First, something to play with: a Morse receiver simulator with BFO dial. Start CW and fiddle with the dial to tune the receiving tone with the spot tone:
By the way, CW means "Continuous Wave", also coloquially known as "Morse code".
The first time a guy told me that a five-watt Morse radio could reach the whole world, I wrote that off as ramblings of an aging ham radio operator. My former boss was a flamboyant man, to say the least.
But it is actually easy to prove it is possible, and long distances may be achieved with 2W, 1W or even less. QRP (low-power communications) has a huge fanbase among radio enthusiasts. In the case of Morse QRP, the basic ingredient is the narrow bandwidth.
AM broadcast radio needs 20KHz per station. SSB radio needs 5KHz. In principle, CW needs zero. CW transmission is like a laser: all the power is concentrated on a single color hue, while SSB is like white light, spread thin over all colors.
In practice, the CW bandwidth is (at most) 100Hz for Morse code, because CW is not really CW; it is keyed on and off. It is equivalent to the AM-SC modulation of a very low frequency data signal.
Given the same transmission power, CW has a gain of at least 20 times or 13dB over SSB (5000 divided by 100). Colloquially, people say that 5W CW is equivalent to 100W SSB. But Morse can pass through bandwidths even narrower than 100Hz, in particular when operators are practising QRS or "slow code", and then the gain is even higher.
Digital modes are also very bandwidth-efficient, with PSK31 using just 31Hz! Other modes need more bandwidth in exchange for faster data rates. The narrow bandwidth and automated reception (no need of a Morse-trained human) create even more opportunities for QRP. Low-power transmitters also mean cheaper equipment and the possibility of using renewable power sources.
In a Zen koan (roughly analog to Christian parable), the master asks the apprentice what is the sound of one hand clapping. Likewise, the CW signal has no sound on its own. It is a pure carrier. An SSB receiver tuned exactly to this carrier will hear 0Hz sounds, which is just silence.
In order to hear Morse code, the receiver must be slightly off-tune. For example, if the transmission is made at 9998.0KHz, the receiver should be tuned at e.g. 9998.8Khz. The product of the local carrier (known as BFO) and the remote signal generates 800Hz tones (9998800 minus 9998000). Receiving at 9997.2KHz would work as well.
The math is exactly the same as if the radio was receiving a pure 800Hz tone over SSB. In CW, the standard is to take the incoming signal as if it were upper sideband (USB). The CW signal comes in 9998.0Khz, so the hypothetical carrier, that we should set our BFO onto, is 9997.2KHz.
The CW etiquette also specifies that the first transmitter (the guy that calls "CQ CQ CQ...") owns the frequency. Whoever answers to the CQ is in charge of tuning their own BFO.
The CQ guy is transmitting at 9998.0Khz, but he must also receive in an offset frequency in order to hear audio. Let's say he prefers 1000Hz tones, so he tunes his radio to 9997KHz. But he will only hear 1000Hz tones if the responder transmits on 9998.0KHz as well. If the responder does the math wrong and transmits in 9998.8KHz, the CQ guy will hear unpleasant 1800Hz beeps.
Modern CW radios can be configured a "spot tone", meaning the desired beep pitch. The display shows the center CW frequency, and the receiver is automatically offset. The only thing the operator must do is to fine-tune the frequency while pressing the SPOT button until the local beep has the same pitch as the received Morse beeps.
The operator must also be sure that pitch *decreases* when frequency is slightly increased — meaning that USB convention is being followed and BFO frequency is below CW frequency.
Once these adjustments are fine, you are "zero beat" with the transmitter. That is, your transmitting frequency is exactly the same as the other side — allowing the remote operator to hear Morse in their own favorite pitch.
If your radio does not have a sharp filter around the BFO frequency (it is often the case of simple kit transceivers), you will hear other Morse conversations, with different pitches. This is not always a problem; the wider band is useful while looking for counterparties, and the different pitches give a rough idea of their frequencies. In good radios, the filter is configurable so you can switch between wide and narrow easily.
The applet at the beginning of the page generates a beep-beep CW with random frequency around 10KHz. The spot tone is fixed at 800Hz. The slider allows you to set your transmission frequency, and Spot Tone button plays (unsurprisingly) the spot tone.