Wednesday, Mar 05, 2003 at 23:20
G’day CJ,
Just a few notes on what has been said so far in reply to your question. All of the answers are partially correct in a way, but if you want to spend a minute reading I will elaborate a bit further.
Hope this is not too long winded.
The term A.M. on the front panel of a receiver or transceiver can be a bit confusing to the uninitiated.
In actual fact A.M. stands for the type of modulation used to get your signal [your message] from you to wherever [and vice versa].
A.M. stands for Amplitude Modulation [as opposed to FM which is Frequency Modulation], and was the first type of modulation used in radio.
The greater proportion of radio transmission on the High Frequency [HF] Band uses Amplitude Modulation in one form or another. There are other forms of transmission, but A.M. wins hands down.
Without getting too deep with technicalities, there are basically 3 types of AM transmission, Double Side Band [DSB] which is commonly referred to as A.M., Single Side Band [SSB] and Independent Side Band [ISB]
Confusing isn’t it?
In a DSB Amplitude Modulation transmission, there are always two “side bands” present – i.e: modulation is present above and below the prime carrier wave frequency of the transmission . These areas are commonly referred to as the Upper [USB] and Lower [LSB] Side Bands
This modulation in each side band is actually a mirror of what is happening in the other side band and is a great waste of “Bandwidth” [the width of the signal from the top of the upper sideband to the bottom of the lower sideband] on a very crowded radio spectrum..
The most recognisable form of double Side Band transmission is your ordinary everyday broadcast stations like 2UE, 3AK, 4BC etc – commonly known as “AM” stations.
As somebody has said in another post, DSB is a waste of bandwidth, and with this in mind, radio scientists in the mid 20th Century developed a method of transmitting the signal using only half of the bandwidth – i.e. only one of the side bands - and SSB radio was born.
SSB is quite conducive to speech transmissions as most DSB transmissions had a bandwidth of about 6 Kilohertz, and the normal speech frequency range audible to the human ear is less than 3Khz [much less]. So, a human voice could be modulated on one sideband only, and clearly understood by the receiving station.
In certain scenarios both sidebands can be utilised by 2 identities emanating from the one transmitter simultaneously [e.g. one transmitter – 2 operators]
This is known as Independent Side Band [ISB] and is usually only seen in professional radio networks. A common example of this would be where a station is transmitting numerous teleprinter signals within the lower side band, and is using the upper side band for voice chat to control the engineering of the circuit with the distant receiving station.
Not something that would be experienced by the average off-roader :) :)
So, to sum up, for all practical purposes, the greater majority of
HF radio transmission utilise A.M. – in one of the forms described above.
The terms “A.M.” and “SSB” on the front panel of any HF receiver or transceiver refer to the types of A.M. transmissions, the radio can receive or transmit.
The term A.M. on the front panel of a set simply means A.M. using Double Side Band.
A quick note on the use of the term “CB”. CB is simply an area of the radio spectrum set aside for Citizens Band [CB] radio communications. CB in the HF band has been allocated part of the 27Megahertz [Mhz] portion of the band for the simple reason that nobody else wants it. It is bloody useless - that area of the spectrum is totally unreliable.
The statement made earlier that HF is becoming the dinosaur of modern communications is not quite correct. HF may be seen to be becoming a dinosaur with respect to RFDS type radio scenarios in the Australian bush, but it will never become redundant in the professional radio world. It carries too much long distance commercial, diplomatic and military traffic from the majority of the world’s smaller countries for starters. Not every little country has the wealth of the U.S., U.K., Australia, and the like, and as such cannot afford the massive use of communications satellites. Until they can, the HF band will continue to be crowded with a myriad of seemingly weird and wonderful transmissions.
As I said at the start, I hope that was not too long winded or confusing :) :) :)
Enjoy the bush
DennisN
AnswerID:
14536
Follow Up By: Rob - Thursday, Mar 06, 2003 at 13:38
Thursday, Mar 06, 2003 at 13:38
Just curious, after reading your reply, which sideband does the RFDS use, USB or LSB.
Regards
Rob
FollowupID:
8786
Follow Up By: CJ - Thursday, Mar 06, 2003 at 15:34
Thursday, Mar 06, 2003 at 15:34
G'day Dennis,
Perfect answer, solved the confusion over the am/ssb dial. Was not longwinded but have printed it to help explain it to friends. I'm pretty sure the new HF portables don't have that dial. My portable is that old (possibly late 60s to mid 70s) it doesn't even have the make on it. It's got the old mesh style speaker and army green box although on the inside there's what appears to be some fairly modern circuitry.
Thanks again
cj
FollowupID:
8787
Follow Up By: Old Soldier - Thursday, Mar 06, 2003 at 20:55
Thursday, Mar 06, 2003 at 20:55
CJ,
Glad to be of some help, your radio certainly sounds a bit "ancient" :) , but as long as it works
well , who cares??
Rob,
To tell you the truth, i would not have a clue, I have never worked RFDS radio.
I'm a retired professional radio operator, and just passed a bit of lightweight tech info along to add to the already good stuff that had been posted on this subject already.
However, I can remember listening to the the RFDS on numerous occasions as I wandered the frequency spectrum over the years, but could not say for certain which Side Band they generally work.
Normally though, the unwritten convention around the world used to be to start on the "upper" and take it from there.
Maybe one of the blokes on the
forum who has RFDS experience might be able to answer that one for you :)
Enjoy the bush
DennisN
FollowupID:
8806
Follow Up By: Bob Y. - Saturday, Mar 08, 2003 at 21:58
Saturday, Mar 08, 2003 at 21:58
Thanks for the informative run-down on HF, Dennis, and for filling in the gaps. I take your point about HF not quite being a dinosaur yet, but as you would understand, I was coming from the RFDS and rural angle. On many properties, UHF has replaced HF, and even some of western Qld shires have gone completely to UHF, using repeater networks.
Had to do a bit of digging for the USB/LSB info, and you score a prize again, Dennis, it is USB.
While one of the earlier posts seemed to take umbrage at my previous comments, I often miss the versatility of HF days, when one could talk to locals, or other stations, up to 3-500kms away, or even further. Have even talked to head office on Radphone, while mustering, and describing condition of cattle being mustered. Now we've got 2 repeaters, and a mob of UHF's, and the HF's are in a cupboard. Hooroo...
FollowupID:
8879
Follow Up By: Old Soldier - Saturday, Mar 08, 2003 at 23:17
Saturday, Mar 08, 2003 at 23:17
G'day Bob Y,
Totally agree with your comments on missing the HF days.
I finished my career with 5 years in the sterile world of UHF, SHF and communications satellite technology - boring.
Give me the good old HF any day.
Nothing matches it for good long distance communication [with attitude :) :)]
I loved
HF radio. Loved the "snap, crackle, and pop" of sunspot activity, ionospheric tantrums, skip fade in and fade out, and the "community" feel any active radio network such as the rural ones you describe.
Most of all I loved the Morse Code, but sadly that is disappearing into history.
Enjoy the bush mate
Dennis
FollowupID:
8886