How Much Bandwidth Does CW Really Occupy?

Novice radio license exams sometimes have a query concerning the bandwidths taken up by varied modulation sorts. The idea behind the query is fairly apparent — as guardians of the spectrum, operators actually ought to understand how a lot house every emission kind occupies. Because of this, the budding ham is left figuring out that steady wave (CW) alerts take up a mere 150 Hertz of valuable bandwidth.

However is that basically the case? And what does the bandwidth of a CW sign even imply, anyway? To know that, we flip to [Alan (W2AEW)] and his in-depth look at CW bandwidth. However first, one must see that CW alerts are a bit particular. To ship Morse code, the transmitter isn’t producing a tone for the dits and dahs and modulating a service wave, moderately, the “bare” service is simply being turned on and off by the operator utilizing the transmitter’s keyer. The audio tone you hear outcomes from mixing the service wave with the output of a separate oscillator within the receiver to create a beat frequency within the audio vary.

That appears to counsel that CW alerts occupy zero bandwidth since no info is modulated onto the service. However as [Alan] explains, the motion of keying the transmitter imposes a low-frequency sq. wave on the service, so the occupied bandwidth of the sign depends upon how briskly the operator is sending, in addition to the RF rise and fall time. His demonstration begins with a sign generator modulating a 14 MHz RF sign with a easy sq. wave at a 50% responsibility cycle. By controlling the keying frequency, he mimics completely different code speeds from 15 to 40 phrases per minute, and his fancy scope measures the occupied bandwidth at every pace. He’s additionally in a position to change the rise and fall time of the sq. wave, which seems to have an enormous impact on bandwidth; the quicker the rise-fall, the bigger the bandwidth.

It’s a stunning consequence given the inventory “150 Hertz” reply on the license examination; the truth is, not one of the situations [Allen] examined got here near that canonical determine. It’s one other nice instance of the refined however necessary particulars of radio that [Alan] makes a speciality of explaining.

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