Why? 20 kW is very common for FM radio, which is also just streaming shitty ads to people. Warsaw had a 1 MW transmitter.
Power density scales with inverse of distance squared. At 1km, 20 kW is 0.001 W/m2. At 500 km it's a few nW/m2.
Power density scales with inverse of distance squared. At 1km, 20 kW is 0.001 W/m2. At 500 km it's a few nW/m2.
2-25Mhz reflects off the atmosphere and returns to earth and the multiple allocated bands in the range are typically less then 6KHz for broadcast AM and other bands often <3KHz SSB down to 100Hz CW can be completely trashed with spurious emissions. 60, 80m and 160m can be difficult to use already due to all sorts of noise from notionally low power devices in the mW range on nearby, or sometimes nowhere near, frequencies. The wide range is probably to enable use regardless of the current maximum usable frequency (MUF) to ensure atmospheric reflection. MUF varies, sometimes considerably, by time of day, season etc. However the reflections do not stop when you receive the signal, they continue to skip and be received in many additional locations that are not the intended destination.
Someone pumping 20KW across the range could cause an incredible amount of disruption, essentially worldwide. See Ringway Manchester's great series of videos on the Russian Woodpecker (it was speculated in the MW power range) https://www.youtube.com/watch?v=wB_jiigRq-c
HF signals can be reflected by the surface of the Earth after hitting the ionosphere and repeat this process until they've gone around the world (sometimes more than once!). This is not a rare quirk; amateur radio operators take advantage of this on a daily basis, with an absolute maximum power limit of 1500W. (But 1500W is uncommon; my radio does, at max, 10W. And I can communicate with Europe from NYC fairly regularly.) The result of all of this is you have to be careful about HF signals; you can ruin it for everyone in the world if you make a mistake. VHF, whatever.
VHF does occasionally propagate over long distances. It used to happen with analog TV and was quite the rare and amazing sight to behold (I've never seen it).
https://en.wikipedia.org/wiki/Tropospheric_propagation#Tropo...
https://en.wikipedia.org/wiki/Sporadic_E_propagation
TV moved from VHF to UHF during the digital transition, so this doesn't happen anymore.
Finally, there is a lot more VHF frequency available than HF. For example, one band that amateurs have is the 40 meter band, which is 7MHz to 7.3MHz. That is 300kHz of bandwidth for everyone in the world. Meanwhile, just 1 FM radio station is 270kHz!
In the US, I don't think VHF ATSC is going anywhere, because there are only 23 UHF channels remaining (14-36) and VHF adds 12 more. VHF is less popular because you need a larger antenna to receive it.
It's only higher of the two previously existing TV bands (channels 7-13, 174-216MHz) that is still allocated for terrestrial broadcasts in the US The old 54-88MHz (channels 2-6) are no longer used in the US for ATSC.