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#17
June 13th 09, 12:05 AM
posted to rec.radio.amateur.antenna
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Coax + Ladder Line
Owen Duffy wrote:
wrote in : ... Even with a 80/1 mismatch, ladder line has low loss. 30m of Wireman 551 with a load of 5+j0 has VSWR=80 at the load end, VSWR=28 at the source end, and the transmission loss is 4.6dB... 65% of the input power is converted to heat. Speaks to your meaning of "low loss". Mythical properties are ascribed to ladder line, some of attributable to ARRL publications. Or, I think more properly, to statements in ARRL publications taken with a different set of underlying assumptions.. For instance, ladder line between a tube amp output tank and a doublet is probably "reasonably" low loss in most common practical applications.. And in an inappropriate application (reactive load with weird Z to 50 ohm transmitter) it's probably lower loss than RG-58. Maybe it's that subjective term "low loss"??? Half a dB per foot is low loss at 32 GHz, but you'd be hard pressed to find a commercially available transmission line with that kind of loss at 7 MHz. (maybe that funky 1000 ohm Z delay line stuff?) Ham applications in any case are kind of an odd thing, efficiency-wise, since the limit is on RF power at the transmitter output connector. Pretty much every other user of RF power amplifiers sets the system measurement plane somewhere else.. in space (EIRP or ERP) or at the power source (power budgets). For instance, you can trade increased power consumption against lower feedline loss. |
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