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Radio Shack BNC 50 ohm terminator as Dummy Load
On Sun, 20 Jan 2008 12:29:19 -0500, "Bob"
wrote: "Cecil Moore" wrote in message ... Tam wrote: Almost certainly, the power rating of the terminator is either 1/8, 1/4, or 1/2 W. Does Radio Shack not specify the power rating? -- 73, Cecil http://www.w5dxp.com Hey engineer! here is link: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. 73's Richard Clark, KB7QHC |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Jan 20, 10:15 am, Richard Clark wrote:
On Sun, 20 Jan 2008 12:29:19 -0500, "Bob" wrote: "Cecil Moore" wrote in message ... Tam wrote: Almost certainly, the power rating of the terminator is either 1/8, 1/4, or 1/2 W. Does Radio Shack not specify the power rating? -- 73, Cecil http://www.w5dxp.com Hey engineer! here is link: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. 73's Richard Clark, KB7QHC On the contrary, I've measured some of the Caddock resistors similar to those and they're quite useful to 2 meters and beyond. We were using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz successfully. YMMV, and as I wrote in this thread before, it does help to have a reliable analyzer to test them with. Much more useful, IMO, than a lot of the drivel that goes on here would be info on ways to bootstrap yourself into some decent measurements. It can be done with very inexpensive home-brew equipment, but it takes some thought and careful construction. Cheers, Tom |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Mon, 21 Jan 2008 10:50:29 -0800 (PST), K7ITM wrote:
http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. On the contrary, I've measured some of the Caddock resistors similar to those and they're quite useful to 2 meters and beyond. We were using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz successfully. YMMV, and as I wrote in this thread before, it does help to have a reliable analyzer to test them with. Much more useful, IMO, than a lot of the drivel that goes on here would be info on ways to bootstrap yourself into some decent measurements. It can be done with very inexpensive home-brew equipment, but it takes some thought and careful construction. Hi Tom, Did you put the MP9100 against a heat sink? The large surface area, thin construction for heat flow, bound on both sides by metal clamping, a high dielectric constant, all point to a considerable sized capacitor (I visualized a 0.01µFd ceramic cap). 73's Richard Clark, KB7QHC |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Jan 21, 10:58 am, Richard Clark wrote:
On Mon, 21 Jan 2008 10:50:29 -0800 (PST), K7ITM wrote: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. On the contrary, I've measured some of the Caddock resistors similar to those and they're quite useful to 2 meters and beyond. We were using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz successfully. YMMV, and as I wrote in this thread before, it does help to have a reliable analyzer to test them with. Much more useful, IMO, than a lot of the drivel that goes on here would be info on ways to bootstrap yourself into some decent measurements. It can be done with very inexpensive home-brew equipment, but it takes some thought and careful construction. Hi Tom, Did you put the MP9100 against a heat sink? The large surface area, thin construction for heat flow, bound on both sides by metal clamping, a high dielectric constant, all point to a considerable sized capacitor (I visualized a 0.01µFd ceramic cap). 73's Richard Clark, KB7QHC Surely you don't think they use high-K ceramic for their substrate. Assuming alumina, A8 grade if you wish, the thermal conductivity is sufficient, I believe, to allow a 2mm thick piece 13mm square to serve the MP9100 at better than the data sheet thermal performance. Relative permittivity of that alumina is about 10 at most. Even if the 13mm square were silvered both sides, it represents a whopping 7.5pF. Assuming that's distributed evenly across the resistive element, it gives you better than 20dB return loss out to beyond 800MHz. For a ham dummy load, that's generally fine. I'll bet this simple back-of-the-envelope calculation yields a worse result than you'd see in practice with an MP9100. Please note that I didn't say I either had, or have used, an MP9100. It was a 25-watt part in a TO-220 style package from Caddock, and I believe it was a precursor of the series with exposed ceramic backs. It was about 20 years ago; I'd have a hard time finding the data at this point. Please feel free to get one of the MP9100s and measure it yourself. I'm confident the capacitance won't be anything like 0.01uF. Cheers, Tom |
Radio Shack BNC 50 ohm terminator as Dummy Load
