Diego From Italy, joined Apr 2001, 135 posts, RR: 0 Posted (11 years 3 months 3 weeks 4 days 22 hours ago) and read 4932 times:
Hi folks I am trying to understand the functioning of the HF radio and scrolling through the B747 300 cockpit photos I bumped onto a picture of an overhead panel showing the HF radio. The radio has a knob with 6 different positions: OFF-USB-LSB-AM-DATA and CW. Although the OFF position is kind of self-explainatory the others raise a few doubts such as when do you select the USB or the LSB and what AM DATA and CW stand for. In my previous post airplay answered a few questions about the LSB and the USB even though no mentioned was made about the using of the radio equipment itself.
Klaus From Germany, joined Jul 2001, 21592 posts, RR: 53
Reply 1, posted (11 years 3 months 3 weeks 4 days 19 hours ago) and read 4907 times:
They are different modulation settings:
USB: upper side band (AM with carrier and lower side band suppressed)
LSB: lower side band (AM with carrier and upper side band suppressed)
AM: amplitude modulation (the same kind used for civilian "AM radio": carrier plus upper and lower side bands)
DATA: probably used for data link; There are multiple possible modulations. I don´t know which one would be used there.
CW: "continuous wave" (morse telegraphy)
For speech transmissions, USB/LSB are the most efficient ones; AM is probably used for compatibility (and for listening to civilian radio stations ).
CW filters make it easier to discern morse signals over a noisy background.
Can´t give you operational info; This is just as much as a radio amateur can tell you.
Skydrol From Canada, joined Oct 2003, 1047 posts, RR: 9
Reply 2, posted (11 years 3 months 3 weeks 4 days 19 hours ago) and read 4905 times:
USB = Single Sideband reception mode UPPER SIDE BAND
LSB = Single Sideband reception mode LOWER SIDE BAND
USB/LSB are forms of 'SSB' communication. SSB provides extra-long range communication capabiliy, but sacrifices audio quality to do so. SSB also requires tuning a balanced modulator to re-insert the missing carrier to allow the audio information to be used. This is finicky and mis-tuning can result in reception of voice communication sounding like Donald Duck. Note: throughout North America, most HF SSB transmissions are USB only, and many newer aircraft HF radios only support USB.
AM = AMPLITUDE MODULATION (standard braodcast - both sidebands, carrier frequency not suppressed). AM basically provides acceptable audio quality without extra clarification processes, but doesn't provide the extra-long range communication power associated with SSB.
DATA = I'm not completely certain here, but this will likely invoke filters which help block noise from interfering with reception of digital tramissions like RTTY (Radio TeleType) or FAX (weather maps)
CW = CONTINUOUS WAVE mixes a BFO (beat frequency oscillator) tone with the received signal to make morse code transmissions easier to decode (especially if they are unmodulated)
Iakobos From Belgium, joined Aug 2003, 3317 posts, RR: 33
Reply 5, posted (11 years 3 months 3 weeks 4 days 8 hours ago) and read 4885 times:
Ground > air and vice-versa voice transmissions in the HF spectrum are using USB exclusively. Every a/c has also its own SELCAL (selective calling) identity (similar to a telephone subscriber number) so that it can be called individually.
Most a/c HF transmitters have a power output of 150W (pep SSB).
When in flight, the performance of the HF antenna is poor due to the lack of ground (which otherwise acts as a reflector).
Rgds fm another ham (w/practical experience of HF onboard a/c)