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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 |
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