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Hello, New to the group. Here¡¯s a dummy load I created.? It¡¯s a 50 ohm 250W resistor connected to a BNC bulkhead in an aluminum case (you can see the model number in the first photo).
Happy to have found the group!
73, Dave KC3GPM
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That won't handle more that 50W continuous and probably has terrible SWR above 20MHz. Quick and dirty solution with RL > 40db up to 50Mhz good for 100W continuos (with cooling): On Tue, 19 May 2020 at 21:24, Dave via groups.io <dave.kc3gpm@...> wrote:
Hello,
New to the group. Here¡¯s a dummy load I created.
It¡¯s a 50 ohm 250W resistor connected to a BNC bulkhead in an aluminum case (you can see the model number in the first photo).
Happy to have found the group!
73,
Dave
KC3GPM
Sent using Zoho Mail
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Some suggestions Reduce the length of the wire to almost zero Bold a heatsink to the side with the resistor -- HBTE Files section:? /g/HBTE/filesErik, PD0EK
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Very neat.? I think I have some scrap aluminium in the garage that has been waiting around 10 years to become useful, and there is a similar 50R in the component box.? A nice project for the weekend, in between g-qrp club convention sessions.? Thanks for sharing it.
Dave
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On Fri, Sep 4, 2020 at 12:20 PM, Dragan Milivojevic wrote:
That is a very nice construction. Do you have a RL measurement for it?
The SO239 socket really needs to be an N-Type socket. The PCB track from socket to 50R element should be 50R (3mm track on 1.6mm FG) so that ought to be OK but the measurement just done (as below) it's not as good as was hoped for, but is most likely down to the liberal use of S0239 and PL259 connectors in this test. The sweep is 50kHz to 1.5GHz - the green smith chart line.  
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Better than 30dB up to 100MHz is not too bad considering that it is a transmission line sandwiched between ground planes and air dielectric.
On Fri, 4 Sep 2020 at 14:28, OneOfEleven < cmoss296@...> wrote: [Edited Message Follows]
On Fri, Sep 4, 2020 at 12:20 PM, Dragan Milivojevic wrote:
That is a very nice construction. Do you have a RL measurement for it?
The SO239 socket really needs to be an N-Type socket. The PCB track from socket to 50R element should be 50R (3mm track on 1.6mm FG) so that ought to be OK but the measurement just done (as below) it's not as good as was hoped for, but is most likely down to the liberal use of S0239 and PL259 connectors in this test.
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Even better without the short coax. Need to replace the S0239 socket with an N-Type and use a straight SMA to N-Type to the dummy load to improve it I think. This is from 10kHz to 900MHz, considering it's going through an SO239 socket, PL259 plug, BNC socket, BNC plug, SMA socket, SMA plug, another SMA plug and finally another SMA socket, I don't think it's doing too bad really ..  
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After calibrating the VNA with the usual 50R SMA dummy load in place of the big one, this is what I get. The dummy load SO239 definitely needs to be an N-Type socket.  
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For my load, I used a R50 @ 250W resistor.
Some time in the lathe and I was able to have a good charge for my use.
73!
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I like that Igor, that's nice !
You'll have to show us a smith plot.
Having a lathe must be extremely handy/useful for all sorts of work.
I've used my 50W load element (with the heat sink arrangement I showed) with nearly 200W PEP SSB without problem, though I think the whole heat sink gets quite hot on 100W continuous. So these little 50R elements can take quite a bit of power.
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Information my load R50 -?#1561
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On Sun, Sep 6, 2020 at 12:03 AM, Igor Jobim wrote:
For my load, I used a R50 @ 250W resistor.
Some time in the lathe and I was able to have a good charge for my use.
73!
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This is my attempt.? Different design due to different scrap material in the garage and SMA scavenged from some unidentified thing from the club junk box.? 100W element.? Dimensions are 35x35x15mm.??    By itself 10W is about the limit as it gets HOT.? Attached to a scrap CPU heatsink the hottest part gets to 70C at a continuous 100W, so well within the 100C limit from the datasheet.  The cover was left off for the tests so the resistor flange temperature could be measured. Thanks for the prompt into action!? Nice to do some mechanical stuff. 73 Dave M0WID
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Better than mine, the SMA is a much better connector than the SO239 I used.
I suppose if we had the base of the enclosures made from a piece of copper then the heat would spread much more easily and so not get so hot.
Yours is neater than mine anyway :)
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Perhaps Aluminum has? a better thermal conductivity than copper.
Sarma vu3zmv
On Thu, 17 Sep 2020, 11:43 pm OneOfEleven, < cmoss296@...> wrote: Better mine! .. the SMA is a much better connector than the SO239 I used.
I suppose if we had the base of the enclosures made from a piece of copper then the heat would spread much more easily and so not get so hot.
Yours is neater than mine anyway :)
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I was wrong.? Copper ranks better.?
Perhaps Aluminum has? a better thermal conductivity than copper.
Sarma vu3zmv
On Thu, 17 Sep 2020, 11:43 pm OneOfEleven, < cmoss296@...> wrote: Better mine! .. the SMA is a much better connector than the SO239 I used.
I suppose if we had the base of the enclosures made from a piece of copper then the heat would spread much more easily and so not get so hot.
Yours is neater than mine anyway :)
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I'm not sure about the physics but amplifiers often use copper "heat spreaders" between the active devices and the aluminmum heat sink.
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Very desirable to flatten the base of the resistor and the Al block, very few commercial parts are really flat. A sheet of 1200 wet and dry will reveal how flat the surfaces are. Tape the abrasive to a piece of flat glass or a surface plate then gently abrade the surfaces. The ?very fine scratch pattern will reveal the high spots and burrs. ? A smear of thermal compound will help, only a VERY thin film is desirable, most people use far too much, the aim is to displace air between the surfaces with a more conductive material. ? The heat emission from a CPU is in the middle of the package, the cooler provides best results where the CPU package normally contacts the cooler. A CPU cooler is very much more effective ( and cheaper) than a massive Aluminium heatsink as long as the fan is turning! ? ? 73 Alan G8LCO ?
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Igor - PY3IG
