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#8
June 20th 10, 11:07 PM
posted to rec.radio.amateur.antenna
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Tapering open wire feedlines?
"Owen Duffy" wrote in message ... " wrote in news:6d1a078a-e5f4- : ... Though the traditional approach has been to use a 4:1 voltage balun at the rig to feed these things, there is good argument to use a 1:1 Guanella balun (current balun), and it can be located outside the shack and inboard shield effectively grounded to deal with common mode current. You still need to minimise the length of coax operated at high VSWR, and it would not be necessarily absurd to think about low loss coax. Owen Okay, then it might make sense to do this: Plan II instead of using the balun is in the tuner, there is no reason why I couldn't mount another 4:1 balun outdoors, and use a short run of low loss coax back through the conduit into the operating room. That would be about 15 feet of LMR-400 cable. I could go to 1/2 heliax but would there be any benefit?? So, I would have balanced open-wire feedline from the dipole to the 4:1 balun followed by a 1:1 common-mode RF choke with the requiste RF ground for decoupling, followed by the short run of low loss coax going back to a unbalanced ant. port on the tuner. I'd still have coax in the mix, but the 4:1 balun should hopefully tame the high impedance excursions across the antenna operating range to reduce the coax's losses at those points? Otherwise it is going to be hard to get balanced line into the shack, unless I pull it through the PVC conduit with the other coax cables. Pete |
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