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Improved feed line
I have been trying to improve the SN, that is to reduce
the noise as much as I can, while prserving the desired signal. This has been an on going battle evry since radio began. The Doty antenna (http://www.anarc.org/naswa/badx/antennas/) is a good starting place. This link (members.aol.com/WA1ION/nrants.pdf) is a good improvement. I dropped his matching resistors and isolated the braid on the output of the 9:1 step down. A minor improvement. Took care of slme stubborn birdies from my equipment. I was troubled by some odd "bleed through". While lloking into RG174 from a friend's "hidden" antenna installation, I found a refference to an article by John Brynt "Is Your Coax Lead-In Actually An Antenna" (www.dxing.info/equipment/coax_leadin_bryant.pdf) that suppled some answers that really helped me understand what is hhappening. I did some experiments with "normal", or at least more common braid only coax and compared it to braid+foil coax. The foil really helped reduce the engress from one stuborn local MW. But it wasn't completly removed. I was able to recude it to the level of leakage directly into my reciver by placing a ferite toroid as Sugested by John Brynt in the "coax" link. A BIG step forward. But I still had a lot of QRM. While changing my coax I decided to do some simple experiments. I was replacing my antenna wire at the same time, so I snipped the horizonatal section off leaving only the the vertical. My signal strength was down a little, but the noise was as loud as ever. So I ran some coax up to the support and reattatched the horizontal wire to a 9:1. This helped but I still had a lot of QRN, not near as much as before but I decided to see what I could do tho help. A Torroid as used by Brynt really helped. So I decided to see what Triax would do. Triax is coax with an outer insulated additional shield. I fed the antenna wire to the hiZ hot, the outer shield went to the HiZ ground, and the low z was connected to the inner conductor and inner braid. At the base I grounded the outer braid, and use a 1:1 to couple the inner conductors to the coax that makes the run to the house. This really reduced the noise. I tested it by placing an especially nasty wall wort next to the feed line. Always before I would get a rapid jump in RF noise as the wall wort was placed near the feedline. No RF creeping back p the feedline to enter then antenna. I used 50 Ohm Triax, but tested a piece of 75 Ohm triax and there was no difference between them on a 20' run. &% Ohm is a lot more common, almost every TV station in the USA uses it and as they switch to fibre', it should become very common and CHEAP. You could probably get a 30' piece just for the asking(that is how I got mine). Forget Triax connectors, they are around $100 and look somewhat like a BNC on steroids. I tried Triax last fall but must have wired something wrong, becuase it just didn't work bery well. Now if the never to be cursed T-storms would let up for a few days I might be able to enjoy the reduction in the local QRM. And this link shows why I hate wall worts. http://home.computer.net/~pritch/shortwav.htm Wall worts are the worst but any device with un-bypassed Si diodes can be nasty. Terry |
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