I am not an antenna expert, but I would guess that so long as you used an appropriate impedance transformer to get to your system impedance, then the efficiency should still be pretty good. The 30 ohm impedance design was probably done to exploit the maximum power handling characteristic of such antennas (see History of 50 Ohms). As long as your system does not really need the 30 ohm impedance, and you can build or buy a low-loss transformer, then you should be OK with the 21 ohm antenna.
History of 50 Ohmshttp://www.rfcafe.com/references/electr ... 0-ohms.htm
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster