On Mon, 16 Aug 2004 11:42:19 -0700, Roy Lewallen
wrote:

It's not clear to me how you could get quantitative data from
measurements in a styrofoam container. Let's say you put 100 watts into
the antenna for five minutes and the temperature rise (of the coil?
helical winding? air?) was 10 degrees C. How would you calculate the
loss or efficiency from that information?


First of all, the test should not be run for 5 minutes, but until a
thermal equilibrium inside the container has been established,
possibly mixing the air inside the container with a fan. This solves
the thermal mass problems when equilibrium has been established.
Measure the power fed to the antenna.

The temperature outside of the container should remain stable during
the test. Then the temperature difference across the container wall is
known, as well as the wall surface area and thickness and hopefully
also the thermal conductivity for styrofoam (from the manufacturer).
Now the thermal resistance of the container as well as the temperature
difference is known and the power dissipation can be calculated. This
is very similar to calculating the heatsink requirements for
transistors.

If the thermal conductivity for the styrofoam is not known, run a
predefined amount of power to the antenna until equilibrium has been
reached and measure the temperature. Inside the container, disconnect
the antenna, attach the feedline to a dummy load (possibly attaching
it to some large metallic part of the antenna to act as a heatsink).
Close the container and feed RF-power into the dummy load at an
adjustable power level, until the same stable temperature is reached
as with the actual antenna. Measure the power fed into the dummy load.
This will eliminate the feedline conductivity problem as well as
offset errors in the power meter (but not linearity errors).

This way the power dissipated by the dummy load is the same as the
power dissipated in the antenna losses in the first place and thus,
the efficiency can be calculated.

To avoid any power meter linearity problems, reconnect the antenna
again, feed in the original full power and verify that there is a
thermal equilibrium at the original temperature and outside
temperature. Insert a calibrated step attenuator ahead of the power
meter and adjust it until it reads the same value as in the dummy load
test.

You can then read directly from the step attenuator, how many dBs the
total full transmitter power was compared to the dummy load power
which is also the antenna thermal dissipation value. A 10 dB setting
would indicate that the thermal losses are only 10 % of the total
power or 90 % antenna efficiency, a 3 dB reading would indicate that
half of the power is radiated and the other half is dissipated in the
antenna, thus 50 % efficiency. However, you would need a step
attenuator with 0.1 steps to measure the worst antennas :-).

Paul OH3LWR