On Fri, 2 Jan 2009 01:35:16 -0500, "NoSPAM"
wrote:
However, the Cantenna relies on natural
convection of the oil for cooling and the higher viscosity of commonly
available silicone oils will limit the power handling capability of the
Cantenna.
So, a low density fluid would be the better choice? If that were the
prime criteria, I would think that water or antifreeze would be the
best. If having convection currents agitate the fluid is important,
it would be easy enough to install a paddle stirrer and thermometer.
Oh wait. This is ham radio. Forget the thermometer.
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.
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. 50/50 water and antifreeze won't work. That raises the boiling
point but ruins the thermal conductivity. Pure ethylene glycol looks
tolerable. Other than the health and ecology issues, any reason that
100% antifreeze won't work?
Incidentally my theory is that the Heath engineers of the 1960's used
mineral oil because that's what's inside a Lava Lamp. They were
probably designing Heathkit Lava Lamps but when that failed, they had
to do something useful with the mineral oil.
Remember that the Cantenna must be de-rated when used for long
duty cycles, and a high viscosity oil will lower the power rating still
more. Also remember that silicone oils are not cheap (and my buddy at Dow
would only send me small samples).
Well, if you want expensive and near perfect, there's 3M Fluoinert.
http://en.wikipedia.org/wiki/Fluorinert
http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxTE5X46EVuQEcuZgVs 6EVs6E666666--
About $400/liter for recycled FC-77 and $2,200/liter for the new
FC-770 stuff from various 3M dealers. A 1 gallon cantenna required
3.78 liters or about $1,500 in recycled FC-77. For those hams that
want the very best. Gold plated paint can is optional.
I would suggest using modern RF terminations made by Bourns and other
companies. These are designed to be bolted to a large heatsink. The
CHF9838CNF series is rated for 50 ohms, 250 watts, VSWR below 1.1 from DC
to 2.2 GHz. It only costs $27.50 in single lot quantities. I think this
is higher than the continuous rating of the Cantenna. I don't know for
sure as I disposed of my Cantenna years ago. Digi-Key sells these Bourns
terminations if you want one.
Nice. Thanks.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CHF9838CNF500L-ND
http://www.mouser.com/Search/Refine.aspx?Msid=65210000&Keyword=CHF9838CNF500L
http://www.bourns.com/data/global/PDFs/CHF9838CNF.pdf
I kinda prefer to use 4ea 200 ohm loads in parallel. In a previous
power amp design, I had to dump 200 watts into a load if the antenna
failed. The thermal resistance of the flange mount was too high for
one device to handle the load at the rated temperatures. However, 4
devices did the trick.
Barry L. Ornitz, PhD WA4VZQ
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558