(The following is from: Greg Smith (LiveTV@en.com).)
Most people mistakening believe that the larger the antenna the better the received signal. The truth of the matter is that each element of the antenna must be cut to a precise length depending upon the frequency of the signal you are attempting to receive. Further more, each element must also be spaced a precise distance away from the others. This creates what is commonly called a "directional array". (see diagram below) By providing enhanced reception (gain) in the direction the antenna is pointed, it also provides decreased reception from the sides and back. (directivity) This prevents "ghosting" which is caused by the same signal arriving at the TV at a slightly different time because the signal bounced off of some structure on it's way to your set.
If you use the house wiring as the antenna, the length will be random and the orientation to the received signal will also be random. Therefore it will pickup the bounced/reflected signals just as well as the primary signal. IE: lots of ghosting = very poor picture quality.
Any kind of directional antenna, even a small one, whether inside or outside, should provide a superior quality picture to that from the device you are talking about. Even a cheap "rabbit ear" antenna mounted on top of the set allows you to orient it in the best direction.
If you only receive the VHF channels (2-10) in your area then buy a VHF ony antenna. If you only receive the UHF (19-60+) then buy a UHF only. If you get some of each then make sure that it is a combination antenna. If your set has separate inputs for VHF/UHF make sure you also get one with the proper splitter.
------------------- ------()------- -------- ---- ^ | direction of signal