Pete, I forgot to mention the reason I believe the IRE dummy antenna
tries to look like a 400-ohm resistive load at shortwave instead of a
certain length of wire. You can see why he

http://users.tns.net/~bb/antlen.gif

This is an old RCA chart showing the behavior of an end-fed wire for
lengths to 125 feet. It indicates the many resonances and
antiresonances a short wire can exhibit. Since the antenna impedance
and the resulting performance vary so markedly with wire length, and
because it would be unrealistic to expect all listeners to use one
particular length, I think the dummy antenna is just intended to
roughly match the input impedance of a typical radio. I'm not sure
about consumer radios, but old communications receivers all seem to
have a specified input impedance of either 300 or 400 ohms.

On the broadcast band, any wire shorter than about 150 feet will look
capacitive. It won't exhibit the resonances you see in the chart at
shortwave. So even if the wire is shorter than the 100-foot length the
dummy antenna seems to model and exhibits a higher capacitive
reactance, it won't affect the radio's RF tracking that much. On the
broadcast band, most radios seem to use rather loose antenna coupling
to minimize mistracking. This allows them to accomodate antennas of
various length. There is an interesting discussion about antenna
coupling strategies in the Radiotron Designer's Handbook.

Brian