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5ghz WiFi in AP Mode
I just purchased a RPI3B+, thinking I would be able to get 5Ghz WiFi in AP mode. It seems I'm still only getting 2.4Ghz. Is 5Ghz AP mode possible?
Yes but it requires changing the value assigned to the hw_mode parameter in the file /etc/hostapd/hostapd.conf

You can read about the hostapd.conf file at the link below. Search for "Operation mode" or "hw_mode".

Another decent link for hostapd is below.
Ok, thanks!

Hopefully it will work on both bands.
Does this mean the RPI can't create 2 APs (one on 2.4g and one on 5g)?
Haven't had any luck getting the AP mode to work at 5ghz.

hw_mode=a # didn't work

Then added:
channel=0 # didn't work

Then added:
ieee80211n=1 # 802.11n support
ieee80211ac=1 # 802.11ac support

No go. Never saw the SSID on my scanner.

Put the hosted.conf back to it's default and the Moode SSID is back, on 2.4g

Anyone have any suggestions?

This is an RPi/Raspbian question. A quick Google search turned up a mixed bag of posts, most reporting failure but a few claiming success. 

Have a look at the Raspbian github issues, 802.11ac 5Ghz wifi AP in RPI 3 B+, for example. There's a number of posts to as well. Lots of posters seem to be changing parameters randomly---the hacker equivalent of the mathematician's "drunkard's walk". 

WiFi technology is a mess. The clash between requirements of different regulatory regions is even worse for the 5GHz band than it was for the 2.4GHz band.

Thank you for that additional info.

Looks like channel=0 was actually the problem. Has to be set to a channel for it to work in AP mode.

There's a lot of other parameters in the link you posted, but it seems those weren't needed in my case.

Thanks, everyone!

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