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[TowerTalk] Re: Force12

 To: [TowerTalk] Re: Force12 Mike"
 ```Hey Tom, See my comments below: ----- Original Message ----- From: "Tom Rauch" To: ; "Mike" Sent: Thursday, October 11, 2001 4:26 AM Subject: Re: [TowerTalk] Re: Force12 > > What you say is true if the characteristic impedance of the line is > > equal to the system reference impedance (Zo). In other words, if your > > are changing the length of a 50 ohm feedline in a 50 ohms system, then > > the only thing that can improve VSWR would be additional loss from the > > line (e.g. the VSWR will be constant along the length of a lossless > > line). Thus, system bandwidth will be the same at any point along the > > length of the lossless line whose Zo matches the system Zo. > > What Wes is saying is ALWAYS correct Mike. > > > > If the line Zo is different than the system Zo (e.g. the classic case > > of 75 ohm CATV cable used in a 50 ohm system), then the direction of > > the load reactance change versus frequency can have a significant > > bearing on the system bandwidth. In this case, the length of the line > > also plays a significant role. > > Except for effects of losses: > > 1.) The SWR on the 75-ohm line remains constant with length of > the 75-ohm line. > Yes, but only if you are measuring VSWR referenced against 75 ohms. If your are in a 50 ohm system, then the VSWR measured (relative to 50 ohms) will change as a function distance along the 75 ohm line > 2.) The SWR on the 50-ohm line remains constant with length of > the 50-ohm line. > > None of this has anything to do at all with 50-ohm line length > changing, and the SWR or bandwidth improving on that 50 ohm line > as the length is made longer. > > A 50-ohm transmission line can perform NO matching function to a > 50-ohm transmission line when it is connected in series as a > normal transmission line, with the exception of an SWR > improvement through additional loss. > I am confused, I thought that is what I said?? > Let's say the load is 35 ohms and the transmission line 50 ohms. > Any length of transmission line will have a 50/35= 1.43 : 1 SWR > anywhere along that line regardless of length. > Yes, quite correct. > If the line is 75 ohms, SWR on the line will be 75/35 or 2.14 : 1. > 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. > This is why the impedance appears as a circle centered on the > normalized impedance of a Smith Chart! I certainly hope we are not > telling people that the Smith Chart, used before we we born and > still used today, is wrong! No, I haven't given up faith in the Smith Chart. Its a great tool. I think the nature of our disagreement may have been due to my capricious response to your original post. I wasn't following the thread and didn't realize that you were talking strictly about the length of a 50 ohm cable in a 50 ohm system. And even then, I am embarassed to say, it took my more than a few seconds to convince myself that it was strictly true in that case :) > > The only exception is loss. In the case where the line Zo matches the system impedance, I would agree. > > 73, Tom W8JI > W8JI@contesting.com 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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