Build a 50W Dummy Load for Cheap

Need a dummy load to test transmitters but don’t have the money for an expensive commercial one? Don’t worry, you can build one for ~$20 (or less!) easily.

Parts you will need:

• 24 x 1.2kOhm 3W axial film resistors (I ordered 25 on digikey to get a better bulk price) — $5.94
• 1 x BNC female connector, panel mount (I got these in a pack of 20 for $8.90 on amazon, or you can get them individually ~$2 each) — $2
• 2 x 150mm x 100mm double side FR4 copper clad board (pack of 5 for $12.99 on amazon, about $5.20 for two) — $5.20
• A ~2 inch piece of 12-16 gauge solid copper wire. (or you could just use a resistor’s leg if you don’t have one)

And that’s pretty much it! Make sure you get resistors that aren’t wirewound, as those are inductive. Mine were metal film and they worked fine. It’s likely you already have the BNC connector or copper clad board if you have homebrewed before. You don’t have to use 24 resistors, you can use more or less to accomodate more/less power. If you use n resistors, just remember for the equivalent resistance to be 50 ohms, your resistors should have a resistance of n x 50 ohms. (in this case 24 resistors x 50 ohms = 1.2kOhm)

First cut your 2 copper clad boards into 100mm x 100mm squares. You can cut FR4 by scoring along a line with a utility knife/dremel/needle file, then snapping the board.

Next stack your two square boards on top of each other, and clamp them onto something you can drill onto. Drill a uniform 5x5 grid of holes with a 1/8" drill bit, through both boards at the same time. (makes it 2x faster)

Cut one lead on each of the 24 resistors so the lead sticks out about half inch from the resistor’s body. Now for each resistor, insert that trimmed end into every one of those drilled holes (except the middle one), and solder them into place as you go:

Next, on the other square board, enlarge the middle hole with a 3/8" drill bit to accomodate the BNC connector. Install the connector.

Next comes the hardest part. Take the board with the BNC connector and place it on top of the other board that has the resistors sticking up, and align the resistor legs to fit into the holes of the upper board. You may need to do this at an angle, one row of resistors at a time. When you get to the middle row of resistors, take your ~2" piece of solid wire, and solder one end to the center conductor of the BNC jack, and the other end to the bottom board. This effectively sets the distance between the two plates. Then once all the resistor legs are inserted into the top holes, solder them to the top board, and cut off the excess leads that may be sticking out with a flush cutter.

The dummy load is now complete. Remember that the bottom board is electrically connected to the center conductor of the BNC (via that 2" wire in the center), and the top board is connected to the outer conductor of the BNC. These conductive surfaces are exposed, so please don’t touch them while transmitting into the dummy load.

Verify your dummy load has a DC resistance of 50 ohms with a multimeter. My multimeter reads 49.5 ohms, which is close enough to 50. Transmitting into this dummy load, my SWR meter reads 1:1 for all bands 1.8-50MHz, meaning any parasitic inductance/capacitance this dummy load has is insignificant at HF. It can take 20W continuous keying down, or 50W SSB without overheating.

Parasitic Capacitance

Just for fun: You could roughly estimate the parasitic capacitance of this dummy load as it looks like a parallel plate capacitor:

Using A = 0.1m x 0.1m = 10-2 m2, d = 0.05m, e = 8.85 x 10-12 m3 kg-1 s4 A2 , k = 1 for air gives C = 1.8pF.

Probably less because of edge effects/a non-uniform electric field. Cool to actually use the physics I learn!