คำถาม :: จาก Robert ในกิจการวิทยุสมัครเล่นเวลาใช้ระบบเสียงพูดทำไมความถี่ 14 MHz และสูงกว่าถึงใช้ระบบ SSB แล้วความถี่ต่ำกว่าละทำไมถึงใช้ LSB ยกเว้นความถี่ 5 MHz นะที่ใช้ (USB)
Robert, KB5QN, asks: In amateur single sideband (SSB) voice operation, why is the upper sideband (USB) used on 20 meters and higher frequency bands while lower sideband (LSB) is used on the lower bands (except the 60 meter channels)?
ตอบครับ :: This goes back to the early days of SSB. At that time, the early 1950s, there was no 40 meter phone band (it was opened to voice on February 20, 1953), so the majority of SSB activity was on 75 and 20 meters. One common design configuration used an SSB generator that produced an upper sideband SSB signal at 9 MHz using a filter or phasing SSB generator. The 9 MHz signal was then heterodyned with a VFO covering 5 to 5.5 MHz, often made from a (then) $5 WWII surplus ARC-5 transmitter.
The additive (9 + 5 = 14, 9 + 5.5 = 14.5) translation to 20 meters maintained the upper sideband. The subtractive translation (9 – 5 = 4, 9 – 5.5 = 3.5) reversed the frequency relations (and the VFO tuning direction) to result in LSB. By just using those sidebands, they did not need to buy a second carrier oscillator crystal and worry about sideband filter symmetry, or put switching into their phasing rigs. It could have just as easily gone the other way, I guess.
It just went on from there. 40 meters went LSB and the upper bands went USB. The US military appears to have settled on USB on all HF frequencies, so the “green radio” guys with SSB-capable military surplus gear use USB, especially on 40 meters. Compatibility with the government protocol explains why we are required to use USB on the five 60 meter channels that we share with government users. Other than that requirement, there is no regulation specifying which sideband be used on any band.
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