He has a strong Aussie accent, a bit hard to understand. It’s a 28 min. YouTube video. He says BD139s have a fT of 250 MHz, but the datasheet I have does not specify any fT because the D in BD139 means they are a low frequency device, not meant for VHFs or RFs in general. This fT might be specified in some manufacturer’s datasheet, but it was never meant to be used at RFs so the experimenter could buy some completely legitimate BD139s that cannot amplify frequencies above 3 MHz.