Full Wave Loop

This table and the complete article are available directly from DX Engineering . NEC modeling for a full-wave loop at 25 ft above ground shows the feed point impedance ranging from 31 to 312 ohms, with the minimum at 3.7 MHz. On 40 m, the same loop's feed point impedance ranges from 104 to 362 ohms. Thus, the 300-ohm balanced line is not a bad match for those two bands.

Across the 6 bands above 40 m, the feed point impedance ranges from 273 to 1327 ohms.
60 m is the odd animal -- the feed point impedance for the same 3.7 MHz loop ranges from 10,162 to 14,880 ohms! That illustrates why so many hams have had trouble covering 60 m using a multiband antenna! 
(Special thanks to WA5MLF for NEC results!)