[TowerTalk] Open Wire line measurements
rstealey at hotmail.com
Wed Sep 30 06:51:26 PDT 2009
"If these antennas are all that touchy mine should be near a fiasco but it works quite well. "
Ask yourself this, "How do I know it works quite well? Could it possibly have 3,4,5 db of excess loss and how would I know?" In fact, next time you are on the air and someone tells you your antenna is working great, ask him how he knows. Think about it.
I've always wondered about the losses we get in balanced line in the real world. A few years ago I did an experiment. I built a couple of 9:1 baluns to match coax to balanced line and ran 435 feet of #14 stranded wire balanced line out along some trees and back in a big circle to a point 100 feet from where it started. I fed it from coax in the shack, carefully calibrated with a 8640B signal generator and HP power meter. I measured the matched loss on the 15 meter band. Then I substituted 320 feet of the brown, common window line in place of part of the homemade line.
So the configuration is - signal generator, coax from the shack, balun, balanced line, balun, coax back to the shack, HP power meter. Coax loss was accounted for. The results on 21 MHz:
1). Loss in open wire - 1.3 db
2). Loss with 320 feet window line substituted - 0.8 db
3). Spray 100 feet of the window line with a hose - 1.3 db
I didn't record the data (sorry) for when I let the window line lay on the ground but as I remember the additional loss was insignificant. I was surprised and impressed that the window line was actually better than the homemade line, but I really didn't take a lot of care to install spreaders along its length. I ran it from tree to tree and had 1X1 pieces of wood as mounting points.
One of the reasons I wanted to do this measurement was out of curiosity as to how homemade line might actually perform. When I was a kid in the mid 50's in WVa we had a TV antenna mounted several hundred yards from the house up in a tree on top of a hill, with a preamp mounted at the base of the tree. The objective was to get Pittsburgh tv stations 125 miles away. We had #18 electric fence wire as transmission line running up the hill, through brush, then into the woods to that tube type amp. When our picture became unusable we would "walk the line" looking for branches that had grown into it, or where the wind would have caused the wires to become crossed. The system must have (obviously) been sending dc up the line to run the amp. We were constantly changing 6CB6s and testing them in the grocery store. So based on my test at 21 MHz with 400 ft of similar line we probably had 10 db or so of loss at Ch 2-6 frequencies but the preamp could have taken care of that.
Pretty good system design, I think. And my dad didn't know anything about rf design, someone told him how to build this setup. 6-7 years later we had local stations and I took that #18 wire for my ham antennas!
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