Are your TV channels on 20 and up? If so this will allow the use of channels 14, 16, and 18.
https://www.solidsignal.com/m/product.aspx?p=nf-469
This model would allow use of channels 65, 67, and 69.
https://www.solidsignal.com/m/product.aspx?p=lpf-750
You will also need the 20 dB attenuator on the output of the modulator.