Posted by
Ron D'Eau Claire-2 on
Oct 07, 2006; 5:22pm
URL: http://elecraft.85.s1.nabble.com/Baluns-wires-feeders-oh-my-tp394281p394290.html
The usual approach to measuring the loss of any device like this is to
measure the heat. Basic physics - the first law of thermodynamics - tells us
that the energy going into the device will equal the energy going out. The
question is how much of that energy emerges as RF. In this case there is
only one other significant form of energy involved: heat. (Yes, if the
device is arcing over there will also be light and sound, but I'm assuming
it *seems* to be working okay; no arcing and buzzing <G>.)
So you put the device into a very well insulated test chamber. Dense
urethane foam is commonly used for this. Then it is operated under the exact
conditions you want to evaluate (high SWR through a BALUN at normal power
levels, etc.) and the heat rise in the chamber is noted on an accurate
thermometer over a specific period of time.
Then you can calculate how many watts of RF were turned into heat. Not into
doing the math? You can figure it out empirically by putting a resistor in
the box with the device, let the whole thing return to ambient after
measuring the temperature rise over time operating the device under test.
Then do not apply RF to the balun (or tuner or whatever you're testing) but
instead put DC current through the resistor sufficient to raise the
temperature the same amount in the same time. The power dissipated by the
resistor (volts times current) will be equal to that lost from the balun,
ATU or whatever. Obviously, it might take several runs to find the exact
value but you can learn something very useful from a limited test. For
example, if you run 10 watts through a balun and the temperature goes up 2
degrees in 10 minutes, then you put 1 watt through the resistor and note
that the temperature goes up more than 2 degrees in 10 minutes, you've
demonstrated that the loss is less than 1 watt, since that amount of loss
would cause a faster temperature rise.
Does all that sound like a little bit "much" for a test on a quiet Saturday
afternoon? That's why most Hams ignore the whole issue, hoping that the
losses aren't too high. That's easy to do since in most on-air operating,
especially on HF, a loss of even 10 dB is hard to spot unless once can do
side-by-side testing with a distant station. So unless the losses are truly
large, we'll never know for sure they are occurring. And if they are, we
notice that the balun feels warm, the cores of the toroids cracked, or it
arced over, Hi!
On the other hand, arranging these experiments and working through them are
a wonderful way to better understand not only Ham radio, but your equipment
and physics as well.
Ron AC7AC
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