A wattmeter built around a directional coupler always has to deal with finite coupler directivity. Making the problem more difficult is that we expect a wattmeter to be accurate over a rather wide frequency range, 1.8 to 30 or even 50 MHz. This places an even greater burden upon the directional coupler.
If the directional coupler has 30 dB directivity--a very good number to be maintained over a wide frequency range--then 1 KW forward power into a perfect load will show 1 watt reflected power, corresponding to an SWR of 1.065:1 instead of the expected 1.0000... for the theoretically perfect load. It is possible to measure the phase and amplitude of the coupled signal to "calibrate out" coupler imperfections. This is what is done with a vector network analyzer when the standard "open/short/load" calibration is applied. The VNA measures the phase and amplitude of the coupled signal when the through port is operated into an open circuit, a short circuit and a known value (resistance and stray L & C known) termination for each test frequency. The VNA then computes and applies an appropriate correction factor to correct for coupler errors. O/S/L calibration has been supplemented by more advanced techniques in newer VNAs. (There's a very good Application Note AN 1287-3 from Agilent on this subject available at http://cp.literature.agilent.com/litweb/pdf/5965-7709E.pdf. Bird Electric has a less technical Application Note on the effect of coupler directivity on SWR at http://www.bird-technologies.com/techapps/app_notes/StraightTalkAboutDirectivity.pdf) A wattmeter using diodes to measure RF voltage used with a directional coupler cannot apply sophisticated error correction to compensate for finite coupler directivity. At most, one can tweak a balance pot or trimmer cap to null the reflected signal at a single frequency and power level. Further complications result from the forward and reverse diode detectors being operated at different points on their sensitivity curve, etc. Hence, it is far from surprising that different wattmeters will show different SWR under ostensibly identical test conditions. Larry's LP-100 wattmeter operates with a different methodology and I'll leave it to him to explain the differences and how coupler directivity is considered. Jack K8ZOA www.cliftonlaboratories.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
