Hi to all,
Hi Thibault,
Please have a look at Phil Nelson's page on using a Blonder-Tongue Broadcast modulator like yours for home broadcasting.
If you scroll down the page, there is a description of a dipole antenna with the correct length for each wire of the dipole for US VHF channels 2-13.
Phil gets good TV signal coverage over his large house, so you should too :
https://antiqueradio.org/HomeTVTransmitter.htm
You mention getting only UHF to work. Do you have the Blonder-Tongue user manual which gives the position of the dip switches to program the channels?
I suppose you don't have the latest model with red LED channel display & up/down pushbuttons.
Best Regards
jhalphen
Paris/France