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Part 3 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Toroidal Current Transformer Example & Estimating Transmitter Resistance

Author: R.J.Edwards G4FGQ © 28th October 2000

Program Notes (Continued)

Example of Toroidal Current Transformer
The ferrite ring core is of a size which when wound with 24 AWG wire can be slipped over the coaxial polythene insulant. Typical dimensions: OD=1/2"=13mm. ID=1/4"=7mm. Thickness 1/6"=4mm. With a permeability of 200, 1uH needs 3.4 turns. 15 turns gives 19.5 uH which has a reactance of 220 ohms at 1.8 MHz. A 33-ohm shunt resistor gives a satisfactory reactance to resistance ratio of 6.7 This transformer would be suitable for a 30 to 100 watt, 50-ohm SWR meter.

How to Estimate Transmitter Internal Resistance (program asks for value)
For accuracy do the measurements on the lowest frequency band, say at 1.9 MHz. Two dummy loads are needed, at least one of which has the standard Zo for the transmitter. The other should be of the same or preferably a higher value. Set the PA drive to give roughly 1/2 full rated power into Zo and measure voltage V1 across Zo. Use T-junctions as necessary. Do not readjust Tx drive again.

Now use a second T-junction to connect the second dummy load in parallel with Zo and measure the load voltage again which will fall to V2. Keep all coaxial leads short. Let the resistance of the two loads in parallel be R2.

Source resistance, Zs = (V1-V2)/(V2/R2-V1/R1) ohms. Depending on usage of ALC and type of output circuit Zs may lie between several 100-ohms and Zo or less.

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