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#22
September 26th 05, 10:39 PM
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Owen Duffy wrote:
On Mon, 26 Sep 2005 20:38:01 +0000 (UTC), wrote: Reg Edwards wrote: I have not insisted, not even mentioned, that SWR can exist only on 50-ohm lines. All I have said, somewhere, is that the usual SWR meter gives the correct answers only on 50-ohm lines. ---- Reg. If I connect a 100 Ohm antenna through a 100 Ohm transmission line to a SWR meter designed for 50 Ohms and then to a transmitter which expects a 50 Ohm load, does the meter read correctly with respect to the desired transmitter loading? Of course it does. 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. Yet we would expect the "SWR meter designed for 50 Ohms" to which it is connected (on the load side) to read VSWR=2:1, so is it measuring the SWR on the load side of the meter as you earlier stated? Yep. Perhaps typical SWR meters are actually sampling current and voltage on a very short section (almost a point sample) of transmission line that is part of the instrument, and they are indicating what the observed SWR would be on an extended line of that type, provided that attenuation was insignificant, and that the extended line was sufficiently long to allow full development of the standing wave pattern. The SWR that most people care about is that of the antenna. Usually you have a 50 Ohm transmitter connected to 50 Ohm line, and then to an antenna you hope is 50 Ohms. To accurately measure the antenna SWR without any error being introduced by line losses, you have to put the SWR meter at the end of the line adjacent to the antenna. This is usually impractical and we normally put the SWR meter near the transmitter. In this case the meter measures the SWR of the entire system, i.e. the line going from the meter to the antenna and the antenna. The net practical effect of the line loss for real line and real antennas is that the observed SWR will indicate a lower value than if the meter were directly connected to the antenna. The inference of what is happening on adjacent line is ours, not the instruments, as demonstrated by your example above. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
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