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ANTENTOP- 02- 2003, # 003

Old computer's PSU


The RF ammeter has two drawbacks. Firstly, it works on a limited frequency range, to 7 MHz for my test. Secondly, the RF ammeter is too frequency dependent. I eliminated these defects to a certain extent. A low- resistance resistor R1 bridged to the meterís winding reduces the frequency dependence and extends the frequency range. Fig. 10 shows the circuit for the "linear" RF ammeter. A germanium diode was used in the RF detector of the "linear" RF ammeter. A d.c. meter had a 100μA full scale deflection and 320-Ohm resistance. I picked up a value of the R1 that while at 1.5 MHz the control RF ammeter showed the RF current of 0.1A the detector"s meter had a full-scale deflection of 100μA. R1 had 110 Ohms in this case. All other measurements were made at RF current of 0.1A and R1 of 110 Ohms. Data for the measurements are shown in Tab. 5. Using data from this table I constructed diagrams "meter reading vs. frequency" (see Fig. 11).


Table 5 Linear RF ammeter


Frequency, MHz
















Meter reading,















Note: The RF current through a current loop is constant on all frequencies and equals to 0.1A




Fig. 11 shows that the RF ammeter (Fig. 10) provides almost linear measurement of the RF current on ranges of 160- 30 meters. It is possible to expand the measuring range of the RF ammeter with the help of a variable resistor connected in serial with the d.c. meter. Fig. 12 shows such a RF ammeter with an expanded scale.


Meter reading vs. frequency



RF ammeter with an expanded scale


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