Home |
Search |
Today's Posts |
#7
|
|||
|
|||
Frank Gilliland wrote, among other things, "The point is that the error
is insignificant when the directional coupler is much shorter than the wavelength." Certainly "directional couplers" for HF may be built at essentially zero length, and ideally would have exactly zero length, monitoring the current and voltage at a single point on a line. Then SWR or reflection coefficient magnitude or even complex reflection coefficient may be calculated under the assumption we know the desired reference impedance. But if the equipment combines the voltage and current samples in the wrong ratio, you will get the WRONG answer. Even if the coupler looks like a perfect 50 ohms impedance section of transmission line (with some attenuation), the error _in_measurement_output_ can be significant indeed. Just because the coupler looks like a 50 ohm line to the line it's hooked to doesn't mean it will read zero reflection when IT's presented with a 50 ohm load. And by the way, not everyone who measures and cares very much about SWR (or reflection coefficient) cares a whit about field strength. Not all loads are antennas. Indeed, as Reg says, we might do better in amateur applications to consider the SWR meter as an indicator of the degree to which we're presenting a transmitter with the desired load. That's really what we're using it for, most of the time. It may ALSO be interesting to know the field strength, but please be aware that a transmitter's distortion products may be significantly higher if it's presented the wrong load impedance, even though the power output may be increased. Field strength alone is not acceptable to me as a means to adjust an antenna load to a transmitter, or as a way to adjust the operating point of the transmitter. Cheers, Tom |