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#11
July 9th 15, 07:06 PM
posted to rec.radio.amateur.antenna
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An antenna question--43 ft vertical
Jeff wrote:
On 09/07/2015 17:03, Ralph Mowery wrote: "Jeff" wrote in message ... By 'apparent SWR' he means as indicated SWR on the meter, and yes it can change at various point on the line due to inadequacies in the meter; the 'real' VSWR will of course remain the same at any point on a lossless line. Jeff That is what I mean Jeff. If there is any SWR, by changing the length of the line, the voltage/current changes in such a maner that at certain points you may get a 50 ohm match at that point. Absolutely NOT. By changing the length of a transmission you will NEVER create the situation where you get a 50 ohm match from an initial mismatch. This is clearly demonstrable on a Smith chart. Take any starting point other than a pure 50 ohms and add a length of transmission line. What you will find is that as you increase the length of line your point will merely rotate around the chart at a fixed radius (known as a constant VSWR circle), it will never spiral into the centre which is 50 ohms and where it must be for a perfect match. The only time that it will start to spiral inwards is if the line is lossy, but you will need a very long length, and the spiralling inwards is due to the loss in the coax NOT any matching characteristics due to the length of line. If such an effect as you are talking about is observed it is merely due to the finite, and often poor, directivity of the SWR meter giving you a false reading. Also it is worth noting that achieving 50 ohms as a magnitude |Z| of the complex impedance (Sqrt(R^2+X^2)) is not the same as getting a good match with 50 ohms resistive. Even if |Z| = 50 ohms it will have a VSWR greater than 1 if Z0. Again, plot the point on a Smith chart and you will see that it can never be in the centre of the chart. Jeff That is easy to disprove Jeff. If I have a 50 ohm load and use a 1/2 wave of any impedance line other than 50 ohms, the swr will be greater than 1:1, except at 1/2 wave multiplies of the line. At this point there will be a 50 ohm match. The swr of the line will not actually be 1:1 but some greater value. That is correct, but not the situation that we are discussing, we are talking about matching a load to a 50 ohm transmission line. In that case changing the length of line will NEVER result in a match. Using a *different impedance* length of coax as a transmission line transformer is a totally different case, and as you say will result is a standing wave on the line and associated losses. Jeff So you are only interested in special cases? -- Jim Pennino |
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