There isn’t a way to do it, digitally. It’s all radio waves. The lower the frequency, the longer the wavelength, the less information it can carry, but the longer it can travel and get picked up on an antenna. AM operates and can have audio heard/understood at a much lower frequency than a digital signal. AM operates in the KHz range. FM is in Mhz, and digital radio operates in Mhz, as well. Specifically, digital radio is broadcast at around 175Mhz, on the low end; while AM radio is around 1,000 Khz.
Thing is, it takes 1,000Khz to equal 1Mhz. That means that digital radio signal is flinging out data around 175 times more than the AM frequencies could, so it can’t reliably transfer nearly as far. There’s also a hard line on digital signal interpretation. Once there’s a threshold hit for not picking up enough signal to fully interpret it, you drop straight to getting nothing interpreted. It’s like how now that all tv signals are digital, so you either get a picture that looks perfect, or you get nothing. Back in the analog days of television you would often get fuzzy pictures and audio. The TV was able to interpret whatever signal it did manage to get, even if it wasn’t all of the signal.