>Not knowing the innards of the Cantenna, may I suggest a quick test for
>the usefulness of simply a bigger can of oil. The test would be to see
>if the sides of the existing Cantenna are warm after the "10 or 20
>seconds" at 2KW claimed to be the maximum before VSWR change. If the
>outside of the can isn't warmer than before the test started, then a
>bigger can filled with more liquid will not help. It would seem that
>one would have to get the oil circulating around the resistor better to
>justify a larger reservoir. My guess is the can surface will NOT be any
The can does get warm after running 1500 watts on it for a bit.
Most RF resistors have limitations in power handling due to their ability to
get heat out of the resistive junction. If one has an infinite and ideal heat
sink, one can theoretically dissipate 100 Watts in an RF chip resistor that is
0.1" x 0.2". We used to spec our parts this way (the entire RF resistor
industry specs stuff this way.). However, this was very confusing for
customers. So now we just say that with a heat sink temperature of 100° C and
a film temperature of 150° C, the part will safely disspate 10 Watts.
So...if you can get the heat out of the part, you can run more power through it
for longer time.
Now, the oil in the cantenna acts as a heat sink to pull the heat away from the
resistor. This oil has some certain capacity to absorb heat effectively (I
forget the proper scientific name). For a given volume of oil, it can absorb
so many calories until it, in effect, saturates thermally at which point it
will no longer absorb heat. Hence, the derating curve. More oil in the can
creates a bigger volume of oil to heat up which will allow you to run more
power for a longer period of time. It is simple thermodynamics.
The water cooled loads are especially good because you always have a fresh flow
of water over the device. Therefore, the heat absorbing capacity of the water
is never reached. I believe that oil has a better absorption capacity
otherwise, they would use water in the cantenna. They use water in the "water"
cooled loads cause it's easier to hook to a water spigot than an oil spigot.
The cantenna may indeed have a 100 Watt rated resistor. But for what
temperature and operating conditions? It's probably 100 Watts in free space
with no air flow or heat sinking. However, add the oil and you've increased
the dissipation capacity of the resistor tremendously. The temperature of the
resistor is what really matters and is what will cause a failure.
>Is it true that the Cantenna's VSWR changes appreciably after 10 to 20
>seconds of 2KW?
Not sure. I've never put 2KW into one. However, it wouldn't surprise me that
as the oil heats up it's dielectric properties change and you also might have
some heating effects in the resistor itself.
"A life lived in fear is a life half lived."
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