It used to be so easy. When DIRECTV and DISH started, there was one satellite location. You pointed your dish at it, and you were done. Now DIRECTV has four primary locations and two secondary ones, while DISH has two sets of locations, each with three satellites. How in the heck is a DIY’er to get all those satellites?
The answer is, of course, an oval dish. The oval dish actually acts like several round dishes all next to each other, and the signals are designed so that when they overlap each other on the dish they don’t affect each other.
There’s a little more to it than that, and in order to understand it, you need to know two letters: Ka. We’re not talking about the Las Vegas nightclub here, either. DIRECTV uses a separate radio band to carry its HD signals called Ka, pronounced “kay, aaay” not “kaaa.” DIRECTV is the only broadcaster in the US to use any Ka broadcasting frequencies, and they did this to make sure they have plenty of room to grow as needed. In contrast, DISH uses the regular satellite broadcast frequencies that everyone else in the Western Hemisphere uses, called Ku (“kay, you”.) That’s part of the reason they need two sets of satellites, because Ku-band satellites are pretty crowded up in the sky, relatively speaking. They’re still thousands of miles apart.
So a regular round dish picks up Ku-band but not Ka-band, at least for DIRECTV, so there’s just no way it would get DIRECTV broadcasts, even if it were pointed right at the satellite. You may be asking yourself, why not just change out the Ku-band LNB for a Ka-band one… there’s only one company that licenses the manufacturing for those and it’s DIRECTV. They don’t want you doing that so you’re going to have an issue there. They want you to use a Slimline dish, so use a Slimline dish you shall.
This gets to the bottom of why it’s a little easier and cheaper for DISH customers to get HD on their RVs and boats. An HD dish for DISH is really just a big round dish with the top and bottom carved out, and that means that fewer electronics are needed to get that in-motion dish to work right. Folks are pretty amazed by what an in-motion HD dish costs, but it takes a lot to make that work for an HD DIRECTV dish, and complexity costs money.