Part 4 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - More NotesAuthor: R.J.Edwards G4FGQ © 28th October 2000
Program Notes (Continued)
Ideally, the ferrite core of the current transformer should have a permeability large enough to need only one turn on the primary winding. The single turn is then the inner coaxial conductor plus polyethylene with a short gap in the braid. The gap in the braid should be no longer than is necessary to obtain a connection from the inner conductor to the voltage divider. Stray capacitance between the exposed inner and the secondary winding can be included in the divider chain with careful layout. A frequency range from 1.8 to 30 MHz should be possible.
Accuracy at the higher frequencies falls off due to stray capacitance in the divider chain, to self-resonance in the transformer secondary winding and to increase in ferrite core loss. However, core loss is equivalent to a shunt resistance across the winding and adverse effects can be reduced simply by experimentally increasing the value of the actual shunt resistance above the computed value.
Core permeability should exceed 100. 200 is preferred. 500 may be too high due to associated high core loss. As a guide secondary turns should be between 10 and 30. Few turns results in greater power dissipation in the shunt which has to be physically small, preferably 1/2-watt or less, for a good HF response.
Use metal film resistors. The shunt resistor should be between Zo/4 and 3*Zo, the smaller values for high power transmitters, higher values for QRP.
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