In article ,
Jeff Liebermann wrote:

I'm also wondering if the electrical characteristics of water are an
issue for an HF dummy load. Even if someone dumps salt water into the
paint can, the conduction losses to ground from the resistor to the
can ground can't be all that much below 30MHz.

It's probably not worth much, but here's some anecdotal evidence. About 50
years ago several of us at W8LT (The Ohio State University amateur radio
club) needed a low-power dummy load to be used at moderate frequencies (e.g.
20 meters). I came up with the idea of immersing a 2 watt, 51 ohm carbon
resistor in a large glass of Columbus, Ohio tap water. We found that at 14
MHz putting the resistor under water made negligible change in observed SWR,
which surprised me because Columbus water (unlike the stuff we're blessed
with here in the Vancouver, BC area which comes right from the mountain
snow-pack) was fairly hard. This also indicated that the water didn't put
too much capacitance in parallel with the resistance. Water-cooling did
allow the 2 watt resistor to absorb a fair bit more power than 2 watts, at
least for a while until the water heated up too much.

Thermal Conductivity Viscosity
W/mK cSt @20c
Water 0.6 0.9
AF (glycol) 0.24 2.0
Water+AF 0.8(?) 1.5 50%/50%
Silicon Oil 0.1 varies radically
Mineral Oil 0.138 34.5
Fluorinert FC-77 0.063 0.75

Ok, I see why. Water has 1/5th of the thermal conductivity of mineral
oil.

It's still a heck of a lot better than air.

As I recall, we were in a hurry, we had the resistor, and we didn't have any
suitable oil on hand.

David. ex-W8EZE

--
David Ryeburn

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