I discovered a lot of pulse transformers. They appear to have primary
and secondary inductance of about 1.5mh and a ratio of 1:1ct

I wonder if they could be used as broadband rf transformers
to make homebrew diode ring mixers?

I don't think they will work very well, you really need transmission line
transformers wound on suitable cores for diode ring mixers.
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Perhaps not suitable for higher RF frequencies ,but these transformers can
be readily used for AF ,for example to provide galvanic separation between
a PC (soundcard) and a radio for operating digital modes like PSK31 etc .

Frank GM0CSZ / KN6WH