30
Appendix A Cont’d
The peak modes should not be used for critical power measurements because of the chance for transients when initializing transmission. If critical
peak
power measurements are desired, you should use the T mode, or W mode with a hold setting 0.25 second, and take several readings to avoid random
peaks. It also might make sense to start at a lower power and raise power up to the desired level.
When I developed the LP-100A, I was at first confused by the occasional high peak readings, thinking there might be a problem with the meter. I built a
test setup just to test this, consisting of my HP-8640B signal generator, RF Communications Model 805 10W instrumentation amp and a Totsu RF relay.
This setup always produced the correct peak reading for either steady state or chopped signals, indicating that the occasional high peaks from my ham
transmitters where in fact real.
Transmission Line Z vs. Antenna Feedpoint Z:
The LP-100A measures the SWR/Z at the Load connector of the coupler. This relates to the actual antenna Z based on the type and length of
transmission line. For a 50 ohm antenna load, the only difference will be determined by line loss. For a higher SWR load, the complex Z (R+jX) of the
antenna will be converted by the transformer action of the feedline to some other value, which could even reverse the sign of the reactance. I am
planning to add entry screens for both the VCP and Plot programs to allow for this conversion to be done automatically for a number of user saved
feedline/antenna combinations. I also plan to incorporate manipulation of the Smith Chart display to allow easy display of the effects of delay lines and
matching stubs sometime in the future.
For a comprehensive review of the principles and math involved in transmission line theory, line transformations, etc., visit
http://www.cebik.com/trans/zcalc.html
This is copyrighted material published by L.B Cebik, W4RNL. He tackles a very complex subject and makes it as
understandable as possible.
Another excellent source of reading is the site of Dan Maguire, AC6LA,
See particularly
http://www.ac6la.com/stss.html
for a
treatment of the Smith Chart and other topics. Dan wrote one of my favorite antenna modeling programs, MultiNEC, unfortunately no longer available.
Many of the formulas used in impedance transformation are also included in the HamCalc collection by George Murphy, VE3ERP.
Sign of X and Phase:
The LP-100A is not capable of determining the sign of X or Phase on its own. With a VNA this is possible because they use quadrature signals and
multiple detectors to eliminate the phase ambiguity. There are one or two antenna analyzers which also do this in a similar or different way with good to
excellent results. This is relatively easy if you are generating the signal internally, but in the case of the LP-100A, the source is a high power signal out of
the control of the LP-100A. I have developed a circuit for sign detection, but it would require a rather substantial change to the PCB. I may change to it in
the future, but it wouldn’t be for quite awhile.
I have provided an automatic sign detection algorithm in the LP-100A Plot program which works quite well. It is based on detecting slope of both
magnitude and phase. I also provide a means to easily reverse the displayed phase/reactance of errant datapoints before saving/printing. With a little
practice, it will become easy to recognize and fix these points. The Plot program requires a serial interface between the PC and rig to allow control of the
transmitter.
Содержание LP-100A
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