"Richard Clark" wrote in message ... On Mon, 21 Jan 2008 10:50:29 -0800 (PST), K7ITM wrote: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. On the contrary, I've measured some of the Caddock resistors similar to those and they're quite useful to 2 meters and beyond. We were using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz successfully. YMMV, and as I wrote in this thread before, it does help to have a reliable analyzer to test them with. Much more useful, IMO, than a lot of the drivel that goes on here would be info on ways to bootstrap yourself into some decent measurements. It can be done with very inexpensive home-brew equipment, but it takes some thought and careful construction. Hi Tom, Did you put the MP9100 against a heat sink? The large surface area, thin construction for heat flow, bound on both sides by metal clamping, a high dielectric constant, all point to a considerable sized capacitor (I visualized a 0.01µFd ceramic cap). 73's Richard Clark, KB7QHC Hi Richard, Would you check out this link: http://www.bourns.com/components.aspx?cmsphid=7631383|7163299|3412571 Which one will be the best for let say, 400 Watts dummy? Thank you. Regards. Bob |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Tue, 22 Jan 2008 02:56:36 -0500, "Bob"
wrote: Hi Richard, Would you check out this link: http://www.bourns.com/components.aspx?cmsphid=7631383|7163299|3412571 Which one will be the best for let say, 400 Watts dummy? Thank you. Regards. Bob Hi Bob, There is every probability that either of your links will work, although I still have a sample request that remains to be filled (but I am in contact with their engineers). However, backing up to the actual application, this is still not a done deal if you are trying to push them to their "ratings." The larger part of success is in the details, and for Heat Transfer, that is a devil. The practical solution will require a sink with fins and forced air in ambient to still find it derated by 60%. Ham applications rarely subject a dummy load to full power continuously, however. So, a good heat sink and a one minute application of full rating in ten minutes "may" be tolerated. You can always water cool them. 73's Richard Clark, KB7QHC |
Radio Shack BNC 50 ohm terminator as Dummy Load
Richard Clark wrote:
On Sun, 20 Jan 2008 12:29:19 -0500, "Bob" wrote: "Cecil Moore" wrote in message .. . Tam wrote: Almost certainly, the power rating of the terminator is either 1/8, 1/4, or 1/2 W. Does Radio Shack not specify the power rating? -- 73, Cecil http://www.w5dxp.com Hey engineer! here is link: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. 73's Richard Clark, KB7QHC Actually, those Caddock MP9 and MP8 non-inductive resistors are good to HF (allowing for the physical size). The inductance of the big ones (MP9100, 100W units) is about 20nH, about 3 ohms at 28 MHz. Half that for the smaller ones. The challenge is in getting the heat away from them. the thermal resistance of the MP9100 is 1.5 degrees/W, so at 100W, the resistor is 150 degrees hotter than the heatsink, and with a max temp of 175C, you'd better keep that heatsink at 25C. |
Radio Shack BNC 50 ohm terminator as Dummy Load
Richard Clark wrote:
On Mon, 21 Jan 2008 10:50:29 -0800 (PST), K7ITM wrote: http://www.caddock.com/Online_catalo...e/Current.html MP9100 Dont lure good people to smacks shops! Just DIY! Unlikely to be useful above the LW to AM band. On the contrary, I've measured some of the Caddock resistors similar to those and they're quite useful to 2 meters and beyond. We were using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz successfully. YMMV, and as I wrote in this thread before, it does help to have a reliable analyzer to test them with. Much more useful, IMO, than a lot of the drivel that goes on here would be info on ways to bootstrap yourself into some decent measurements. It can be done with very inexpensive home-brew equipment, but it takes some thought and careful construction. Hi Tom, Did you put the MP9100 against a heat sink? The large surface area, thin construction for heat flow, bound on both sides by metal clamping, a high dielectric constant, all point to a considerable sized capacitor (I visualized a 0.01µFd ceramic cap). 73's Richard Clark, KB7QHC It's about 1/2 cm x 1 cm. Say it's 1/2 mm from sink to innards. 8.85 pF/m * 0.5E-4 m^2/0.5E-3m = 1pF.... pretty small. |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Wed, 23 Jan 2008 16:24:01 -0800, Jim Lux
wrote: It's about 1/2 cm x 1 cm. Say it's 1/2 mm from sink to innards. 8.85 pF/m * 0.5E-4 m^2/0.5E-3m = 1pF.... pretty small. I posted the actual specifications yesterday. 73's Richard Clark, KB7QHC |
Radio Shack BNC 50 ohm terminator as Dummy Load
On Wed, 23 Jan 2008 16:20:28 -0800, Jim Lux
wrote: Actually, those Caddock MP9 and MP8 non-inductive resistors are good to HF (allowing for the physical size). The inductance of the big ones (MP9100, 100W units) is about 20nH, about 3 ohms at 28 MHz. Half that for the smaller ones. The challenge is in getting the heat away from them. the thermal resistance of the MP9100 is 1.5 degrees/W, so at 100W, the resistor is 150 degrees hotter than the heatsink, and with a max temp of 175C, you'd better keep that heatsink at 25C. This was covered yesterday. 73's Richard Clark, KB7QHC |
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