I'm looking for a resistor about 25 Ohm at 13.56 MHz (this is our operating frequency). The resistor must withstand at least 50 W (100 W is better!) at ~ 1 kV voltage amplitude. As I will use this for compensating the phase difference between the measured voltage and current from our voltage probe and a current transformer, It also must have a low capacitance and inductance. I originally thought a wirewound type resistor would be a good choice due to its strength for high power and high voltage, but I immediately realized that it might have bad inductance and, possbily, capacitance. What type of resistor is the best choice for me?
Resistor, 25 ohm, 200W, non inductive, RF, SOT-227
This resistor is made to be used with a heat-sink. In order to size the heat-sink you need to know how much wattage you need to eliminate and how much the temperature is allowed to rise across the heat sink.
Lets assume that you are working in a lab with ambient temperature Ta = 25C. The resistor datasheet also says the maximum allowed junction temperature of Tj_max = 155C. Therefore your maximum allowed temperature rise is...
dt = (155C - 25C) = 130C
Your post says that you have a maximum dissipation of W_max = 50W max. In that case the maximum allowed Junction to Ambient Thermal resistance R-ja-max is...
R_ja_max = dt / W_max = 130C / 50W = 2.6C/W
The resistor datasheet says that it has a thermal resistance of Rth_resistor = 0.35 Kelvin/Watt between the resistor baseplate and the heat-sink. R_ja_max includes all the thermal resistances along the whole thermal path. In a simple case this is Rth_resistor + Rth_heat_sink. Therefore the maximum thermal resistance of your heat-sink must be...
Rth_heat_sink = R-ja-max - Rth_resistor = 2.6C/W - 0.35C/W = 2.25C/W
You can pick any heat-sink that has less than 2.25C/W thermal resistance, so long as it fits mechanically. Alternatively if you have any large metal cabinets that are part of your equipment you can bolt the resistor to the frame (which will almost certainly have very low thermal resistance).
The old school way of doing this was to use many resistors in a series / parallel combination .Old ARRL handbooks describe this well .Wirewound resistors were not used due to inductive effects .Carbon resistors were used when I was a toddler .Metal film resistors would be suitable .When you bunch a lot of resistors together they will not do anywhere near thier power rating .I used a small 12V computer fan to cool my bunch of thru hole phillips PR03 metal film resistors .This worked fine for me at 3MHz .You could also try TO220 power resistors .They cost but you will not need so many of them and they easily bolt on to a heatsink just like a standard TO220 power transistor and better still the tab is isolated .
I am sensing an XY problem here, 13.56MHz is not that high a frequency, and 100W is not that much power, so standard RF methods should be applicable for most measurements.
While plenty of folks make thick film TO220 resistors in reasonable power ratings (Watch the temperature derating on these, they tend to specify them at 25 degrees case temp), I cannot help but suspect it is the wrong answer.
What are you actually trying to achieve?