedance will be needed for good performance on 630 mtrs. 
I did not find core specs on the FairRite site for frequencies below 1 mHz for 73 material.
Is 73 material the best choice for a transformer to be used on both 630 and 160 mtrs?
Would twice as many turns be sufficient for 630 mtrs, i.e. 6:24 turns on one BN-73-core, or 3:12 turns on 4 BN-73-302 cores, or 3:12 turns on 2 BN-73-6802 cores (which have twice the mu)?

Also, is the 73 material still suitable for a broadband transformer up to 30 mHz? I am using 2:2 bifilar turns on a BN-73-302 core as a 1.8 to 30 mHz broadband 1:1 transformer in a receiver. A test shows good swr over this freq range, but I do not know if the loss is low at 30 mHz.

Thanks for the help.