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#1
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Robert Lay W9DMK wrote:
Just today, I made a careful measurement on an RG-8/U line of 5.33 meters length at 30 MHz and terminated with a 4700 + j 0 load. The Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and its Velocity Factor is between 0.75 and 0.80 The input impedance was actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25. The SWR at the load end was 94. Those two SWR's establish a total loss on the line of 0.15 dB. If one were to blindly apply the formula in The Antenna Book on page 24-9, the result obtained would be 4.323 dB. For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. -- 73, Cecil http://www.qsl.net/w5dxp |
#2
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On Sun, 28 Nov 2004 09:38:28 -0600, Cecil Moore
wrote: Robert Lay W9DMK wrote: Just today, I made a careful measurement on an RG-8/U line of 5.33 meters length at 30 MHz and terminated with a 4700 + j 0 load. The Matched Line Loss of that line at 30 MHz is 0.9 dB per 100 feet, and its Velocity Factor is between 0.75 and 0.80 The input impedance was actually measured at 2.45 -j15 ohms for an SWR at the input of 22.25. The SWR at the load end was 94. Those two SWR's establish a total loss on the line of 0.15 dB. If one were to blindly apply the formula in The Antenna Book on page 24-9, the result obtained would be 4.323 dB. For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. -- 73, Cecil http://www.qsl.net/w5dxp Dear Cecil, I hope I'm not misinterpreting your values - I assume that you are starting with a theoretical open circuit and a theoretical RG-8 line and calculating a theoretical impedance seen looking into that line of 0.57 + j 0. From that you then calculate a theoretical 0.2 dB. When I say calculate, I assume that you may instead by using a nomogram. Anyway, based on all of that being the situation up to but not including the loss figure, when I take the 0.57 + j0 and calculate the SWR as 87.7 I get a loss more like .05 dB, theoretically, so I'm not sure in what ways we are coming up with these numbers. I can explain exactly how I got mine, which was via measurements followed by a theoretical cacluation of loss based on the two SWR's formula which is built into all Smith Charts. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
#3
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Robert Lay W9DMK wrote:
..For your 1/4WL open stub on 10.6 MHz, with a stub impedance of 0.57 ohms, I calculate total losses of about 0.2 dB. I hope I'm not misinterpreting your values - I assume that you are starting with a theoretical open circuit and a theoretical RG-8 line and calculating a theoretical impedance seen looking into that line of 0.57 + j 0. From that you then calculate a theoretical 0.2 dB. When I say calculate, I assume that you may instead by using a nomogram. Not using a nomogram but everything is 100% theoretical. It doesn't matter what line is being used as long as it's Z0 is 50 ohms. Matched line loss didn't enter into my calculations. It's only total loss. Anyway, based on all of that being the situation up to but not including the loss figure, when I take the 0.57 + j0 and calculate the SWR as 87.7 I get a loss more like .05 dB, theoretically, so I'm not sure in what ways we are coming up with these numbers. Is that the additional loss due to SWR or the total loss? My theoretical loss is total loss and the matched line loss need not be known. The measured resistance of the resonant stub is all one needs to know besides Z0. I can explain exactly how I got mine, which was via measurements followed by a theoretical cacluation of loss based on the two SWR's formula which is built into all Smith Charts. I can't remember where the following formula came from. I think it was from an RF guru at Intel, but I can't be sure. I have a hand- written notebook of useful formulas covering 25 years but I didn't record where they all came from. The formula for theoretical TOTAL losses in a *resonant* stub: Total loss = 10*log{[(Z0-R)/(Z0+R)]^2} where R is the measured resistance of the resonant stub and Z0 is the characteristic impedance of the stub material. You can see the [(Z0-R)/(Z0+R)]^2 term is akin to a virtual rho^2 at the mouth of the stub. Since rho^2 = Pref/Pfor, the losses in the stub are equivalent to the losses in an equivalent resistance equal to the measured virtual resistance at the mouth of the stub. -- 73, Cecil http://www.qsl.net/w5dxp |
#4
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Cecil Moore wrote:
The formula for theoretical TOTAL losses in a *resonant* stub: Total loss = 10*log{[(Z0-R)/(Z0+R)]^2} where R is the measured resistance of the resonant stub and Z0 is the characteristic impedance of the stub material. You can see the [(Z0-R)/(Z0+R)]^2 term is akin to a virtual rho^2 at the mouth of the stub. Since rho^2 = Pref/Pfor, the losses in the stub are equivalent to the losses in an equivalent resistance equal to the measured virtual resistance at the mouth of the stub. In other words, replace the stub with a resistor having the same value of measured resistance as the stub, and calculate the I^2*R losses in the resistor. That will be the same value as the total losses in the stub. -- 73, Cecil http://www.qsl.net/w5dxp |
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