At the museum today, we got into a discussion of the reason for including both the Upper CW (UCW) and Lower CW (LCW) on the Omni VII radio. The explanation wasn’t on the tip of my tongue, so I decided to research this a bit.
On Icom radios, they call the Upper CW mode “CW-Reverse,” or CW-R. Here’s the explanation of how it works from the IC-756PROIII manual:
CW reverse mode
CW-R (CW Reverse) mode receives CW signals with a reverse side CW carrier point like that of LSB and USB modes. Use when interfering signals are near a desired signal and you want to change the interference tone.
If you have properly zero beat the signal of a station with which you are in contact, then the other station’s signal will sound the same in either mode. An interfering signal will, however, sound much differently, and, as shown, may be out of the passband altogether.
If you don’t zero beat properly, though, being in different modes will affect the way that the other station hears you. Say, for example, that both you and the other station are in normal CW mode (LCW on the TenTec). The other station’s transmit frequency is set to 7030 kHz. Your sidetone frequency is set to 500 Hz (meaning that the BFO frequency is 500 Hz above the receive signal), but you set your transceiver to hear a 600 Hz tone. To hear that 600 Hz tone, you will have to set your transmit frequency to 7030.1 kHz.
If you’re in UCW mode, or CW-R mode as Icom calls it, then the BFO frequency will be 500 Hz below the received signal, and to hear a 600 Hz tone, you’ll have to set your transmit frequency to 7029.9 kHz. In the first case, the other station will hear a 500 Hz tone when receiving your signal. In the second case, he’ll hear a 700 Hz tone.
To minimize the frequency difference between the two stations, it’s a good idea to check the setting of your sidetone frequency and zero beat as closely as possible with the other station. I would also avoid shifting between CW modes while in contact with another station, or if you do change modes, don’t change your transmit frequency while in contact. Use the RIT control to change the frequency of the tone you hear.