[Elecraft] K3 - SWR Indication

[Jack Smith]

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