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#2
August 30th 09, 11:15 PM
posted to rec.radio.shortwave,rec.radio.amateur.equipment
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Can I use wirewound resistors to build an HF attenuator?
In rec.radio.amateur.equipment dave wrote:
wrote: In rec.radio.amateur.equipment dave wrote: wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thanks, Jim. The thing works like a champ. I added a couple of $2.00 relays to bypass the pad when receiving. Good to hear I didn't drop a decimal... -- Jim Pennino Remove .spam.sux to reply. |
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