The 2019-2023 General License question pool asks the meaning of split mode operations:
G4A03: What is normally meant by operating a transceiver in “split” mode?
A. The radio is operating at half power
B. The transceiver is operating from an external power source
C. The transceiver is set to different transmit and receive frequencies
D. The transmitter is emitting an SSB signal, as opposed to DSB operation
Single sideband (SSB) operations on the HF bands sometimes require slightly cleverer operating techniques than we’re accustomed to with most FM or digital ops on the VHF/UHF bands. Operating in split mode is one of those techniques that can come in handy in a difficult situation. Here’s the scoop…
Suppose you’re dialing across the 20-meter band one fine sunny afternoon and you hear a station calling from Mauritius, an island off the east coast of Madagascar. What a DX catch that would be! But after hearing the snippet of the station operator’s call followed by “QRZ,” your receiver gets blasted with a pile of strong, local transmissions, all in response to the Mauritius call. And some of the US stations go on and on for many seconds, striving annoyingly to make their call signs heard to Mr. Mauritius, the highly desired, rare DX contact. As a result of the cacophony of strong-signal responses in the US, no one was able to hear the relatively weak signal of Mr. Mauritius responding with one particular station ID that he desired for a contact. Everybody loses because of this operational inefficiency – even though contacts will be made within the chaos, a greater number of contacts could be achieved in the same time with smarter ops.
There is a better way to handle such a pile up. The highly desired station that is running the pile up can operate in split mode. In split mode the operator adjusts the transceiver to transmit on one frequency and receive on a different frequency. With the other stations complementing this operational arrangement, the pile up chaos is reduced significantly. Follow the reasoning…
Mr. Mauritius establishes a 5 kHz split: He transmits on 14.250 MHz, but he receives on 14.255 MHz.
The US stations complement by transmitting on 14.255 MHz while receiving on 14.250 MHz, exactly inverse of Mr. Mauritius.
Mr. Mauritius picks out a US station ID heard on 14.255 MHz and responds with a transmission on 14.250 MHz.
The strong-signal US stations are no longer transmitting on 14.250 MHz, so the weak signal of Mr. Mauritius is heard by the US stations without all the strong QRM masking his signal.
The lucky US station identified by Mr. Mauritius gets the contact, the other stations hear the station ID and end their raucous transmissions to await the next “QRZ” utterance from Mr. Mauritius so they can try again.
With this operating technique the pile up runs smoother and more efficiently, and more contacts can be squeezed into the air time made available by Mr. M and the often capricious atmospheric conditions of our planet. However, this is not the end of the story.
In order for split mode to work well everyone has to play along. Just as with your imagined scenario, there will be other stations dialing along and discovering this exceptional DX contact opportunity over time. And those stations will jump into the fray, transmitting in ignorance on 14.250 MHz instead of 14.255 MHz because that’s where they hear Mr. M transmitting. The whole problem again rears its ugly head.
So, Mr. M has to run the pile up properly. The highly desired station should include the split mode information in its transmissions very frequently so that new stations stumbling across the party understand the game. For instance, Mr. Mauritius may want to state at the end of a contact, “This is 3B8ABC listening up 5 kHz. Listening up five. QRZ?” This way, any new stations wishing to attempt contact will know to set up a split mode and transmit 5 kHz above the listening frequency.
Other splits are common, perhaps 10 kHz or even greater, and the split may be listening down as well as up, as in our example. If you wish to set up a split mode for your transmissions, just be sure to make the split greater than the typical SSB bandwidth of 3 kHz so that there is absolutely no overlap of transmit and receive SSB bands. Also make sure that both frequencies in the split are not otherwise in use, so to avoid interfering with other ongoing QSOs.
You’ll find split mode in operations quite common, especially when propagation is good and the DXers have come out to play.
The answer to General Class question G4A03, “What is normally meant by operating a transceiver in “split” mode?” is “C. The transceiver is set to different transmit and receive frequencies.”
-- Stu WØSTU