Before we begin using the transmitter, we must adjust it to ensure it is connected to an antenna appropriate for the frequency in use.
Ideally the antenna should be a good match for the transmitter. The quality of this match can be assessed using a SWR meter. If the antenna is not connected, or the antenna is inappropriate for the frequency in use or otherwise a poor match then the SWR meter will show a high reading. Conversely, an antenna which is a good match for the transmitter will show a low SWR reading.
It may be that the most appropriate antenna we have for the frequency in use is not a very good match for the transmitter and shows a high SWR reading. In this case we can use a device called an antenna tuning unit (ATU) to assist us in matching the antenna to the transmitter. The ATU is adjusted until a low SWR reading is obtained. This process of adjusting the ATU is known as "loading the antenna" or "loading up".
SWR measurement and ATU adjustment should always be done using as low a power output from the transmitter as possible to avoid causing interference to other users.
These topics are discussed further in the section on antennas.
Transmitting with a high SWR, or with no antenna at all, will damage your transmitter. The repair bill for such a fault is likely to be very high.
An excessively large input signal will cause the peaks of an AM signal to become too large and reduce the troughs to zero. This will make your signal sound very distorted to the person receiving it. The effect for the listener is similar to turning the volume of a small cheap domestic radio (or one with flat batteries) up well beyond the point at which distortion begins. It also is likely to cause interference to users of adjacent frequencies.
An excessively modulated FM signal is also likely to cause interference to users of adjacent channels. If operating through a repeater, the system may fail to retransmit your audio if the deviation is excessive.
Over-modulation of a transmitter may also cause the radio to output signals outside the amateur bands, particularly if you are operating close to a band-edge. We must remember that it is a condition of our licence not to radiate outside these bands, or to cause interference to other users.
The most common cause of over-,modulation is the incorrect setting of the microphone gain control, and the volume control of external amplified microphones. Remember if you change the microphone, you must reset the microphone gain control. Also if you use a TNC to connect your computer to your radio, then its output may need to be adjusted to prevent over-modulation.
Shouting into the microphone (and laughing loudly) will also cause this effect and should therefore be avoided - turn away from the microphone when laughing loudly