Hi Ed,
Yes I am surprised that such a detector worked.
Apparently the effect was discovered when Lyden jars where discharged and it
was noticed that near by Lyden jars of similar size and with a similar spark
gap discharged even though they where not charged. The pictures I have seen
show a Lyden jar with a circular shorting bar (the transmitter I assume) and
a loop of similar size (no Lyden jar) with a spark gap (the receiver).
Presumable such an arraignment did not have the same resonant frequency. It
must have taken Hertz a very long time to tune his transmitter and receivers
to the same frequency. Thinking about that statement. Its based on the
assumption that the loops would have high Q which is probaly not true
because of there radiation resistance. So perhaps his receivers only needed
to be with in a octave or so of the frequency of the transmitter.