During the design and construction of my 160 mtrs power amplifier the need arose to have a well defined 50 Ohm load. This article describes the construction of such a 50 Ohm load capable of sinking 4 kW with 100% duty cycle for very long period of time. Constructed is water-cooled load consisting of 18 times 250 Watt high power flange mount resistors.
Mainstream dummyloads for HF bands are limited to 1 kW of power capabilities. If you need more powerhandling this design is a way to get into the 4-5 kW region with regularly available components. Used are 18 times 100 Ohm high power flange mount resistors. Care should be given to adequate cooling of the resistors in order to keep the mounting tab temperature below 100 degree °C. With an ambient temperature of 25 degree this will lead to a heatsink with heatsink-ambient thermal resistance below 0.02 °C/Watt. To achieve this with standard heatsink design a very big heatsink with many vins is needed, even with additional forced air cooling.
Besides power capability of more than 4 kW continues this dummyload will provide adequate return loss accross all HF bands below 30MHz. Due to the rather high self capacitance of the flange mount resistors (approx. 6 pF) return loss will deteriorate quickly above 50 MHz.
This design is using 18 times 100 Ohm high power flange mount resistors capable of 250W dissipation each. Resistors used are ATC FR10975N0100J01. These resistors are placed in 6 parallel rows consisting of three 100 Ohm resistors in series. This will lead to 50 Ohm input impedance (3 times 100 divided by 6). Resistors are mounted on a aluminium plate/sheet construction. Aluminium construction consists of two 10mm think aluminium plates in which a single turn copper tube is milled (milling cutter). By running water trough this tubing adequate cooling can be achieved. To get rid of the absorbed heat by the water a small heat exchanger is used along with a couple of ventilators to force a decent flow of cool air through this radiator. In principle the heat exchanger could be omitted by using a bigger water reservoir. In this design the water reservoir is quite small (a 50cm tube of 11 cm diameter), holding approx 4 liters of water. A small water (aquarium) pump, submersed in the water reservoir will provide some waterflow.
Added is a -50 dB output coupling for measurement purposes. This is done by adding a small resistor in series with one of the 100 Ohm resistors and pick off the voltage via a 50 Ohm series resistor to get the coupling output return loss low.
For more pictures of the complete assembly go to:
30 MHz , better than 25 dB return loss (RL) , SWR better than 1 : 1,12
50 MHz , 18 dB RL , 1 : 1,3
100 MHz , 10 dB RL , 1 : 2
150 MHz , 8 dB RL , 1 : 2,3