Can a HD antenna be connected to the ultra box since we do not have a smart tv
No. The Roku gives you the same features a smart TV would have. Connect the antenna to your TV.
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No, the HD antenna would be connected to your TV for OTA(Over the Air Channels). Usually in the coaxial port/some on a HDMI port.
The Roku Ultra is connected to your TV via the HDMI port for streaming. (Helps make your TV resemble a smart tv with streamable apps/channels).
Then you use the various Inputs on your TV to toggle between the two.
(ie INPUT 1/ANT would control your antenna OTA channels (when you want regular TV)
INPUT2/HDMI would control your roku. (when you want to stream content - internet connection)
Hope this helps clear things up.