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swr question
Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being
defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... |
Fred W4JLE wrote: Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... Hi Fred, It would still be 1:1. Doesnt matter about the characteristic impedance of the transmission line as long as you use 1/2 electrical wavelength. Gary N4AST |
Sounds right to me, the 50 ohm load of the ant is "reflected" back by a
1/2 wave line... .... so, I am probably wrong... grin Warmest regards, John wrote in message ups.com... Fred W4JLE wrote: Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... Hi Fred, It would still be 1:1. Doesnt matter about the characteristic impedance of the transmission line as long as you use 1/2 electrical wavelength. Gary N4AST |
"Fred W4JLE" wrote in message ... Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... Hi Fred, Assuming that when you say the antenna is 'perfect' with a 1:1 SWR on a 50-ohm line it means that the input impedance to the line is 50 + j0 ohms. Since the line is 1/2 wl,and presumed lossless, the terminal impedance of the antenna is also 50 + j0. However, when you replace the 50-ohm line with one of 72 ohms the SWR on the line changes from 1:1 to 1.44:1. But since it's length is 1/2 wl the input impedance is still 50 + j0 ohms. The 1.44 SWR results from the 1.44 mismatch between the 72-ohm line and the 50 + j0 ohm impedance of the termination, the terminal impedance of the antenna. The 1/2 wl consists of two 1/4 wl lines in series. At the point 1/4 wl toward the input the line impedance is transformed up to103.68 + j0, (72 x 1.44) which is then transformed back to 50 + j0 (72/1.44) along the 1/4 wl section returning to the input of the line. Walt, W2DU |
This is a good illustration of one of Reg's hot buttons -- that "SWR
meters" don't actually measure the SWR on a transmission line, but rather are reporting the degree of match or mismatch at the meter's insertion point. In the case of 50 ohm line, it turns out that the "SWR" reported by the meter is actually the SWR on the 50 ohm transmission line, assuming that the meter is designed for use in a 50 ohm system and reads properly in that environment. When you substitute the 72 ohm line, the SWR meter will still read 1:1 because the impedance at its insertion point is still 50 ohms, but the SWR on the 72 ohm transmission line is actually 1.44:1. So the answer depends on what you mean by "what would be the SWR": The SWR on the 72 ohm transmission line will be 1.44:1 The SWR meter will read 1:1 If there is a 50 ohm line between your rig and the 72 ohm line, its SWR will be 1:1. Roy Lewallen, W7EL Fred W4JLE wrote: Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... |
"The head of the pin is now open for dancing..."
.... ahhh, missed this, so you now much choose: 1) I am going to use this for a practical application--so I really don't care how it works (like time)--I simply need something which works and functions on a equiv to a 1:1, frequency specific, voltage transformer. 2) I want to debate the physics of one-half wavelength lines. .... since my basic motto/bmod (basic method of operation) is "whatever works!" ... my choice is clear... grin Warmest regards, John "Fred W4JLE" wrote in message ... Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The head of the pin is now open for dancing... |
Fred W4JLE wrote:
Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The SWR on the 72 ohm feedline would measure 1.44:1. You do have an SWR meter calibrated for 72 ohms, don't you? :-) Your 50 ohm SWR meter will measure 1:1, but as Reg says, it is merely measuring the degree of match to your transmitter designed for 50 ohm loads. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil Moore wrote: Fred W4JLE wrote: Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being defined, as I am feeding it with exactly 1/2 electrical wave length of 50 Ohm feedline and it is 1:1 SWR measured at the source end. What would the SWR be if I substituted the 50 Ohm feedline with a 1/2 wavelength of 72 Ohm feedline? The SWR on the 72 ohm feedline would measure 1.44:1. You do have an SWR meter calibrated for 72 ohms, don't you? :-) Your 50 ohm SWR meter will measure 1:1, but as Reg says, it is merely measuring the degree of match to your transmitter designed for 50 ohm loads. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- It was stipulated in the original post that the swr was being measured at the source end for the 50 ohm line. If all one changes is the impedance of a 1/2 electrical wavelength line, and nothing else, the answer is 1:1. Did not say anything about measuring the swr anywhere but at the source. Whatever the antenna Z is in the 50 ohm example that gives a 1:1 match, determines the swr bridge characteristics. If the antenna Z was 72 ohms, then the bridge is 72 ohms for a 1:1 match. Change the transmission line to 72 ohms, still a 1:1. Gary N4AST |
wrote:
It was stipulated in the original post that the swr was being measured at the source end for the 50 ohm line. If all one changes is the impedance of a 1/2 electrical wavelength line, and nothing else, the answer is 1:1. Nope, not if a 72 ohm SWR meter is being used. An SWR meter calibrated for the transmission line Z0 of 72 ohms will read 1.44:1 even if the 50 ohm transmitter is happy with the 50+j0 ohm virtual impedance being presented to it. I have SWR meters calibrated for 50, 75, 300, 450, and 600 ohms - doesn't everybody? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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