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(Last part of an article by Bill Orr W6SAI, previously published in QST June 1993)
What the SWR meter does NOT tell you
Let's take an example of an antenna having a feedpoint resistance at resonance of 16.7 ohms (not an uncommon value for a multi-element yagi). The antenna is fed directly by a 50 ohm line. We will assume there is no unwanted coupling between the line and the antenna. Using a RF impedance bridge, the resistance and reactance are measured along the line at 1/16-wave intervals. For 20 metres that is a distance of about 4.3 feet. The results are read out in terms of resistance and reactance, both expressed in ohms. The theoretical SWR on the line is the ratio of line impedance to resonant feedpoint impedance, in this case 50/16.7 or SWR =3:1. And, assuming the line is lossless, that is what a good SWR meter at the station end of the line will indicate. The RF impedance bridge however reveals a more complex picture along the line. Note that if the line is a quarter of three-quarters wavelength long, the impedance at the measuring point is 150 ohms. But at one-eighth and three-eights points, the impedance is 30 ohms plus or minus a reactive term of 40 ohms. Based on these observations, how could I improve my transmitter loading on 10 meters? I could only achieve about 35 watts output at the resonant frequency of the antenna. At this point the indicated SWR was 2:1. I did not want to fiddle with the antenna. As a test I added three feet to the transmission line, right after the SWR meter. The line from the operating position to the antenna is now three feet longer than before. Power output of the transceiver went up from 35 watts to 70 watts. The SWR remained constant at 2:1. I then added an extra 2.5 feet, making the transmission line 5.5 feet longer than the original length. Bingo! The transceiver went up to full power output of 120 watts. Again, measured SWR was still 2:1. The SWR meter wasn't any help to me in this situa
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