SSB Transmit Power: Audio Required!
Recently, I was talking with a new ham about a problem he was experiencing when trying to “tune up” his transmitter. While operating SSB (single-sideband) on 10 meters, he pressed the Push-To-Talk (PTT) button on his microphone but no RF power came out of the transmitter. He was confused and started to think that his transmitter wasn’t working properly. What’s going on with SSB transmit power?
To understand what is going on, we need to look at the various types of modulation we use with ham radio. The figure below shows the RF output for Morse Code (Continuous Wave or CW), Amplitude Modulation (AM) and Frequency Modulation (FM). The Morse Code (CW) case is the simplest as we are just turning the transmitter carrier on and off as we press the key.
The AM waveform is more complex because the RF carrier is modulated by the microphone audio. On the peaks of the audio, the amplitude of the RF carrier also peaks. SSB is a particular variation of AM, but with a critical difference. Conventional AM has a carrier that is always present and the voice modulation causes that carrier to increase and decrease instantaneously. SSB is a form of AM that has the carrier suppressed. This is one of the reasons that SSB is more efficient than AM— no power is “wasted” in the carrier. This also means that when there is no audio from the microphone, there is no RF output from the transmitter. This is what our new ham experienced: press the PTT button and nothing comes out…until he speaks into the microphone.
With FM, the modulation is applied to the frequency of the carrier and the amplitude remains constant. Press the PTT button on an FM transmitter and you will get a nice steady RF output. Speaking into the microphone causes the frequency of the carrier to change but the amplitude remains constant. This is probably the behavior our new ham was expecting: push the button and transmit a carrier.
There are many reasons we might want to key up our transmitter. To use an SWR meter to check our antenna system, we usually need to have a steady carrier coming out of our transmitter. If we are using an antenna tuner, we normally need a carrier to drive the antenna system while adjusting the tuner settings. Older radios (or amplifiers) may have vacuum tubes in the output and a matching network that needs to be tuned up, requiring some power transmission to affect the tuning process. Or maybe we just want to key up the transmitter to see how much RF output power we are putting out. Whatever the purpose, it is handy to have a way of quickly transmitting a steady RF carrier.
So how do we do this? Well, it depends. It depends on the specific design of your transmitter and how your ham shack is configured. If your rig has an internal antenna tuner, it probably has a “tune” button to activate the transmitter and adjust the tuner, as shown in the image here. Often, these tune buttons automatically reduce the transmit power to minimize interference on the bands, but outputs a carrier as required for the tuning operation. (You should always try to “tune up” on an unused frequency.)
If you have a Morse Code key connected to the radio, you can just flip to CW mode and operate the key to produce a steady carrier. Some transmitters allow you to use the microphone PTT to key the transmitter in CW mode. Just flip to CW mode and press the PTT button. If your station is set up for RTTY or other digital modes, you can probably use those modes to transmit a steady carrier. Another option is to use AM or FM to output a constant carrier. (Be careful to not have any audio coming into the microphone, else you won’t have a steady carrier on AM or you may inadvertently transmit FM in a band where it is not allowed.) You’ll need to fiddle with your radio and maybe even read the manual to figure out what works best with your rig.
-- Bob KØNR