I am using RG 11 coax connected to receive radio. Center wire to one post shield to other post. Can it be used as a long wire antenna by it's self?
It will work after a fashion - like that - but as the "shield" is trying to stop signals reaching the "core" you would be best placing the shield connection to the centre of the receiving plug lead.
What length of RG-11 are you using ? As it is not good for HF (<30 Mhz) especially is you have 100 feet or so of it.
You would probably be best just making a long-wire receive antenna and not worrying about the coax at all.
Yes, RG-11 coax can be used as a receiving antenna wire by attaching the braid outer conductor to the antenna connection post on your receiver. You ignore the inner conductor as it serves no useful purpose in this application.
However, why? Why would you want to use a more bulky, less flexible, coaxial cable for receiving antenna when a small thin wire can do the job just as well. I mean, if you have this coax lying around and if it is in sufficient quantity then sell it on Craigslist and then go out and buy some small gauge wire, say #18, insulated so you can run it anywhere, and string that up in some fashion. Experiment with position and length of the wire for the receiving bands of your interest.
I use standard home wiring insulated copper wire for all my wire antenna applications -- both transmitting and receiving. It is strong and gets the job done. In the US, it is something you find at big box discount stores. I buy it in the 500 foot spool size.
RG11 can work as well as any other wire as an antenna, but you wouldn't want to connect the shield to one post, with the center conductor to the other post. The reason is that any current which is on the center conductor but which is not matched by an equal and opposite current on the shield migrates to the shield via skin effect. So, you are effectively connecting both posts to the same wire, which won't work especially well.
Better would be to connect one post to the shield, and connect the other post to ground, like a cold water pipe. Or really any other metal thing. Be sure to disconnect it in lightning storms. Utilized in this fashion the coax is essentially a fat wire. If you had ordinary wire I'd use that instead, but if you already have the coax, I don't see why not.
Another thing you can do is remove the outer insulation from a quarter-wavelength section of the coax, then bunch up the shield and fold it over on itself, effectively making a sleeve or bazooka balun. Check out this answer for some more pictures. Utilized this way, you are essentially making a dipole.
As others have said, using coax for a long wire is workable but it's a waste of money. There are antennas that specifically use coaxial cable to advantage though