I spent a few hours reading about 125k antenna formulas and designs to try and better understand the relationship between desired frequency, loops of wire, wire gauge, and the value of the tuning capacitor.
I worked through some of the formulas in this datasheet, plugging in 100 turns, 66mm, 30 gauge to see what it would come up with for inductance, ideal # of turns, and capacitor value, to see if they would match the 1nf & 100 turn values specified in scanlime's blog entry. I wrote a simple java program to calculate the values for me after not finding what I thought I was looking for on various online antenna calculators (they seemed to be all too complicated).
I found this interesting note along the way which I had not seen elsewhere: "... For copper wire, the loss is approximated by the DC resistance of the coil, if the wire radius is greater than cm. At 125 kHz, the critical radius is 0.019 cm. This is equivalent to #26 gauge wire. Therefore, for minimal loss, wire gauge numbers of greater than #26 should be avoided if coil Q is to be maximized....".
So, #26 gauge wire next time!
Unfortunately my program isn't putting out believeable numbers yet, so I have more work to do.
In the meantime, I also discovered this other datasheet for a neat little 0.32 cent RFID chip (the Philips Hi Tag-S) that has a good description of the details of the encoding methods used starting in Section 7.3 (including our manchester, for future reference)
And here's another good discussion of the antenna design and coupling (different frequencies, same concepts and formulas).
I played with the open source 4NEC2 Antenna Modeler & Analyzer for a while trying the helical generator, but it kept blowing up at simulation time with my RFID design, complaining that the antenna was connected to ground at the end (duh, this is what the tutorial shows, I don't get it!).