Posted by Fred C (W6BSD) on Apr 04 2022

It is often helpful to run tests and take measurements without connecting an antenna. Ham radio operators or anyone who builds or experiments with radio transmitters should have a good dummy load. Since I didn't have such a device, I decided to make one.

In radio, a dummy load1 is a non-inductive power resistor with a value of 50Ω, used in place of an antenna. It electrically simulates a "perfect" antenna. It's used to allow your radio to transmit with full power without radiating a radio signal. The power of the transmitter or amplifier is converted to heat.

I needed to build a dummy load that could handle the full power of my transmitter: 100 Watts. The goal was to give me enough time to take the necessary measurements or make adjustments.

The challenge was to dissipate the heat generated by 100 Watts with a 100% duty cycle. Usually, the resistors are placed on a large heat sink. The heat sink drives the heat away from the resistor and transfers that heat into the ambient air. Sometimes, a fan is used to help transfer the heat into the air. On high power dummy loads, liquids are used instead of air. They allow a more efficient heat transfer.

I wanted my dummy load to be compact. I didn't want to use fans because they require an external source of power. Large heat sinks were not an option because I wanted the dummy load to be compact.

Some ham friends use a dummy load made of a high-power resistor dipped into a one-gallon paint can full of mineral oil. Unfortunately, these cans are made of cheap thin metal, and they all start leaking after a while. The weak point is where the connector is installed on the lid. Any stress on the connector introduces metal fatigue. I also have an example of someone dropping the can on the ground. With the impact, the lid opened, and his radio shack floor became a big oily mess.

I wanted to use a sturdy box smaller than a gallon can for my dummy load. I chose a BUD Industries air-tight Aluminum enclosure with four screws to hold the lid in place. The walls are 5mm thick. A continuous silicone gasket ensures the box is air-tight.

I found a couple of 100Ω 250 Watts high-frequency resistors on eBay. (You can see an excerpt of the resistor's datasheet at the end of this post)

A thick 3" x 3" aluminum plate holds the two 100Ω resistors. They are mounted in parallel to give a total value of 50Ω. The 250 Watt resistors mounted in parallel can handle 500 Watts, as long as I can maintain a temperature below 150 ̊C. Considering the size of the box and the coolant volume, I won't be able to use this dummy load at 500 Watts, but I should be able to send up to 250 Watts for several minutes before the resistors become too hot.

A Type-N connector is mounted with four machine screws on the side of the box. There is silicon glue around the connector and the screws to make sure that the oil won't leak.

I use a coaxial cable from the connector to the resistor. It ensures that the 50Ω impedance is maintained close to the resistor.

## Parts list

Description Quantity Price
RF Termination Flange Mount 250W 100Ω Resistor 2 $27.17 BUD Industries AN-1304-AB IP68 Aluminum Enclosure 1$22.40
Chassis Mount Type-N Female RF Connector 1 $4.80 Total:$54.37

The total of \$54 does not represent the device's total price. You need to add the following costs: the hardware, the oil, and the few inches of RG316 coax. I already had these parts from another project.

RG316 is a thin, flexible coax for applications requiring high power and small coax.

## Performance

The formula to calculate the VSWR from the return loss is:

$$VSWR = \frac{1+|\Gamma|}{1-|\Gamma|}$$

with:

$$Rl_{(dB)} = -20 \log |\Gamma| \\ \Gamma = 10 \cdot \frac{-Rl}{20}$$

I use a NanoVNA to measure my dummy load return loss. The graphs below show a return loss below -30dB on the HF portion, giving a VSWR below 1.065:1.

On the VHF bands, the return loss is less than -30dB on the 6-meter band and rises to -28dB for the 2-meter band, which gives a VSWR of 1.083:1.

On UHF, for the frequencies I am interested in, the return loss is -34dB, giving a VSWR of 1.04:1.

This dummy load is perfect for an amateur radio operator. It provides an excellent 50Ω match on all the ham bands from HF to 440MHz.

## Resistors datasheet

Flange mount 250 Watt resistor specifications.

SPECIFICATIONS
--------------
1.0 ELECTRICAL
Resistance Range: 10 - 400 OHMS
Resistance Tolerance: ±5% standard 1% and 2% available
Typical Capacitance: 1.64 pF
Input Power CW: 250 watts @ 100 ̊C heat sink, derated linearly to zero power at 150 ̊C
Peak Power: 2500 watts (based on 10us pulse width and 1% duty cycle)
2.0 ENVIRONMENTAL
Operating Temperature: -55 ̊C to +150 ̊C
Non-operating Temperature: -65 ̊C to +150 ̊C
Temperature Coefficient: +/-200 PPM /  ̊C max