The antenna foil has a very narrow bandwidth and resonates at one frequency. On other frequencies, they are less well matched, so reception is not as good (and the transmission may damage your transmitter). Making a wide element out of a piece of foil increases bandwidth; it also increases the aperture, so you can receive more incoming radiation. TV signals are particularly sensitive to smooth bandwidth response because they are 6 MHz wide digital signals, and if there are phase and amplitude interferences in the antenna due to strange resonances, your signal will be garbled and potentially unwatchable.
The material the foil antenna is made of doesn't matter. As long as it conducts electricity well and has good mechanical properties. Outdoor antennas are made from thicker aluminum because it is cheap, lightweight, and resistant to corrosion.
One of the best foil antennas is a bow tie, which you can make from coat hanger wire or aluminum foil, supported with cardboard if desired, and hung on the wall. These elements are examples of simple fractals. The active area is smaller (closer to the center) at higher frequencies, but the electrical performance looks the same at lower frequencies. I have built bow tie antennas down to 3 MHz and they match the transmitter reasonably well.
