You likely won't even have to set the portal. I was having issues when I tried to set a portal to connect with a 24" computer monitor, as the hertz are much higher. Until then the portal had been hooked up to my TV capture card in the computer via S-Video to RCA input. I finally set the portal back to S-Video; disconnected the S-Video, and only hooked up the monitor (HDMI) and rebooted the portal. It autodetected the settings and works fine now at 1080i.
Also, do not let the above posting confuse you, the portals do full 1080.