I made a dummy load before using a bunch of resistors, a paint can, and added some electronics so it could measure the radio’s power output. Unfortunately that dummy load is only good through 2 meters and a lot of my stuff is done on 70cm now.
This dummy load is a bit different and is based on a design from Nuts & Volts Magazine, 2019, Issue 3.
|100w 50ohm RF Resistor
|SO-239 Panel Mount Connector (or other suitable connector)
|4-40 Bolts ½ inch Long
|4-40 Bolts 1 ¼ inches Long
|Washers that fit 4-40 bolts
|1in long piece of heat shrink tube Size 1/4in Wide
|12V PC Fan 80mm
|Aluminum Sheet 10in X 4in X 1/16in Thick
|PowerPole or DC Connector
|Solder Wick / Copper Braid / 16AWG Solid Copper Wire 1in Long
|Stick on Rubber Feet
|Good Quality Thermal Paste (Arctic Silver or similar)
- Drill & Drill Bits (3/32 Inch & 5/8 Inch)
- Soldering Iron & Solder
- Wire Strippers
- PowerPole Crimping Tool (If you use PowerPoles)
- Screw Driver
- Socket or Pliers for the 4-40 Nuts
- Block of wood at least 5in long on one side
- Permanent Marker
- Ruler or Tape Measure
- Measure 4 inches in from the end of the aluminum sheet (this will make a 4in square on the sheet) and draw a line.
- Next measure ½ inch from the other end of the aluminum sheet and mark that line. These lines are where you will bend the aluminum, but don’t do that yet.
- Now mark the center of the 4in square you created with that first line, by measuring in 2 inches from each side or use a ruler to draw two diagonal lines from the corners of the aluminum sheet to the corners of the line you drew. Where these lines meet should be the center of your square.
- Next place the aluminum sheet on your block of wood and clamp it down.
- Drill a 5/8 inch hole through the aluminum sheet on that center mark. The wood will ensure you don’t drill through your good desk or table. 🙂
- Now ensure the SO-239 will fit in that hole. You may need to file the edges a bit with a round file.
- Once the SO-239 fits in the hole, mark the 4 holes around the connector and drill them out with the 3/32 inch drill bit. I would place the wood under the holes you’re drilling.
- Now place the aluminum sheet over the block of wood, lining up the line you drew with the wood and bend it over. It might be easier to do this in a tabletop vice.
- After that, use 4 of the ½ inch 4-40 bolts and 4 nuts to secure the SO-239 into the aluminum sheet.
- Now place the resistor on the inside of the sheet as close to the SO 239 connector as possible. Remember that your nuts and bolts must fit. Mark the two holes for the resistor.
- Drill two 3/32 inch holes for the resistor. DO NOT drill through the holes already in the resistor, remove it first.
- Apply thermal paste to the bottom of the resistor.
- Ensure that the center pin of the resistor (the little foil tab) is facing the SO-239.
- Put two ½ inch long 4-40 bolts through the resistor and the holes in the aluminum sheet. Add one washer to each bolt against the aluminum sheet. Then add one 4-40 nut to each bolt and hand tighten only. You just want it slightly snug, don’t break the resistor.
- Next, add flux to the foil tab of the resistor and solder a small piece of copper braid (or wire) to the foil tab, slip the heat shrink tubing over the wire and foil tab and then solder the other end of the braid to the center pin of the SO-239.
- Place the aluminum sheet over the wood again, lining up the other line with the edge of the wood or edge of the vice and bend it up.
- Place the fan against this lip of the sheet and mark the holes. Drill two 3/32 inch holes.
- Place the fan so that it blows air towards the resistor and put the fan on the inside of the lip.
- Next using two 1 ¼ inch long 4-40 bolts, 2 washers, and 2 nuts, bolt the fan to the sheet with a washer between the fan and the nut.
- Next add a PowerPole or DC connector to the fan’s wires.
Now you have a 100W dummy load that should work from 160m through about 70cm. Use the least amount of power possible when testing. I wouldn’t use this over 50w except for very short durations. The more power you use, the shorter the life of the resistor and you could burn it up.
So how does it stack up? Check out these SWR graphs. Under 1.2 to 1 SWR from 1.8Mhz to 148Mhz is good enough for me. On 70cm it has an SWR under 1.46 to 1, so it’s not the best, but it’s usable.