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## [TowerTalk] Amidon UNUNs

 To: [TowerTalk] Amidon UNUNs k4ab@avana.net (k4ab@avana.net) Tue, 18 Mar 97 17:35:15 PST
 ```Just a little note on this thread as it regards converting the 75 ohm CATV cable to 50. I,m sure all of you are aware than an electrical half wave of transmission line, or any multiple thereof, will repeat the impedence it sees on one end at the other end, regardless of the impedence of the line. In other words, a .5 electrical length of line of any impedence may be connected to a 50 ohm load at one end, and it will measure 50 ohms at the other. All that is really necessary is to ensure that the CATV line is a multiple of 1/2 wave electrically and the cable will repeat the antenna impedence at the shack end. As a matter of fact, this is true of any impedence cable, open wire line, ect. While you might be unfortunate enough to have to add about 100' at 3.8, ( if you came up inches short ), it's only about 50+ feet for 40, 25' at 20 and so forth. (using .8 for a velocity factor.)) In essence, 25' +/- at 20 rolled up in a coil outside the shake adds practically nothing in terms of db loss, and is one heck of a lot cheaper than transformers, the necessary connectors ect. This principle is also true even if the .5 ( or multiple ) wavelength of line is made of coax of different impedences. In other words, you could have 50 feet of 50 ohm line, joined to 50 feet of 75 ohm line, and it will also act as is the line were an identical section of cable. Just one word of caution; published velocity factors cannot be trusted. I have seen VFs vary by as much as 10% on the same type line. If you've got the MFJ 259, it's a snap to get the line right in almost no time....but remember, while you're doing that, see what the effect of cutting about .75" of cable off the end does....because the PL-259 or whatever, is going to substract that. If you measure 20khz for 1", cut the line for 20khz above where you want it. 73 Ed ------------------------------------- Name: Ed Sleight E-mail: k4sb@avana.net To: Date: 3/18/97 Time: 5:35:15 PM ------------------------------------- -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Sponsored by: Akorn Access, Inc. & N4VJ / K4AAA ```
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