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On Wed, 29 Jun 2005 14:07:26 -0700, Frank Gilliland
wrote: On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore wrote in : Frank Gilliland wrote: Impedance matching of an SWR meter is generally unimportant since most SWR meters used for HF have a directional coupler that is much shorter than the operating wavelength. Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms. The point is that the error is insignificant when the directional coupler is much shorter than the wavelength. In a word, baloney. The error is independent of length. A zero length bridge calibrated at 75 ohm is in error when measuring in a 50 ohm system. Period. The error is even more insignificant when there are a host of variables and confounds between the SWR meter and the transmitted field that can (and frequently do) affect the objective -- field strength. Often, field strength is of zero importance. What do you do when the device under test isn't supposed to radiate? The simplest example of this would be a CATV system, yet VSWR is *extremely* important in cascaded networks. It's much simpler (and just plain logical) to measure the field strength directly instead of measuring an abstract value halfway towards the objective and relying on nothing more than speculation that the rest is working according as expected. More baloney and it isn't even sliced. |