To test the transformer by itself, I suggest the following:
A simple experiment can be done if one has an antenna analyzer.
1. Put a precision 50 ohm (or 49.9 ohm) resistor on the output
2. Drive the full primary with the antenna analyzer and sweep from your lowest desired frequency to highest.
3. Plot SWR across the band of interest. If it deviates much from 1:1 across your band of interest then you will see a problem.
If your antenna analyzer can determine complex impedance, you can measure the impedance seen by the primary with a 50 ohm load at the secondary for any frequency you might be interested in.
Out of curiosity, I will try this in the next few days and do the same test with a coilcfraft WB1-6TSLB up to 50 MHz, the limits of my antenna analyzer. I'll post the plots when I have them.
Alternately, you can use a PHSNA with return loss bridge to test a separate transformer over the frequency band of interest.
"Digital Steve", K1RF