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#24
September 27th 05, 03:25 AM
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Owen Duffy wrote:
On Tue, 27 Sep 2005 01:36:15 +0000 (UTC), wrote: Owen Duffy wrote: On Mon, 26 Sep 2005 21:39:27 +0000 (UTC), wrote: Jim, that seems inconsistent with your earlier statemetn "No, the SWR being measured is on the load side of the meter." The load side is the side with the load, i.e. the antenna, on it. In the example you quoted with a 100 ohm load on a 100 ohm line, were the line loss low, and the line long enough to be sure to sample a fully developed voltage maximum and voltage minimum it would be found that the VSWR was 1:1. Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a meter calibrated for a 50 Ohm system. I am sorry Jim, the VSWR is a property of the transmission line and its termination, and the VSWR on that 100 ohm line with a 100 ohm termination is 1:1. The VSWR could be *MEASURED* on that line by sampling the magnitude of the voltage at different points on the line and it would be found that the magnitude of the voltage was constant, which means VSWR=1:1. No, the measured SWR is relative to the design impedance of the SWR meter which is normally 50 Ohms. The SWR on the line depends on the characteristic impedance of the line and the impedance of the termination of the line. 50 ohms does not come into it. We are not talking about SWR on the line, we are talking about SWR at the input END of the line; big difference. The SWR on your proposed 100 ohm line with a 100 ohm termination is 1:1. If your measurement indicates anything else, then you need to consider your measurement as invalid. Once again, we are not talking about SWR *ON* the line, we are talking about SWR as seen at the input *END* of the line; big difference. Furthermore, the SWR *ANYWHERE* on the line is *NOT* 1:1 for a 50 Ohm reference. Try hooking a length of 93 Ohm line (which is easier to get than 100 Ohm line) terminated with a 93 Ohm resistor to any 50 Ohm SWR meter of any type. Then hook just the 93 Ohme resistor to the meter and tell me what the difference is in the readings. Owen -- If anything is misnamed it is the term SWR. SWR is nothing more than a dimensionless impedance ratio. You do NOT need a transmission line to have SWR in spite of the W in SWR standing for 'wave'. -- Jim Pennino Remove .spam.sux to reply. |
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