150 W Dummy Load with Wattmeter
At our Milford Amateur Radio Club hamfest, we discovered that many local hams are interested in building projects. As a result, we formed the Greater Cincinnati Builders Group (GCBG) to share in each other’s passion for building Amateur Radio gear. Twenty-eight members of the GCBG were able to build the dummy load (DL) described on this page.
See the full article under Hardware – DL Article and the assembly manual and code are under Downloads.
Every ham should use a dummy load, especially with older equipment where a lot of knob-fiddling takes place each time you change frequency. This DL can easily be built in a single session. The circuit is simple, as is the code that runs the DL. The project is fairly inexpensive, at under $20.
The Resistor Network The dummy load is constructed from 20 non-inductive metal film resistors connected in parallel between two copper wire loops. Each resistor (see the lead photo) is 1,000 at 3 W. In order for the resistor network to fit inside a quart can without touching any part of it, each wire loop is about 3 inches in diameter, spaced 2 inches apart. The resistors are wired in parallel, so the network looks like one large 50 60 W resistor. By bathing the resistor network in a quart of mineral oil, we increase its dissipation capability to more than 150 W. We use mineral oil rather than transformer (or other) oil because it is non-toxic.
The circuit diagram shows the wiring for the DL. The small OLED display is controlled by an Arduino Nano. We buy these five at a time on the internet for about $3 each. The OLED is a small (0.96-inch) 128 × 64 display and uses the SPI interface, which minimizes the number of connections between the OLED and the Nano. Our display supports two colors and costs about $4. Cheaper displays are available, but make sure the display supports the SPI/I2C interface. Many suppliers are willing to give small discounts on quantity orders for group purchases.
The Dummy Load Code for the Nano is available on the Download Page. It is a standard Arduino IDE file that can be modified to suit your needs.
The code samples the power input 30 times — as determined by the variable named ITERATIONS — by reading the A1 SENSORPIN analog pin. This allows us to construct an average power reading. We used an average because the applied power could be changing due to tuning by the user or environmental factors, like the oil getting hotter. The 1-second delay at the bottom of the code loop is done to reduce flickering when the display is updated.
Dummy loads should be used whenever you are tuning on a band. If you have an antenna switch, connect one port to the DL. That way, with a quick flip of the switch, you can tune to your heart’s content without affecting other hams on the air. It can also be useful if you’re using a 100 W transceiver in a low-power environment or a contest as a quick way to make sure your settings are correct and that you’re playing by the rules. This 150 W dummy load is a great club build project because it’s useful, easy to build, and inexpensive.