I just went through all this because my dishes will be mounted so far from the electric service entrance in the house we're moving to soon.
DC resistance between one ground rod and the earth must be 25 Ohms or less. If it is more than 25 Ohms, a secondary ground rod must be installed. THe proper engineering design for a second ground rod, and a rule of thumb, is to install the second ground rod no closer to the first ground rod than the length of the ground rods themselves. (8 ft ground rods = 8 feet apart). They can be as far a way from each other as you want, so long as they are bonded together.
The equipment needed to accurately measure a ground rod's resistance is expensive and time consuming to set up. It's far easier to install two ground rods if there is any doubt about the soil conditions.
I drove another 8 foot copper grounding rod. I dug 125 feet of shallow trench to bury a #6 copper wire running between that new rod and the main house electrical grounding rod. The house was totally rewired. All new electrical wiring, new service, new ground rod, etc so I had the phone installer reground their interface box to the new ground and he was more than happy to do so.
I spoke to a master electrician who said this about what I had done, "I'm glad to see that you knew enough to bond your ground rod to the main electrical service ground rod. You are way ahead of most custom installers in understanding this. Many hours have been spent, by many people, trying to figure out why they are getting audio hum or rolling bars on a video display because of improperly grounded & bonded telephone, CATV, satellite, and antenna cabling."
I did a lot of reading, looking for an easier way. In the end, doing it the RIGHT way won.
Keep in mind that your dish receivers are connected to electrical, phone, and (in many cases) antenna. All these systems have to share a common ground (no pun intended) and simply driving a 4 foot rod wherever is convenient is not the right way to do this.