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[TowerTalk] Re: Force12

 To: [TowerTalk] Re: Force12 n4kg@juno.com (n4kg@juno.com) Thu, 11 Oct 2001 09:56:40 -0600
 ```On Thu, 11 Oct 2001 "Mike" writes: BIG SNIP > > > Yes, of course, but switch over to a 50 ohm reference and then > mover your 50 ohm VSWR meter along the length of the line. > At 1/4 wavelength from the 35 ohm load, the 50 ohm VSWR will > be 160/50 = 3.2:1. Move another 1/4 wavelength (total 1/2 > wavelength) from the load and the 50 ohm VSWR will be 50/35 = > 1:43:1 VSWR. > HUH? Where did you get 160 Ohms? The formula for a 1/4 WL transmissionline transformer is Zin X Zout = Zo^2 (Zo squared) Zout = Zo^2 / Zin = 50^2 / 35 = 2500 / 35 = 71.4 ohms at 1/4 WL from the 35 ohm load for a 50 ohm cable. SWR = 71.4 / 50 = 1.43, same as at the load, ignoring losses. OH wait, I see, you are assuming a 75 ohm coax. Then the IMPEDANCE 1/4 WL from the 35 ohm load is 160 ohms but the SWR = 160 /75 = 2.13, same as at the load (75 / 35 = 2.14). SWR is a function of the LOAD impedance AND the LINE impedance. You can't just arbitrarily change the reference impedance unless you also change the line impedance. IF you placed a 50 ohm line after 1/4 WL of 75 ohm line with a 35 ohm load, THEN the SWR in the 50 ohm line would be 160 / 50 = 3.2 as you stated, but the SWR in the 75 ohm line is still only 2.14 IF you place a 50 ohm line after 1WL of 75 ohm coax the SWR = 50 / 35 = 1.4 in the 50 ohm line and is still 2.14 in the 75 ohm line. Using a REFERENCE impedance that is different from the LINE impedance is MEANINGLESS. Tom N4KG ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems, Trylon Titan towers, coax, hardline and more. Also check out our self supporting towers up to 100 feet for under \$1500!! http://www.anwireless.com ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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