AnswerID: 14536 Submitted: Wednesday, Mar 05, 2003 at 22:20
Old Soldier
replied:
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 worlds 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
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