6 Meter QSO

During a 6 meter QSO the op who holds a Technician License and is preparing for her General exam, knowing that I am listed as a mentor by the ARRL, asked me a question.

She referred to a "sample question" found in her study guide. "Why is USB (Upper Side Band) used on 20 meters and LSB (Lower Side Band) usedon 40 meters?

The answer in the study guide said that the choice of which side band is "used by convention"; LSB on 160, 80, 40 and USB on 20, 15, 10 and VHF.

She is in the legal field and said "If I ever said that something is used by convention", the judge and jury would think me a fool - they want a real explanation and so do I.

Well, I know the choice has to do with how our radio circuits are designed. But, she wanted a more detailed answer for her. I told her I would provide her the answer in detail in written form.

In order to find an "easier to understand, detailed answer", I posed the question to KE7OG, Dave Casler, who writes the "ASK DAVE" tech column for QST. He has a way of taking complex answers and making them understandable to people without a technical background. The following quote was his reply.

"There is an historical reason. Many older rigs used a tuning oscillator that was tunable between 9 and 9.5 MHz. This was mixed with the incoming signal to produce a first IF output. Mixing this with a lower frequency (e.g., 40 meters) would invert the desired signal, and with higher frequencies would not. This signal was then mixed with a fixed oscillator (the appropriate values being switched into the oscillator by the band switch), which mixed it down to 455 kHz for the final IF. It went through filters to get the sideband signal. The convention through the communications industry was (and is) to use Upper Sideband, but if you set up the 455 kHz IF for that, the inversion for frequencies below 10 MHz forced Lower Sideband. Ham operators are about the only ones to use LSB.

These days, with digital radios, the choice of sideband is arbitrary, but tradition lingers on.

Hope that helps. 73, Dave, KEØOG"

Well, I hope that also gives you perspective about the USB/LSB choice beyond saying "by convention'.

73, Tom, WB2KLD ARRL Instructor, Mentor and Volunteer Examiner

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