How to interpret power meter readings during SSB transmission
A topic we quite often see addressed in different ham radio fora is how to interpret power meter readings when using SSB. Many often conclude that their transmitter doesn’t provide the rated power because the readings are so low when they speak into the microphone. There are often three different reasons for this confusion: (1) What is really meant by PEP (2) What does a power meter measure (3) And maybe some lack of understanding of signal theory.
Peak Envelope Power is the power related to the peak value of the amplified modulated signal. One simple way to measure your transmitter’s PEP is to apply a single, clean frequency to the mic input. If this tone gives you 100W on normal ALC deflection, your PEP is 100W when you speak into your mike with normal ALC deflection.
I tried this on my FT-2000, and to check the frequency response on the input, I just changed the frequency on the single audio tone. The output was 100 W on 100 Hz, 500 Hz, 1000 Hz, 1500 Hz, and so forth, but dropped some at 3000 Hz. Of course this response is dependent on your EQ settings.
If you apply two tones: Would you expect that your transmitter would be able to supply 100 W to both tones (which would imply that your transmitter would transmit two independent carriers with 100 W in each)? I believe not (unless you have the 200W PEP transmitter). If your watt meter showed 100W when you applied two different audio frequencies to your TX input, wouldn’t that imply that your power meter actually adds the power components in your signal? So if you increased the number of tones to 7, and you still expected that the power meter should give you 100 W – what should then happen if you increased the number of frequencies to 70? Still 100W? When you speak into your mike, you apply a continuum of tones over your available bandwidth, and your power meter will measure the average power in those frequency components – not the sum.
I believe that its common experience that if you increase the bandwidth of your audio signal, your measured average power drops. If you want to be economic, use a narrow audio bandwidth so your available power is concentrated on the part of your voice spectrum that carries information. (That is why CW is so much more effective than SSB).
Several newer transceivers give you the possibility to increase the bandwidth far beyond the standard 2400 Hz that people are used to, and this changes the average power measured with the watt meter on the transceiver. I guess this has confused quite a few hams.
One more trick to try. An ordinary power meter is not able to measure peak power, and it is an instrument that is useless to measure emitted power during speech transmission. One reason is that the needle simply isn’t fast enough.
I applied a single 1000 Hz tone that gave me 100W output. Then I involved a switching of the tone to generate a train of pulses – first slowly, then faster and faster. I knew that the signal was exciting the transmitter to 100 W, but when the duty cycle was less than half a second, the power meter was not able to follow any more. I then turned up the speed by some factors (created a fast train of pulses) and the needle settled at about 20 W – but still the PEP was 100W.
I have a Daiwa CN-101L with a so called PEP selection. During my tests, I proved that this instrument was not able to measure PEP at all. But it has a large condenser that create an overshoot, but that isn’t PEP!