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what is the bandwidth of my CW signal?

This article is from a discussion at my local radio club about how the sending CW speed is a factor of your CW signal bandwidth.

Here I will speak about ‘power bandwidth’. The power bandwidth has been described by L. Couch as the difference between two frequencies defining the frequency « band » where 99% of the power resides. So looking to a signal envelope with a spectrum analyzer, you can identify 2 frequencies (Fl and Fu in the schema below this paragraph) where 99% of the signal power is contained.

Capture

This is similar to the FCC’s definition of the ‘occupied bandwidth’ (FCC Rules and Regulations, 47 CFR 2.202) where the mean power below its lower frequency limit and above its upper frequency limit are each equal to 0.5% of the mean power of considered signal.

Now, when you are looking to a keying wave form, you can see 3 parts, the rising part (1), the constant part (2) and the falling part (3).

keying

Depending of the keying wave  shape, the form of the raising/failing part could be different.

This is an example of a  keying wave shape we can found with some old transceivers:

waveform

Let’s assume we have the same rise/fall shape at different CW speeds (which is generally the case for common CW speeds in most of our transceivers), the constant part will have its time changing depending on the speed, to have shorter or longer dits.

Then we can start using the Fourier  analysis to calculate the Fourier series and the resulting average power for each discrete frequency component of the signal.

W9CF explained very well the calculation and the effect of the speed on the bandwidth of the central peak of the signal. Said quickly, at slower speeds the peak of your signal will be narrower than it is at higher speeds.

In addition, the rise/fall wave shape impact the signal bandwidth. A square-wave keying will increase the bandwidth by about 3 times from what you get with a sinusoidal rise/fall wave shapes.

What does this means practically?

Using a QRP transceiver or calling a DX in a crowded band (or both), you want to have the better signal over noise ratio at the receiver entry of the other station. Reducing your keying speed will help you to increase this ratio by narrowing the central peak of your signal. Moving from 40 WPM to 20 WPM will reduce the bandwidth of your central peak by  2 times, so this will enhance your signal/noise ratio.

Then, beside the key clicks a hard  keying will produce, moving to a sinusoidal rise/fall wave shape reduce the signal bandwidth by up to 4 times and will help to enhance your signal as well.

As a conclusion, the lower speed you will use, the better soft sinusoidal keying wave shape you will have, the stronger signal you will produce.

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what is the bandwidth of my CW signal?

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