If they are mounted against a ground-plane (which is often the case for heat-sinking), I've found that the 100-watt resistors are not too good above 50 MHz. The 30 watt resistors are better, but the 15 watt resistors are best thru 2-meters. I built a series of 50 ohm terminations where the resistors were bolted to a brass plate, and the brass plate was attached to a N-connector. The resistor leads were about 0.2" long (one pin attached to the connector center pin, the other soldered to ground). Measured with an AIM4170 up to 180 MHz.
Phil - AD5X ----- Original Message ----- From: "Don Wilhelm" <[email protected]> To: "Phil & Debbie Salas" <[email protected]> Cc: <[email protected]> Sent: Friday, May 21, 2010 5:50 PM Subject: Re: [Elecraft] OT: heat sinking resistors > Phil, > > Is this recent data? Has something changed? I have had plots of the > Caddock 50 ohm 100 watt resistors taken to 250 MHz and the return loss > indicated that they made very good attenuators and dummy loads at that > frequency - assuming they were mounted with zero length leads directly to > a BNC or SO-239 connector. My data is 2 years old. > > BTW - the facts I have quoted are good only for the 50 ohm Caddock > resistors - I have found that other values were "as stated" non-inductive, > but they certainly were capacitive. A capacitive dummy load is just as > bad as an inductive load. > > 73, > Don W3FPR > > Phil & Debbie Salas wrote: >> "I just received a few Ohmite thick film power resistors for an >> attenuator I'm making, rated 20 watts. They are 15mm x 10mm x 3mm. >> Obviously they have to be heat-sunk (heat-sinked?) if they are going to >> dissipate that much power." >> >> FYI - Both Ohmite and Caddock make versions of these resistors that have >> screw holes for mounting. The lower power parts (15-20 watts) are pretty >> good thru 2-meters. However, the higher power versions (especially the >> 100-watt versions) can start to look bad at 50 MHz due to the large >> package capacitance-to-ground. >> >> Phil - AD5X >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >> > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
