Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

B747 HF Radio

Thu Jan 29, 2004 12:05 pm

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.
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Joined: Wed Jul 11, 2001 7:41 am

RE: B747 HF Radio

Thu Jan 29, 2004 2:29 pm

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  Wink/being sarcastic).
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.  Wink/being sarcastic
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Joined: Fri Oct 03, 2003 12:01 pm

RE: B747 HF Radio

Thu Jan 29, 2004 2:32 pm

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)

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RE: B747 HF Radio

Thu Jan 29, 2004 2:35 pm

Klaus: Looks like we attended the same school!  Big thumbs up
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RE: B747 HF Radio

Thu Jan 29, 2004 2:39 pm

I´m getting a bit rusty on this stuff, though, since I´ve not been "radio-active" for quite some time.  Wink/being sarcastic
Good to air it once in a while!  Big thumbs up
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RE: B747 HF Radio

Fri Jan 30, 2004 1:55 am

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)

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