I just finished a project with an ID-20 and a custom antenna just like what you described.
I wound enameled wire (the red stuff from Radio Shack) around a cylinder to make as tight a ring as possible. You'll have to experiment with the number of turns in respect to the diameter of the loops (diameter of cylinder.) Mine is about 65 turns on an approx 2" diameter. The datasheet says 63 turns on 120mm, that was my baseline.
Basically you connect one end of the wire to each of the antenna pins on the ID-XX board and a capacitor between ground and pin 3 on the IDXX (same pin as one of the two antenna connections).
You'll need a few very small capacitance capacitors to try once you've got your winding done. I had a 1nf, 1.5nf, 4nf and 6nf (or close to that) that I picked up from other broken consumer electronics I had laying around or got from Radio Shack, you'll do better from sources online. I just tried each of them to see which ones gave me the best range.
I have the luxury (well, I paid for it) of having a storage oscilloscope to view the signal in the antenna graphically and with some help from people on here figured out exactly what I was building...
Basically your building a resonating circuit called an LC circuit. The ID-XX is pumping a sine wave into the antenna at 150KHz and it builds up to over 100Volts (very little amperage) by pumping back and forth with the capacitor. (See something like it visually here: http://www.falstad.com/circuit/
Be careful when looking for caps, you can get over 100V if this thing is tuned correctly.
I'll try to put up some pics tomorrow.