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[AMPS] Cantenna Dummy loads

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Subject: [AMPS] Cantenna Dummy loads
From: (Jon Ogden)
Date: Wed, 3 Feb 99 13:34:07 -0600
>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.



Jon Ogden

"A life lived in fear is a life half lived."

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