Dummy Load 50 Ohm 50 Watts

I thought it would be useful to have a dummy load to take power measurements. Rather than buy one I decided to make my own (as usual!) I purchased 10 x 470 Ohm 5 Watt Metal Oxide Film resistors. They are low inductance which is very important at RF. My power is limited but I decided to make it rated for 50 Watts. 10 x 470 Ohm in parallel gives 47 Ohms nominally. Parallel connection means that the power rating becomes 10 x 5 = 50 Watts nominally. (I read one article online which stated 4 x 100 Watt resistors in series gave him 400 Watts. This is incorrect! A common mistake.) The main problem I find with most projects is the hardware, in particular, cases and enclosures. Mooching around, I found this sewing tin with a hinged lid. Using a metal enclosure ensures good screening and prevents any transmissions radiating from the unit. A quick inspection found that the SO-239 socket would just fit on the side, provided I cut the lid away to allow it to close. The metal is very thin, so care is needed when drilling the holes. Sharp drill bits and proper support whilst drilling is the key. When assembled I measured the DC resistance with a multimeter at 47 Ohms. I'm not sure how accurate my multimeter is.

                                        

I assembled the project in about 2 hours. The tin made for a neat compact unit. everything just fit inside really snug. Total cost was about £6.00. A lot cheaper than the commercial ones and it is rated at 50 Watts.

The resistors were all soldered onto 2 pieces of solid copper wire and covered with heat shrink sleeving. A bit of hot glue secured everything in place. The hot glue you can see on top is covering the centre feed wire to the SO-239 to prevent any accidental short circuits. The ground wire was attached via a small crimp-lug and bolted to the holding screw. On reflection, I could have put some sleeving over the wires. I've tested it up to 20 Watts on all HF frequencies and the SWR reading does not change from 1:1. If operating at higher powers, the tin could be filled with dry sifted sand to dissipate heat, and then sealed up.

Update:

I did fill it with sand and seal it up because I noticed that it got quite warm when testing up to 25 Watts. Hopefully, problem solved.