51 ohms is very common , if you look at the data/spec sheets on almost anything , you'll see 52 Ohms , 50 ohms is the norm accepted.... I would have put the SO-239 on the end of the box and the resistor on the side of the box to act as a heat sink... Or add more resistors in the future for a heavy duty load, to handle you bigger rigs.. You could have went to the scrap yard and picked up an old stereo and yanked the heatsink out of it and attach your resistor to it and add 4 standoffs for the SO-239... Unless you intended the box to carry spare items for a field day ETC... 73 from South West Louisiana ...
The SWR you saw at 146 MHz is probably due to the length of wire between the SO-239 and the resistor. Next time mount that resistor right next to the connector and use a very short piece of quarter inch wide brass or copper flashing to connect. That should improve the performance. de Dave, K4TO
That was a fantastic "quick build" for a dummy load! Never thought about using those resistors before....but that is the way to go! Thanks, C.J.! will work great for my little QRP rigs. Ki5jta
I built something similar on the bottom of the box rather than the lid for more thermal mass. I also glued down a small chunk of single-sided protoboard next to the resistor. I soldered the tab of the resistor down to the protoboard, and then the wire to conecttor to the board. So there's no stress at all on that really delicate terminal on the resistor. I can run about 50 watts into it for a few second before the box warms up. But the screw heads... they get too hot to touch after a few second! Still, plenty good enough for lots of testing. It's great little thing to have around. Also in the box is an RF sampler feeding my tinySA or even a scope. So I can monitor the live signal going out to the antenna, or loop the output around to the dummy load resistor in the same box! Real handy..
I am here again watching a video of yours. Thanks for your ideas. I have installed shark sticks on my truck with the same mounts you used on the truck. Now am interested in a dummy load.
If you put the resistor closer to the connector you can get better result especially at higher frequency. Otherwise you can use a short coax length instead of a standard wire. 73s
Very cool! One thing you can do to make your project look a little more slick is to think about the 'lid' as the bottom. Drill your mounting holes in the box. This way, the four screws that fasten the lid to the box will be facing the bench in use.
I used an electrical aluminum box that has a built in heat sink. I added more holes to install a spacer for a small 12v fan. I used CPU carbon based thermal paste. The fan works very well. I broke my last resistor... so I ordered some more... hopefully, it’ll be much better. I’m worried about using smaller gauge wire for larger RF power such as 50 watts.
i would mount the parts on the main body. more metal to soak away the heat. the join between the case and the lid has some thermal resistance. nice simple project though.
I don't know why everyone who builds a dummy load uses a piece of wire between the connector and the resistor instead of a piece of coax. I built a simple one with a piece of scrap RG-174 between the connector and the load to keep the dummy load useful up beyond HF, in fact far into UHF.
Thanks! You can certainly use your antenna but if you’re testing a radio that may have a problem, it’s better to not transmit so that you don’t interfere with someone.
should a dummy load allow for a broadcast? i have two different testers and two different dummy loads and both are allowing a broadcast to my other radios
Most likely leakage from either your radio or cable. Remember, most cable shields only have about a 95% effectiveness. The sensitivity of modern radios is at -130 to-150dbm No amount of shielding will prevent your setup from radiating a signal this small...unless you spend tens of thousands on a mil grade enclosure and cables.
lol, to run any amount of power into a resistor like this you need a heavy heat sink not a flimsy box. Your SWR on 2m most likely comes from the long wire you used, to maintain 50 ohms on 2m the wire needs to be as short as possible with a specific distance to the metal case. It might work for CB or HF but really is unusable for serious measurements anywhere higher in frequency.
I have read some things about how wire-wound resistors couldn't be used for RF dummy loads at high frequencies, but I assumed these types would be ok for it. Its a shame scientific fact, so rarely lines up with our desire to cut a corner here and there. I have noticed the radio hobby keeps you honest, because you can rarely do one good thing without introducing a bad thing to balance it out. Just like in economics, "there is no free lunch".
Fun rainy day project of a dummy load primarily for QRP and low power applications. Have fun!
A 51.50 Ohm resistor is also known as a "Van Halen" or "California crazy" resistor. Only well-rounded geeks will get the references.
51 ohms is very common , if you look at the data/spec sheets on almost anything , you'll see 52 Ohms , 50 ohms is the norm accepted.... I would have put the SO-239 on the end of the box and the resistor on the side of the box to act as a heat sink... Or add more resistors in the future for a heavy duty load, to handle you bigger rigs.. You could have went to the scrap yard and picked up an old stereo and yanked the heatsink out of it and attach your resistor to it and add 4 standoffs for the SO-239...
Unless you intended the box to carry spare items for a field day ETC... 73 from South West Louisiana ...
The SWR you saw at 146 MHz is probably due to the length of wire between the SO-239 and the resistor. Next time mount that resistor right next to the connector and use a very short piece of quarter inch wide brass or copper flashing to connect. That should improve the performance. de Dave, K4TO
Correct sir
That was a fantastic "quick build" for a dummy load! Never thought about using those resistors before....but that is the way to go! Thanks, C.J.! will work great for my little QRP rigs. Ki5jta
I built something similar on the bottom of the box rather than the lid for more thermal mass. I also glued down a small chunk of single-sided protoboard next to the resistor. I soldered the tab of the resistor down to the protoboard, and then the wire to conecttor to the board. So there's no stress at all on that really delicate terminal on the resistor.
