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## [TowerTalk] re: feedline length

 To: [TowerTalk] re: feedline length TexasRF@aol.com (TexasRF@aol.com) Wed, 24 Sep 1997 08:45:05 -0400 (EDT)
 ```In answer to k4oj's question about cutting his 1/2" catv hardline: You can add RG11 jumpers at one or both ends of your 75 ohm hardline to "tune" the overall length to a multiple of 1/2 wavelength. As you know, 1/2 wavelength is the same as 180 degrees of line length. We want to use 180 degrees because the input impedance is repeated at 180 degree intervals along the line. If you want a line that is four 1/2 wavelengths long, then the total line length is 720 degrees. The signal at the input end does not know that this 720 degrees is made up by one or more different pieces of coax. All that matters is that the total length equal 720 degrees and all pieces of the coax are 75 ohms. As a side note, the loss of the coax will shift your 50 ohm load impedance a little toward the 75 ohm figure. I mention this only because at VHF or UHF, where cable losses are higher, someone might try to use this "trick" and wonder where their 50 ohms went. To understand this interesting characteristic, consider a coax cable long enough to have 10 db of loss. If this cable were short circuited at the far end, the reflected power would measure 10db + 10db for a total of 20db attenuated at the near end of the cable. This is the same as a vswr of 1.222, implying an impedance closer to 50 /1.222 (40.9 ohms) instead of the expected zero ohms. In the 20 meter case, the losses are much lower than 10db and the load impedance is much higher than zero ohms so the effect will be very much reduced. This transmission line stuff is great fun, eh! 73 de Gerald, K5GW, Owner/General Manager, Texas Towers -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search ```
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