# 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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