On the HamRadioHelpGroup mailing list, a fellow wrote about the measurements he made on some RG-8U coax. He connected a 60 W, 440 MHz transmitter to one end of the cable, a 50-ohm dummy load to the other end, and measured the power. With 60W in, he only measured 1 W out.
Now, that seemed like quite a bit to me, so I did a quick Web search and found the OCARC Line Loss Calculator and a couple of RG-8U data sheets. Belden specified their 9913 cable, the RG-8U equivalent, at 4.3 db/100 ft. Plugging all the numbers into the line loss calculator, it would appear that the power measured at the other end of coax should have been more than 22 W, not just 1 W. My guess is that he’s got some other problem with that piece of cable.
Mark, K5LXP, pointed the list towards perhaps an even better online resource: VK1OD’s RF Transmission Line Loss Calculator. This calculator not only calculates the loss of a line at a particular frequency, but also takes into account the mismatch between the line and the load. The loss figures given in the data sheets are accurate only when the line and load are matched.
When there’s a mismatch, the loss goes way up. For example, 100 ft. of Belden 9913 has a loss of approximately 2.8 dB at 440 MHz and an SWR of 1:1, according to VK1OD’s calculator. At an SWR of 5:1, however, is about 4.8 dB.