I'd like to perform some simple receiver testing and calibration (mostly for frequency variance, not signal strength) and have been told WWV/WWVH transmissions can be found throughout most of the US.
What would be a good, cheap, small antenna for receiving one or more of these transmissions at 2.5MHz, 5MHz, 10MHz, 15MHz, and 20MHz? I note that it's common for "atomic" clocks to have such an antenna internal, and if I find one lying about I'll use it, but I don't keep such clocks, nevermind random ferrite rods, lying about.