Forget about air-core chokes. G3TXQ's page that Phil links to in his excellent answer here shows just how much superior ferrite chokes are than air core choke baluns.
Please visit k9yc.com and download rfi-ham.pdf. Jim Brown there on page 36 recommends a choke balun made by wrapping 8 turns of the coax at the antenna feedpoint through four stacked 2.4" diameter #31 ferrite cores. They cost about $7 from http://www.mouser.com. And don't wrap the RG-6 too tight! That PDF tells how to properly wrap it.
Below is a similar choke balun that I made, at the base of my 160m inverted-L. Those are the exact Fair-rite #31 cores I mentioned above, with RG-6 coax. You don't need to make the loops as big as I did.
(For 1.8 MHz, it really should have had more turns and cores, but it worked very well nonetheless.) I used this antenna on 160 through 20 meters, with great results.