With Dish Network the local channels are broadcast on what they term a "spot beam" You will receive your local channels within about 150 to 200 miles of your home location on that beam, once you get too far from it you won't get them anymore.
Also, with Dish the zip code entry was only to tell you where to point the dish for signal. It has nothing to do with your programming. Newer receivers for Dish don't even have the zip code calculator in them anymore. I had to download a program for the computer which I used to put the local zip code into and it would tell me the azimuth and elevation for my dish.
When we bought our Georgetown however I installed an automatic Dish Trav'ler system. All I do now is push a button and it finds the birds itself.