because "ac" actually do not add any new frequencies...
simply overlap all previous frequencies.
For example, on Italy the frequency range usable go from this two edges: 5490 - 5710
With 20MHz channel are available 11 channels: no one single channel is overlapped.
5500, 5520, 5540, 5560, 5580,5600, 5620, 5640, 5660, 5680, 5700
With 40MHz channel (20+20 = Ce = eC or 40MHz "turbo mode" as on some wlan chipset ), one channel are fully overlapped with the next or previous.
The total unoverlapped combinations are only 5:
5500 + 5520 Ce (= 5520 + 5500 eC)
5540 + 5560 Ce (= 5560 + 5540 eC)
5580 + 5600 Ce (= 5600 + 5580 eC)
5620 + 5640 Ce (= 5640 + 5620 eC)
5660 + 5680 Ce (= 5680 + 5660 eC)
5700 Ce unusable: 5700 + 20 MHz > 5710
Overlapped channels with previous list:
5500 eC unusable: 5500 - 20 MHz < 5490
5520 + 5540 Ce (= 5540 + 5520 eC)
5560 + 5580 Ce (= 5580 + 5560 eC)
5600 + 5620 Ce (= 5620 + 5600 eC)
5640 + 5660 Ce (= 5660 + 5640 eC)
5680 + 5700 Ce (= 5700 + 5680 eC)
With 60MHz channel (20+20+20 = Cee = eCe = eeC ), one channel are fully overlapped with the next or previous TWO!
Like previous example, the total unoverlapped combinations are only 3, LIKE 2,4GHz BAND!!!
5500 + 5520 + 5540
5560 + 5580 + 5600
5620 + 5640 + 5660
5680 + 5700 + xxx unusable: 5700 + 20 MHz > 5710
With 80MHz channel (20+20+20+20 = Ceee = eCee = eeCe = eeeC ), one channel are fully overlapped with the next or previous THREE!
Like previous example, the total unoverlapped combinations are only 2:
5500 + 5520 + 5540 + 5560
5580 + 5600 + 5620 + 5640
5660 + 5680 + 5700 + xxx unusable: 5700 + 20 MHz > 5710
and if you see the "ac" standard, it can use also 160MHz channels. only one channel available:
5500 + 5520 + 5540 + 5560 + 5580 + 5600 + 5620 + 5640 Ceeeeeee ...... eeeeeeeC
5660 + 5680 + 5700 + xxxx + xxxx + xxxx + xxxx + xxxx unusable: 5700 + 20 MHz > 5710
This demonstrate how 802.11ac make previous unoverlapped 5GHz band,
full overlapped and unusable band worst than 2,4GHz band.
agree, and the things can be worst:
In wlan environment many 5ghz devices do not support DFS channels.
That leaves you only with 8 20 mhz channels 34-40-44-48 149-153-157-161 that is 4 40 mhz channels or 2 80mhz channels.
To avoid co-channel interference and do channel reuse you need minimum 4 non overlapping channels that leaves you limited to 40mhz channels.
Big distance From Marketing to reality on 5ghz