On Tue, 17 Aug 2004 15:50:05 GMT, Richard Clark
wrote:

However, as to the statement above, and the presumptions that follow,
there is no equilibrium to observe as the test implementation has been
described. You are pouring calories into an insulated environment
which can only raise temperature without end (short of destruction, of
course).

As I described elsewhere, this is called a caloric bomb, and as such
should be limited in time for all comparisons.

You seem to assume that no thermal power will flow through the
styrofoam walls of the box.

Your statement is true if you insert a huge amount of power into the
box, in which case the temperature would climb, until the styrofoam
would melt. However, the measurement can be done at much lower power
levels that are compatible with the thermal conductivity of the
styrofoam.

If styrofoam would be an ideal isolator, you could put some deep
freeze food into a styrofoam box, move to your summer cottage and hope
that the food would still be eatable after a week. Unfortunately this
is not true :-(.

When doing antenna efficiency measurements, the power levels should be
set to a level, in which the temperature increase is manageable (below
the melting point of the styrofoam).

To give an example of the power levels required, assume that
you insert a 1 kg (1 liter) bottle of drink at 0 C into the test
container and after 10000 seconds (about 3 hours) the drink
temperature is at +10 C, the energy needed is about 40 kJ, thus the
leakage through the container walls would be about 4 W. With the
outside temperature of 20 C, the average temperature difference would
be 15 C and the thermal conductivity about 4 C/W. This is what you
usually get with some power transistors with a heat sink :-).

If you build an igloo around the "miracle" antenna, you should be able
to measure the heat generated (and thus efficiency) with power levels
well below the legal limit in most countries and with bad antennas
10-100 W should be enough.

Of course, you should make sure that the styrofoam own RF dissipation
is sufficiently small at the test frequency.

Paul OH3LWR