Transmission Mode Efficiencies

Or

Why The Hell Does Your CW Reach More People Than My SSB Transmissions?

 

It’s well known that low power enthusiasts (QRP-ers) prefer to use CW rather than SSB often because it gets them much better distances for their transmissions.  What’s so special about CW (and what’s SSB’s problem)?  This is the subject of this article.

 

There are many modes of communication: AM, FM, SSB, CW, and a whole host of ever growing digital modes (that would be the subject of a multi-part article in itself).  What’s the difference and why do they have any effect on how far they can be received?

 

Let’s take a look at Amplitude Modulation (AM) first:

AM Mode

AM takes a carrier wave (somewhere in the RF Band) and modulates it with an audio frequency signal wave (like your voice) and produces a wide spectrum of frequencies (ranging from the sum of the carrier and audio frequencies to the difference of the carrier and audio frequencies.  The information from the audio frequency is completely absent from the middle Carrier portion of the AM signal.  All the useful information exists as an identical pair of sidebands on either side of the original carrier signal.

All Waveforms

Some bright guy figured out that you could get rid of the carrier signal and one of the sidebands and still have all the information that you would need (the idea of sidebands was known as far back as 1915, but a practical way to remove everything but one sideband was discovered in 1948 by those smart guys at Bell Labs).  Doing that shaved off 2/3 of the required power for the same effective transmission strength.  By effectively channeling all your previous AM signal into a single sideband, it was equivalent to tripling your signal power!  Someone decided on the gentlemen’s agreement that all amateur SSB below 9MHz would use the lower sideband and all SSB transmissions above 9MHz would use the upper sideband and the rest was history!

 

Okay, so that explains why SSB goes farther than AM, but what about the rest?

 

Well, the simplest way to consider transmission mode efficiency is to determine each mode’s relative bandwidth – i.e. for identically powered signals, the narrower the bandwidth the greater the mode’s transmission efficiency, the greater the bang for the buck!

 

Almost every source gives a different bandwidth for each mode and thereby gets slightly different dB gain or loss for a comparative transmission mode.  Let’s say that a SSB signal occupies 2KHz of bandwidth, a CW signal going at 25 WPM roughly takes up 100Hz (I will spare you the math involved).  So the gain of CW over SSB would be something like 13 dB!  That’s better than most expensive linear amplifiers can offer for effective signal gain!  No wonder there are still CW fans out there.

 

Have you ever wondered why FM was marooned to VHF and beyond like some redheaded stepchild?  It’s the bandwidth it hogs and the awful effective signal loss compared to other modes.  We use an FM signal with 5KHz deviation on our repeaters.  That’s a bandwidth of 10KHz!  That’s an effective loss of -7dB over a SSB signal.  There are other factors, such as FM’s intolerance to the background noise that is a constant factor on the HF bands, but it’s poor energy efficiency is an important factor as well.

 

How about them there new fangled digital modes that we OMs read about (what will those youngin’s think of next)?  We’ll just use one as an example today – PSK31:

PSK31

Well, that looks an awful lot like a CW signal and they are very similar.  PSK is computer (or rig) generated and computer (or rig) decoded mode, no well trained operator is required for this mode.  PSK comes in different speeds and as the speeds increase the bandwidth it takes up increases too. The bandwidth of PSK-31 seems to be under dispute.  The ARRL feels it is approximately 60 Hz, but I have frequently read that it is 31 Hz wide.  Even taking the more conservative bandwidth of the ARRL, PSK-31 has a 2.2dB gain over CW!

 

What about Nobel Laureate, Joe Taylor, K1JT’s ultra weak signal mode, JT9?  You practically don’t have to turn your transmitter on with this mode!  It has forward error correction (FEC) built into its design to improve its detection below the noise floor!  But just on the basis of its miniscule bandwidth, I’ll give you a hint, it only takes up 16Hz of bandwidth.  You do the math!

 

So, go out and try one of those real efficient modes!  Who knows, you might get the first QSL card form Mars <HI HI>!

 

-The Editor- (I can be contacted at TheEditor@KCRC.com)