On Sat, Dec 31, 2011 at 9:35 PM, Don Wilhelm <[email protected]> wrote:
Keep in mind that the length of an unmatched feedline is critical to the > success or failure of such a system. That is a factor that is seldom > mentioned in posts that say a particular antenna "works well" and "loads > well" - the simple fact is that the feedline type and length are critical > elements in that antenna system - to say that a 100 foot dipole works > great is not sufficient, one needs to state the type and length of the > feedline as well as the length of the antenna. > Lots of good comments. I think part of the original post asked about keeping RF out of the shack. Assuming one does a reasonable job of keeping both antenna and open-wire feedline balanced, there is a fairly easy answer. Operative word here is "reasonable"--perfection is the enemy of good enough. Don't let all the horror stories stop you from trying. People, including me, have been using balanced feeders and center-fed dipoles with good results for a very long time. Here's how to minimize RF in the shack. Make the length of one leg of the dipole plus the length of the feedline as close as possible (practical) to an odd multiple of a quarter wavelength at the frequency of interest. With a dipole, there is an RF voltage maximum at the far end. One quarter wavelength away there is a voltage minimum, another quarter wavelength away, a voltage maximum. Etcetera. An odd multiple of quarter wavelengths puts a voltage minimum in the shack. In addition to minimizing RF in the shack, this approach puts minimum RF voltage and maximum RF current in the shack. This means relatively low impedance in the shack which means relatively easy job for the tuner. This approach is easy for a single band. For multiple bands, it can be tricky. Again, perfection is the enemy of good enough--just try it. There is a useful chart in the 1974 ARRL Antenna Handbook. Chart is posted at https://picasaweb.google.com/StarrGarretson/Fig353#5692717146409405762 . L in the chart is total length of one leg of center-fed dipole plus feedline or total length of antenna plus feedline for an end-fed antenna. When using the chart, keep in mind, that 1974 was before WARC bands. After all of the above, now my experience. For fifty-six years at eleven locations, my antenna has been an eighty-meter dipole (sometimes very contorted) or inverted-V put up with whatever supports I could muster. Open-wire feedline was whatever length was convenient. Using a Johnson Viking KW Matchbox with either 700 or 1,500 watts output, I rarely had RF in the shack on all contest bands eighty through ten. The few times I did, it was simple to add a bit of feedline to solve the problem. Be a ham, try it, and have fun. 73, Hank, W6SX Mammoth Lakes, California Elevation 8083 feet in John Muir's Range of Light ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
