DX University™

  A Guide for DXers and DXpeditioners

Some Bandwidth Stuff

The WeeklyDX™ Helpful Hints No. 60 from the DX University™*


In the beginning we had little idea about what frequency we were transmitting on. In fact, we were actually transmitting over a wide range of frequencies. Our bandwidth was huge. When crystal control came along, we were restricted to only one frequency – plus and minus limited modulation – at a time. We could transmit on any frequency for which we had a crystal, but only one at a time.

One would think that would make things simple. In the mid-20th century, while transmitting on single frequencies with VFOs we still didn’t know exactly what our frequency was. So, we often used 100 kHz band-edge markers to help us stay inside the allocated frequency bands.

Unfortunately, operating out of the band can still be a problem. While most of us have transceivers with frequency control that synthesizes signals that are accurate – and displayed – to roughly one part in a million, many of us still have difficulty staying within the limits of our authorizations. Why you say? Let me tell you!

Starting in the fifties, hams – particularly phone guys – knew about sidebands. Single sideband was new, and familiarity with the technology was widespread. Now, what we know is that the radio is on twenty meters, and the radio is set to SSB. The frequency is displayed to the nearest one-hundred or even ten Hertz. What more could we want to know? If a DX Station is calling CQ in a contest on 14.349.7 it’s inside the band limit, right?

If you are up to speed on advanced communications engineering, you will say “Wait! On SSB, you will be probably be transmitting outside the band!” Is that true? One well known contester at a big multi-multi station once told me that he was definitely inside the band because the radio displayed 14.349.7 kHz, and that band limit was 14.350 kHz.

So what is going on? The fact is that the single sideband transmission power spectrum generally occupies a range of frequencies roughly equal to the bandwidth of a voice. This bandwidth is necessary to convey the information contained in the transmission. Pure CW – key down – takes no bandwidth; add some modulation, such as keying and maybe a bit of ripple, and even CW takes some non-zero bandwidth, but very little.

SSB on the other hand, takes roughly 2.7 kHz of bandwidth, starting about 300 Hz above the carrier frequency and extending usually to about 3 kHz. Some newer radios limit the frequency at the high end of the signal, thus limiting the bandwidth, allowing you to creep a bit closer to the band edge. This is done either by limiting the audio frequency response or using a narrower filter in the transmitting chain. (If you think that the transmitting bandwidth is affected by the bandwidth setting for the receiver, check again. Only a few radios do that.)

The rule of thumb has always been to stay about 3 kHz away from a band edge of your sidebands extend in that direction. For example, you should only transmit on 14.347 USB or 7.128 LSB.

Knowing where your sidebands might be of interest to others, as well as your regulator. This came to my attention the other day listening to the operator of a major DXpedition who was on 20 meter single sideband. He was on 14.205 kHz listening from 14.220 to 14.230. Now everyone knows not to venture into the slow-scan television frequencies. (Even though these frequencies weren’t occupied at first, eventually, someone came up and started complaining.) The DXpedition operator eventually heard the complaints. He said that he hadn’t listened above 14.230, but he would move down to 14.225. Nice guy! But if he had been listening at or below 14.230, what was the problem? The problem was that to avoid 14.230, one must keep the carrier frequency below 14.227 in order to limit one’s sidebands from extending beyond 14.230. The DX operator should have known this.

Over the years with frequency control so much better than in the past, we have forgotten. Just “set it, and forget it.” At ARRL we resurrected the Frequency Measuring Test partly because of this problem. Accurate frequency readouts don’t tell the whole story. But you know all this, right? This week’s hint: Do some research, and learn how to stay inside the allocated bands. 

- N7NG