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Syllabus Sections:-

RF Power measurements

10d.1 Understand that steady RF power may be determined by measuring the RF potential difference across a dummy load.

Understand the meaning of PEP (peak envelope power) of an SSB transmission and that it may be determined using a peak reading power meter or an oscilloscope and dummy load.

Definition of PEP

Peak envelope power (PEP), is the maximum average power supplied by the transmitter to the antenna transmission line during one radio frequency cycle taken under normal operating conditions.

Peak Envelope Power (PEP) is the only accurate and meaningful way to compare transmitter power levels between stations

SSB POWER METERS.

RF power meters need a continuous carrier to read continuously, which presents a problem with SSB as the power is never continuous, varying with speech. With the addition of a suitable capacitor or time constant the capacitor is charged on voice peaks, holding the reading as the human voice goes up and down, if there is no signal present then the meter reading drifts down to zero.

The diagram below is of an SWR bridge which is also a Peak Reading Power Meter which will be dealt with more in the next section but is added here as in the text above we mention the Capacitors to store the charge.

PEP Peak Envelope Power.

Another and perhaps more correct way to measure pep is to use an oscilloscope, dummy load and an audio 2 tone generator.

Set the audio tones to say 600hz and 2khz (not harmonically related) and the same amplitude, connected to the audio input of the SSB Tx, set the scope to measure the voltage across a dummy load. The pattern shown on the oscilloscope will be approximately as shown below.

The Peak Envelope Power (PEP) is the maximum average power supplied by the transmitter to the antenna transmission line during one radio frequency cycle taken under normal operating conditions.

On the scope screen you need to read the peak to peak voltage say reads 80 Volts peak to peak then PEP = VRMS2/R

Find the RMS of 80V peak to peak / 0.707 = 56.56

With PEP = VRMS2/R ( R = 50 the dummy load)

then,

PEP = 56.562/50

=3199/50

= 63.98 say 64W

The deflection on the oscilloscope is due to proportion of the voltage causing it.

Not everyone has an oscilloscope or 2 tone generator so the pep reading meter is preferred.

THE DUMMY LOAD.

A dummy load is a resistor or a combination of resistors which is used in place of an antenna for the purpose of testing or tuning. The resistor nominally 50 and is of a non inductive material such as carbon.


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