I use a DL made from four 50 ohm / 400W flange-mounted
RF resistors*, in series-parallel. VSWR is very good
though 6m. In the lab at a former employer, I had access
to an RF power source of 660W @ 40.68 MHz. Based on
temperature rise, the useful power rating is almost
1600W @ 25C ambient.
The resistors are mounted on two heatsinks with 140 sq
inches area each. The heatsinks are arranged with the
mounting surfaces facing each other, to minimize connection
length. Copper strips are used to minimize series XL.
The cooling fins are arranged vertically, and a 100cfm
fan blows air upward through the fins. I use a LM35DT
"Precision Centigrade Temperature Sensor" to monitor
heatsink temperature. Some photos can be found here:
http://www.tinyurl.com/wa7prc-dummy
No messy cooling liquids are needed, and it handles all the
power a ham can legally throw at it. Every once in a while,
I pick the dust bunnies out. A future upgrade (when I get a
round tuit) will be RF and temperature sensing to auto-
magically control the fan, etc.
* Similar resistors and terminators can be found on eBay
from Henry Radio (http://stores.ebay.com/Henry-Radio)
as well as others.
vy 73,
Bryan WA7PRC
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