Subject: Re: Elmer Qustion: Antenna feedline
From: James R. Duffey (JamesDuffey@comcast.net)
Date: Sun Feb 29 2004 - 00:32:41 EST
The best way to find out if the performance of your antenna/feedline system has deteriorated is to compare a present SWR or impedance vs. frequency plot with the one taken when you first erected the antenna. You will find this in your log book on the date you erected the antenna. Seriously, making and keeping good records of your antenna when you erect it is very useful. And the log book is a handy place to keep these records as most of us treat these as archival records. I still have my novice log books from 38 years ago! My mother knew these were important to me and did not throw them out or give them away. I record a small sketch of the antenna I build with relatively accurate dimensions in my log book. On this sketch or others I record height above ground and the orientation of the antenna. I also record the length and type of the feedline I use. I record and plot an SWR vs frequency graph on each band I will use the antennaon. For tuned feeder antennas, I record the tuner settings on each band. It does not take much time to The old ARRL log books used to have a nice piece of graph paper in the back on which to record all of this. Somewhere along the line this has been omitted as a log book feature and the price rasied to compensate for the omission. :^)= These days I sketch it on the back of a page of a log book or plot the information on a separate piece of papaer and tape it in the logbook page. With all of that information recorded , you can quickly tell if an antenna is working properly or give details of it to other hams. If you know the length of the feedline you can translate impedance measurements made at the end of the line to those at the antenna. With the antenna orientation, and a great circle map, you can tell where in the world you wil lbe strong and where you will be weak. If you keep an electronic log, consider keeping a small bound quadrille ruled notebook, like you used in freshman Physics lab, to record your antenna projects and othe rprojects as well. Oh yes to your original question. If the SWR plot has changed significantly, either gotten lower or higher, you have a problem, usually with the feedline. If the resonant point (lowest SWR more or less) has changed significantly, like out of band, you have a problem as well. I have rambled. - Dr. Megacycle KK6MC/5 ______________________ James R. Duffey KK6MC/5 Cedar Crest NM 87008 DM65
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