The problem is probably that your antenna can't pick up any of the remaining analog stations in your area. Only low-power stations are still broadcasting an analog signal. If you are using an indoor antenna, you might the signal with an outside antenna with an amplifier. (Check a site like tvfool.com to get the direction to any nearby analog station and make sure that your antenna is oriented correctly. This site or antennaweb.org will also help you orient your antenna for the digital stations in your area.)
You need a set-top digital tuner or a converter box to translate the OTA digital signals (ATSC) and translate them to the analog NTSC format. Low end converter models with only SD output are harder to find but still available. However, there are some converter boxes that have HD output as well. This one, http://www.solidsignal.com/pview.asp?mc=03&p=3500STB&d=iView-HD-DVR-ReceiverConverter-Box-for-TV-Antenna-QAM-(3500STB)&c=Digital%20Converter%20Boxes&sku=
, lets you attach an external drive (USB flash drive or hard drive) . It connects via component video or HDMI. (We've had good luck with our model with a portable (2.5") external hard drive. It replaces the SD converter box and DVD recorder.)
I hope this helps.
(In my area, there is only one low-power station still broadcasting. Its transmitter is nearly 40 miles and can only be picked up with the external antenna and amplifier. If the weather is bad, we will lose that signal. Beware: an amplifier can become damaged and block the signal. The usual check for this removing the amplifier and attaching the antenna directly to the TV. For low-power stations, this test doesn't often help.)