Illinois house, built 1963. Not sure if there is a ground rod or just the water pipes used for ground. Want to put in another/first rod. Can I connect it to the clamp on the electrical meter housing which has been used for other grounds? If not, where do I have to connect it, noting there is no known existing rod. Also, is there a distance limit from the ground connection point (e.g. electrical meter) and the rod? I'm looking at about 50 feet. Adding additional ones between would be problematic due to concrete.
In reading your question, I think we are into that area of all the things we call "grounding" which are actually bonds, jumpers and grounded equipment. What you need to concentrate on is NEC Part III Grounding Electrode System.
I don't expect a DIY to know the entire section, but the highlights goes as follows. The entire GES needs to terminate at the first point of disconnect in your system. If you open up your panel you probably see a large #8 or #6 Bare copper conductor (the Grounding Electrode Conductor) attached to your neutral bus. This is where your cold water ground should be attached. You need to attache your driven ground rod to the bare conductor.
The driven rod you are proposing is known as a supplemental or secondary ground to the cold water ground so you do not need to install two or more ground rods. It may become the main ground if you water pipe begins to get cut up and replaced by non metallic water lines. The NEC recommends that you should use as many methods of grounding as you can to any system.
Hope this helps and good luck.
You'll want to tie the new GEC to the existing GEC using a listed lay-in grounding lug
What you'll want to do in your case is use a listed lay-in grounding lug appropriately sized for the wires in question to connect the ground wire from your ground rod to the existing grounding/bonding wire from the pipe -- this way, you can add your ground rod without disturbing the continuity of the existing grounding system. (ComEd is ambiguous about where GECs go in their service manuals -- some documentation has them routed to the main panel, while other documentation has them routed to the meter base, and some documentation shows both approaches used at once.)
Now would be a good time to add an intersystem bonding termination
While you're at it here, you can fit an intersystem bonding termination device to the existing water pipe GEC/bonding conductor -- its presence is a Code requirement (250.94), and it also provides a convenient place to land the grounding and bonding conductor for the antenna coax when you put that in. These devices have a lay-in type lug for the grounding electrode conductor with a small busbar of terminals sticking out that can be used for grounding conductors from other systems: antennas, CATV drops, telephone drops, and such.
You definitely want to bond the coax shield to the main grounding electrode system
When you are erecting the antenna and feedline, you must connect the lightning protector (antenna discharge unit) ground and the coax shield to the grounding electrode system using a 10AWG minimum wire -- this is required by NEC 810.21, and won't cause any problems since the whole grounding electrode system can be assumed to be at the same potential. (The only time actual paths to ground matter for radio equipment is if your antenna gets zapped by lightning. The reason to have it bonded in with all the other grounds is so that the breaker trips or the utility fuse blows if your antenna somehow finds itself at mains voltage.)