I tried without much success to use a Dish 500 measuring 22 x 24 inches with a Linear LNBF attached for true FTA signals. With a good LNBF, a strong satellite signal painstakingly peaked, short run of RG-6, and no switches or couplers to weaken signal, I can get a few strong transponders on 97w and a few other sats near my true south. If a heavy cloud comes between the dish and satellite, signal is lost.
In my experience you need a 31 inch / 76cm or larger dish for a reliable all-weather signal.