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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Home Made UHF 50 Ohm Dummy Load
Hi,
I wish to make a 25 watt (short duty) 50 ohm dummy load for UHF 440Mhz band. I have a UHF SWR meter and I have made dummy loads before, but they are of little use at UHF frequencies due to internal capacitances in the resistors. The only good dummy load I made that works very well at UHF is a 2 W 50 ohm straight carbon resistor solder on the inside of a PL259 connector. I can use this as a 10W (short duty) dummy load as long as I make a brief transmission. I thought about putting five 2 watt 10 ohm straight carbon resistors in series inside a copper tube which is soldered to the back of a PL259 connector. What are your thoughts about this idea? Thanks in advance, Brad Before you type your password, credit card number, etc., be sure there is no active key logger (spyware) in your PC. |
#2
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Home Made UHF 50 Ohm Dummy Load
"Brad" wrote in message ... Hi, I wish to make a 25 watt (short duty) 50 ohm dummy load for UHF 440Mhz band. I have a UHF SWR meter and I have made dummy loads before, but they are of little use at UHF frequencies due to internal capacitances in the resistors. The only good dummy load I made that works very well at UHF is a 2 W 50 ohm straight carbon resistor solder on the inside of a PL259 connector. I can use this as a 10W (short duty) dummy load as long as I make a brief transmission. I thought about putting five 2 watt 10 ohm straight carbon resistors in series inside a copper tube which is soldered to the back of a PL259 connector. What are your thoughts about this idea? Thanks in advance, Brad Before you type your password, credit card number, etc., be sure there is no active key logger (spyware) in your PC. My initial thought is that your sums are in error. Although 5 x 10 ohm resistors in series makes 50 ohms, the power rating remains 2 watts. Use 20 x 1k 2 watt non inductive resistors in parallel. This gives 50 ohms at 40 watts. The way to construct a dummy load for the 70cms band, is to cut two discs of double sided PCB material, then drill twenty resistor lead-out sized holes in each, plus a 3mm hole in the centre of one, and a 1.5mm hole in the centre of the other, then solder the resistors between the two, soldering to both sides of the pcb material. Take a small paint tin, or similar, and fix an SO239 socket to the lid. Solder a stiff thick insulated wire to the centre pin of the socket, and run this down through the large hole in the top pcb, into the hole in the centre of the lower pcb disc, and solder it Finally, solder multiple braids from the upper pcb disc to the paint tin lid, then put the lid back onto the can. You now have a fully screened pretty much non inductive 50 ohm load. If you want, you can fill the tin up with mineral oil to increase the power dissipation to probably 100 watts short term, but if you do this, make sure that your SO239 is either a sealed type, or seal it with some silicon rubber compound. I have several dummies of this construction in use all the time, and have never had the slightest trouble with them. Mine are actually built into golden syrup cans, which solder easily. Arfa |
#4
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Home Made UHF 50 Ohm Dummy Load
"Brad" wrote in message ... Hi, I wish to make a 25 watt (short duty) 50 ohm dummy load for UHF 440Mhz band. I have a UHF SWR meter and I have made dummy loads before, but they are of little use at UHF frequencies due to internal capacitances in the resistors. The only good dummy load I made that works very well at UHF is a 2 W 50 ohm straight carbon resistor solder on the inside of a PL259 connector. I can use this as a 10W (short duty) dummy load as long as I make a brief transmission. Use about 100 feet of rg58 or 50 feet of rg174 coax. It has enough loss without anything o the end to look like a good dummy load at 400 mhz and above. I have seen (bought some ) dummy loads at hamfests for about $ 15 that are good for 500 mhz . |
#5
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Home Made UHF 50 Ohm Dummy Load
On Tue, 13 Jun 2006 22:54:24 GMT, "Ralph Mowery"
wrote: Use about 100 feet of rg58 or 50 feet of rg174 coax. It has enough loss without anything o the end to look like a good dummy load at 400 mhz and above. Good idea. The problem is that RG174 doesn't really have much power handling capeability. There's chart at: http://www.therfc.com/powerrat.htm which claims 60 watts. I think that's rated at 40C (104F) which is rather warm. The heat dissipation is NOT uniform. The bulk of the smoke will be coming from the radio end of the coax cable, while the other end remains fairly cool. I have seen (bought some ) dummy loads at hamfests for about $ 15 that are good for 500 mhz . Well, I was trying to encourage the OP to build something instead of buying. However, if you must spend money, MFJ as a 25 watt (continuous duty) load for $40. http://www.mfjenterprises.com/produc...rodid=MFJ-260C -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com # http://www.LearnByDestroying.com AE6KS |
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