I can run about 50 watts into it for a few second before the box warms up. But the screw heads... they get too hot to touch after a few second! Still, plenty good enough for lots of testing. It's great little thing to have around. Also in the box is an RF sampler feeding my tinySA or even a scope. So I can monitor the live signal going out to the antenna, or loop the output around to the dummy load resistor in the same box! Real handy..
That wa out standing..I am now going to make my own too.thank you so much..Ron from Reading PA...K3RJW,,,73
I am here again watching a video of yours. Thanks for your ideas. I have installed shark sticks on my truck with the same mounts you used on the truck. Now am interested in a dummy load.
Cool!! So glad the videos are helpful! Thanks!
If you put the resistor closer to the connector you can get better result especially at higher frequency.
Otherwise you can use a short coax length instead of a standard wire. 73s
That was a great and quick project! Even my non-engineer self could put that together! Keep up the great videos!
WJ6F Thanks! This one is my QRP dummy load. I picked up some power resistors to make a regular dummy load. Stay tuned for that one!
Nice vid
Thanks!
Very cool! One thing you can do to make your project look a little more slick is to think about the 'lid' as the bottom. Drill your mounting holes in the box. This way, the four screws that fasten the lid to the box will be facing the bench in use.
Great idea! I'm actually thinking of a higher power version using an electrical box.
Good day 73❤
Well done video, great little project for a raining day, only I suggestion a link to resistor would have been great. Thanks again
Very well done!👍☘️
Thanks! I don't have a dummy load. I think I ran across his channel a while ago. I will check it out.
1crazynordlander He has some great videos up. Big CW QRP buff.
I used an electrical aluminum box that has a built in heat sink. I added more holes to install a spacer for a small 12v fan. I used CPU carbon based thermal paste. The fan works very well. I broke my last resistor... so I ordered some more... hopefully, it’ll be much better. I’m worried about using smaller gauge wire for larger RF power such as 50 watts.
I actually have the parts to build one like that!
Nice project 👍
i would mount the parts on the main body. more metal to soak away the heat. the join between the case and the lid has some thermal resistance. nice simple project though.
Ynot6 Good idea. I have a high power version planned out.
Thank you.
I have a 50w version,very useful.
Excellent.
That's a helluva nice project there. Well done, sir!
Thanks! It was fun to build something.
Perfect !!!
Very cool I will try to find those resistors on eBay
I don't know why everyone who builds a dummy load uses a piece of wire between the connector and the resistor instead of a piece of coax. I built a simple one with a piece of scrap RG-174 between the connector and the load to keep the dummy load useful up beyond HF, in fact far into UHF.
Good point!
I bought one of those resistors and the tab fell off. They are very delicate...😅
Made a couple very similar a couple of years ago.
Tom Crosman Like I said, I never had one in all the years I’ve been a ham. I think the next one will be a little more heavy duty
I would have used a piece of RG58 coax for my hookup wire for a better match. Also a better ground.
Good idea! I’m planning another one. I might just use coax inside. Thanks!
Loved your video, just a question ! Why can't i use my antenna instead of a dummy load ?
Thanks! You can certainly use your antenna but if you’re testing a radio that may have a problem, it’s better to not transmit so that you don’t interfere with someone.
Onto many dummy loads it is written 3GHz. Are good for testing power in 136-500 MHz range ? (radios within VHF and UHF bands)
I would think if it’s rated to 3Ghz, it should be ok in vhf/uhf.
should a dummy load allow for a broadcast? i have two different testers and two different dummy loads and both are allowing a broadcast to my other radios
I have an MFJ-260c, and the transmit range at 5 watts is like a block radius. Dummy loads can still transmit RF.
Most likely leakage from either your radio or cable. Remember, most cable shields only have about a 95% effectiveness. The sensitivity of modern radios is at -130 to-150dbm No amount of shielding will prevent your setup from radiating a signal this small...unless you spend tens of thousands on a mil grade enclosure and cables.
what is the max power that you would recommend
QRP. 10 watts max.
Just built it , thanks , useful item in my shack. G3KMV
Awesome!
Where did you buy your 50ohm resistor ?73 AE4OY WILLIE P.
eBay.
What is the purpose of a dummy load
Testing radios without transmitting thru an antenna
lol, to run any amount of power into a resistor like this you need a heavy heat sink not a flimsy box. Your SWR on 2m most likely comes from the long wire you used, to maintain 50 ohms on 2m the wire needs to be as short as possible with a specific distance to the metal case. It might work for CB or HF but really is unusable for serious measurements anywhere higher in frequency.
I have read some things about how wire-wound resistors couldn't be used for RF dummy loads at high frequencies, but I assumed these types would be ok for it. Its a shame scientific fact, so rarely lines up with our desire to cut a corner here and there. I have noticed the radio hobby keeps you honest, because you can rarely do one good thing without introducing a bad thing to balance it out. Just like in economics, "there is no free lunch".