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Measuring RF power has traditionally been notoriously difficult, except perhaps in the singular case of continuous-wave (CW) sources that produce nice, well-behaved sine waves Even in that limited case, however, some measurement methods are distinctly better than others The peak voltage of a waveform is 100 V (ie, peak-to-peak 200 V) Given that the CW waveform is sinusoidal, we know that the root mean square (RMS) voltage is 0707 V The output power is related to the RMS voltage across the load by (Vrms)2 Zo
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P= where: P is the power, in watts Vrms is the RMS potential, in volts Zo is the load impedance, in ohms
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[2711]
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If we assume a load impedance of 50 , then we can state that the power in our hypothetical illustration waveform is 100 W
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530 Measurements and adjustment techniques We can measure power on unmodulated sinusoidal waveforms by measuring either the RMS or peak values of either voltage or current, assuming that a constantvalue resistance load is present But the problem becomes more complex on modulated signals The various power readings on a Bird model 4311 peak power meter, the peak (PEP) and average powers, vary markedly with modulation type One of the earliest forms of practical RF power measurement was the thermocouple RF ammeter (see Fig 27-11) This instrument works by dissipating a small amount of power in a small resistance inside the meter, and then measuring the heat generated with a thermocouple A dc current meter monitors the output of the thermocouple device, and indicates the level of current flowing in the heating element Because it works on the basis of the power dissipated heating a resistance, a thermocouple RF ammeter is inherently an RMS-reading device Because of this feature it is very useful for making average power measurements If we know the RMS current and the resistive component of the load impedance, and if the reactive component is zero or very low, then we can determine RF power from the familiar expression: P = I 2 Ri [2712]
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There is, however, a significant problem that keeps thermocouple RF ammeters from being universally used in RF power measurement: those instruments are highly frequency-dependent Even at low frequencies, it is recommended that the meters be mounted on insulating material with at least 3 8-in spacing between the meter and its metal cabinet Even with that precaution, however, there is a strong frequency dependence that renders the meter less useful at higher frequencies Some meters are advertised to operate into the low-VHF region, but a note of caution is necessary That recommendation requires a copy of the calibrated frequency response curve for that specific meter, so that a correction factor can be added (or subtracted) from
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27-11 Circuit for thermocouple RF ammeter
dc voltmeter
Selecting and using RF wattmeters and antenna VSWR meters 531 the reading At 10 MHz and higher, the readings of the thermocouple RF ammeter must be taken with a certain amount of skepticism unless the original calibration chart is available We can also measure RF power by measuring the voltage across the load resistance (see Fig 27-12) In the circuit of Fig 27-12, the RF voltage appearing across the load is scaled downward to a level compatible with the voltmeter by the resistor voltage divider (R2/R3) The output of this divider is rectified by CR1 and filtered to dc by the action of capacitor C2 The method of measuring the voltage in a simple diode voltmeter is valid only if the RF signal is unmodulated and has a sinusoidal waveshape While these criteria are met in many transmitters, they are not universal If the voltmeter circuit is peak reading, as in Fig 27-12, then the peak power is P= Vo 2 R1 [2713]
The average power is then found by multiplying the peak power by 0707 Some meter circuits include voltage dividers that precede the meter and thereby convert the reading to RMS, and thus convert the power to average power Again, it must be stressed that terms like RMS, average, and peak have meaning only when the input
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