I was wondering if the built-in Wi-Fi supports 5 GHz networks?
Will it need an external antenna to be attached to work properly?
The network module is a a BCM43438. I'm having trouble finding a source I fully trust, but from a cursory view, it looks like it only supports 2.4 GHz. Since the Raspberry Pi foundation doesn't specify, it'd be normal to assume a lack of 5 GHz.
An image on raspi.tv displays a very compact antenna. Reception with the default antenna may not be stellar, but we won't really know until it starts getting into reviewers' hands.
As @EDP has pointed out, there appears to be only one Wi-Fi antenna. This further implies that there is only support for 2.4 GHz.
According to this article (I'm sorry it's in German) the new Raspberry Pi only supports the 2.4 GHz WiFi network.
edited based on comments
The Pi3 uses a BCM43438 wireless “combo” chip. Broadcom doesn't have a product page (that I could find). But, it appears be in the same family of chips as the BCM4334:
Since, that is an older chip in the same family that supports both 2.5 and 5 GHz, it seems like it was probably at least an available option for the Pi3 designers. Guess, we'll have to get our hands on one to know for sure.
The antenna is built into the board, and the chip likely has a pin for it. According to the product release, "... the only change is to the position of the LEDs, which have moved to the other side of the SD card socket to make room for the antenna."
No, I've read it's 2.4 GHz only.
Based on the FCC documents, it looks like the new Raspberry Pi 3 does support Bluetooth and WiFi, but it only seems to support 2.4 GHz networks, not 5 GHz. http://liliputing.com/2016/02/raspberry-pi-3-to-feature-on-board-wifi-bluetooth.html
No, I've read it has a "chip antenna".
The radio is connected to a small surface mount ceramic antenna. http://www.allaboutcircuits.com/news/raspberry-pi-3-is-here-but-is-it-a-worthy-successor/