Bredhurst Receiving and Transmitting Society

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.

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.

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 drive so the calculated power output is 200 watts into a dummy load. The pattern shown on the oscilloscope will be approximately as shown below

the PEP is twice the mean power ie 400watts or 26dbw.

Other power levels may be determined from this

Say 26dbw = 400 watts = 6 cm pk/pk on the scope.

then transmit using voice the peaks are 3 cm pk/pk.

then,

400 x (3)²/6²

=400 x (3/6)²

= 100watts or 26dbw.

The deflection on the oscilloscope is due to proportion of the voltage causing it and for power the proportion is due to the square of the voltage hence the scopes measured deflection is squared.

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.