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Richard Clark wrote:
On Wed, 01 Dec 2004 10:17:05 -0600, Cecil Moore wrote: As you know, furnishing line losses in dB implies a power ratio. dB is dimensionless. Such generalizations forced into logic become naive paradoxes: Note from the matched line example above where 5 watts of line losses are 0.21 dB to the mismatched line example where 5 watts of line losses are 1.25 dB (for the same forward power of 100w), the same magnitude of loss appears much higher as a dB value referenced to NET source power. with boundary conditions being violated with the substitution of Mismatch Loss for Dissipative Loss - and done poorly too. Well, I think he's assuming dissipative loss in both cases. But the 5 watt numbers were assumed, and there's nothing to indicate that the same piece of transmission line would produce these losses under the described conditions. Seems likely that the line losing 33% would have to be quite a bit lossier. As for the second case, we don't know what the source power was, so it presumably could also be a matched case at lower power. The example, unfortunately, compared apples to oranges. My take on dB attenuation figures in wire and cable table data: dB loss = 10*log(power dissipated in the line / (power dissipated in the line + power dissipated at load)) By way of reference, according to an old Standard Wire and Cable data book of mine, the attenuation factor in dB per hundred feet is: A=4.35*(Rsubt/Zo) + 2.78*sqrt(E)*p*F Rsubt = .1*(1/d + 1/D)*sqrt(F) (total line resistance in ohms per 1000 ft) [*Note: there might be a misprint here. The .1 would seem to indicate a conversion to ohms per hundred feet.] E = dielectric constant p = power factor of the dielectric F = frequency in MHz d = outside dia. center conductor [no units are indicated] D = inside dia. outer conductor [no units are indicated] The manual also discusses attenuation as a funtion of VSWR. But they do not imply that the additional attenuation is due to anything but reflection. In other words, there is no indication that reflected 'power' causes additional dissipative losses. And.....I think Reg makes a good point about reflected power. _____ $0.02 AC6XG |
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