Ham Radio dummy loads

May 19, 2016
Technical Reference : Dummy

A dummy load is just one of those standard tools that you need in the shack. It's a load that has a purely resistive impedance of 50 ohms at all frequencies of interest (HF in my case). You don't need to pollute the airwaves while just tuning or testing your transmitter!

Dummy-loads are easy and relatively inexpensive to build yourself, using one or more suitable resistors. How inexpensive, depends on the dissipation you need (QRP (5-10 W) or (much) more) and how long you want to be able to apply power without frying the dummy load and setting your shack on fire.

So, the basic issues are:

how to get a wide-band purely resistive 50 ohm load with SWR very close to 1:1.

how to dissipate the heat that is generated in the resistor(s).

The first issue is easy to address: use non-inductive resistors, and keep wiring short to minimize capacitance. Options:

single 50 ohm resistor with sufficient dissipation rating

multiple resistors with lower dissipation rating, in parallel or series-parallel configuration. E.g., 20 parallel metal film resistors of 1 k ohm and 2 W rating to get 50 ohm and 40 W. I used four 50 ohm hybrid resistors with 100 W rating (acquired via eBay; total $11 + $15 S&H, mid-2009)" and "2-series of 2-parallel". I have chosen the latter. Note that this type of resistor is also available with 100 and 200 ohm value, and up to 800 watt!

The second issue is not quite as easy to solve. Without additional measures, the resistors may overheat and be destroyed in relatively short time, even at nominal power. The hybrid resistors can only be used anywhere near their power rating, if 1) they are mounted on an adequately dimensioned heatsink, and 2) there is good thermal contact between the resistors and the heatsink. So: the resistors must be mounted on the heatsink with thermal paste (a.k.a. thermal grease or thermal compound).

Dissipation can be further improved if the resistors and heatsink are actively cooled with a fan, or they are immersed in oil. Pure mineral oil (a.k.a. paraffin oil) is a good choice, and is available in drug stores and pharmacies. It is safe for human consumption (though it does have laxative effects...). Baby oil usually is just pure mineral oil with a fragrance (check the label). Mitch, KJ7JA, informed me that mineral oil is also available at farm supply stores. This veterinarian-grade oil is a lot less less expensive than the food-grade variety.

In the past, traditional transformer oil was used. That type of oil often contains highly toxic PCBs! The resistors and heatsink (if you use one) can be mounted in a tin can (from loose-leaf tea, or an empty paint can - some paint stores sell clean, empty cans).

With proper dissipation, using four resistors with 100 W rating should allow me to use my dummy load for 400 W. I have mounted the resistors on a heatsink, immersed in mineral oil in a paint can of 1 quart (almost 1 liter).

Source: www.nonstopsystems.com
Amateur Radio High Power 50 Ohm Dummy Load
Amateur Radio High Power 50 Ohm Dummy Load
MFJ 250 Antenna Dummy Load Video Review by AA1PR Part 1
MFJ 250 Antenna Dummy Load Video Review by AA1PR Part 1
The Upgradeable Ham Radio Dummy Load Part 3: Upgraded
The Upgradeable Ham Radio Dummy Load Part 3: Upgraded

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