Let me add two more sites to the list.
http://www.aa5tb.com/efha.html
AND
http://www.njqrp.org/n2cxantennas/halfer/index.html
As you most likely know, impedance whether simple or complex , is going to be relative to frequency, if you are concerned with a single band it should be fairly simple , but the more harmonically related bands you try for the more complex things become, an antenna analyzer IS the best tool to use here, a "GRID DIP METER" would be the next best .
Good Luck & 73, John