WWVB and Loran-C use low frequencies for phase stability. Ionosphere bounce has bad fading and erratic prop delay; ground wave is very lossy but is much more amplitude and phase stable. Both are being killed by GPS.
1000 watts is enough for SSB communications halfway around the world. You can't do that with a megawatt of ELF.John
Well there's a garbling, since it's far more common to see SSBsc, ie Single Sideband with suppressed carrier. Either someone started with a more complicated example for the sake of it, or it's suddenly been reinforced to support a false notion.
A single sideband, and before going on this tangent the talk was of ELF so if any voice modulation is going to go on down there it's going to be SSBsc, is really easy to demodulate. Beat a signal against it, and the sideband translates down to audio. No problem with mistuning, you simply live with an odd sounding signal, a little retuning will fix that.
Note that even if you started with a DSBsc signal, there are plenty of SSB receivers out there perfectly capable of stripping off the unwanted sideband and then the rest of the receiver treats it like it was an SSB signal. Indeed, the only difference is that you wasted the power used for the extra sideband. Sometimes that's fine, since it makes the transmitter simpler.
But even if the discussion truly was DSBsc, demodulation is easy, and has been well described for 50 years.
You don't look for the highly suppressed carrier, you get the information about where to place the locally placed carrier at the receiver by looking at the sidebands. Simple detectors of thirty years ago would take the IF signal in the receiver, and double it in frequency, giving a constant frequency, and divide it down by two to get the needed frequency and it's right there in the middle, derived from the sidebands.
More complicated methods use a dual channel arrangement, with the VCO locked to the outputs of the product detectors. Webb described a practical circuit in CQ magazine about 1957 or 58, and while it used a lot of tubes, it wasn't excessive. With solid state devices, it's far easier.
A lot of portable shortwave receivers made in the past thirty years use a synchronous detector, just what we are talking about, that work just like that 1957 circuit.
Michael
Nonsense but irrelevant as virtually no one uses double side band supressed carrier and it has nothing whatsoever to do with the previous discussion.
Double side band was played with about 40 years ago and essentially abandoned as ssb is more efficient both in bandwidth and power.
Most all supressed carrier is done single side band.
Vestigial sideband is used extensively as in analog TV broadcast.
-- Jim Pennino Remove .spam.sux to reply.
Not quite; the decommisioning of the Loran system has been indefinetly delayed and the implementation of a new generation Loran system as a backup for GPS is under study.
Much less than 100 W is enough for SSB communications halfway around the world.
And depending on the state of the sun, 1 W is often more than enough.
-- Jim Pennino Remove .spam.sux to reply.
Though oddly enough, the problem with SSB is that it's hard to tune. Not in terms of receiving something listenable to, but to tune it exactly. There is nothing to lock onto, so one always has to make do with "that's about right". It's fine for voice since mistuning only makes someone sound higher or lower pitched. But music is horrible since you do notice when it's mistuned.
The redundant sideband, as I posted about earlier, allows for perfect tuning of the reinserted carrier. Plus the redundant sideband, with the right detector, allows for a certain level of frequency diversity reception, and of course the redundancy means one sideband may arrive at your receiver without interference while you have to live with what you get if one sideband is sent.
The carrier is the main hog of power at the transmitter, eliminate it and you get a far bigger level of efficiency than going whole hog and getting rid of the extra sideband. Sending the extra sideband gives those advantages.
Michael
Why would anyone in their right mind transmit music with SSB?
Music is generally about fidelity which means bandwidth.
One of the primary reasons for using SSB is to reduce bandwidth.
I doubt you are going to see much benefit from a frequency diversity of 6 Khz at 10 Mhz.
Well, it all depends on what it is you are trying to achieve.
-- Jim Pennino Remove .spam.sux to reply.
Because they could, economically. Before the conversion to digital transmission, telecoms transmitted 20kHz audio via SSB coast-to-coast.
Yes, but that doesn't preclude ssb when you know how to do it well.
snipped-for-privacy@specsol.spam.sux.com wrote:
Stone me! What a mess we can get into sometimes here. Lets point out a few facts about VLF signals (below say 50KHz).
Take a look at the RN Radar and Radio Museum pages at
Incidentally, the remark above 'the arc system was inherently limited to low frequencies' is probably incorrect. The arc was just a means of making and breaking a circuit and history records that some of the earliest demonstrations of radio (Lodge and others) were carried out in lecture theatres using a sparking induction coil and a short dipole antenna (two plates) transmitting to a nearby loop antenna. The average wavelength may have been around a metre or less.
Chris
Your efficiency, both in terms of power and in terms of spectrum utilization. You only need one sideband for content, so why send the extra sideband (redundancy aside). Almost forty years ago, shortwave broadcast stations did start talking about and/or playing with SSB, to make better use of their allocated spectrum. Shortwave broadcast stations transmit music as part of their programming.
Which you get when you drop the other sideband, and instant halving of the bandwidth used.
There is nothing inherently narrow bandwidth about SSB. It does tend to be narrow because you are mostly transmitting only voice, and that doesn't take up much bandwidth. And it was certainly easier to restrict bandwdith with SSB, since in the early days those phasing methods were so limited that they couldn't deal with wide bandwidth, and it's easier to make a crystal filter that is narrow than wide.
So long as only voice was used for SSB, nobody gave this thought. But once shortwave broadcasters started to play with SSB, then of course they had to reveal that SSB could indeed be wide bandwidth (yet still narrower than an equivalent DSB signal, since the extra sideband is never sent).
But that is precisely what causes a lot of problems with AM with full carrier over long distances. The sideband(s) arrive while the carrier fades, and then there's not enough carrier to properly demodulate it. You have to reinsert the carrier at the receiver, so all those synchronous detectors have determined where to place it by using the sidebands as the information.
This isn't a guess, DSB when demodulated properly is seen as a diversity method. Obviously not as good as transmitting on two very distinct frequencies, but it's there, and a lot simpler than having two transmitters.
Michael
I seem to recall reading that submarines did generally come close to the surface to receive signals, so I think the buoy system makes sense. And of course, a lot of work has been done on making good receiving antennas at low frequencies, loops and even active elements. You lose too much trying to use a tiny antenna to transmit low frequencies, so you have to keep raising the power and at some point you get virtually no return for the increases. At the receiver, you use amplification to compensate, but it's easier to amplify small signals than power signals.
Spark seems to be what you are talking about, and yes it was inherently wideband.
The arc transmitter came a bit later, and was limited to relatively low freuqencies (though I'm not sure the limitation was noticed at the time; as previously posted for a while the higher frequencies were dismissed as "useless" so everyone hung around a relatively small slice of the spectrum). It was a real CW transmitter, the arc transmitter was a real oscillator.
I seem to recall there were also transmitters that used mechanical generators to cause a CW signal, and those obviously were limited to low frequencies.
Michael
Yeah, and it isn't what I would call listenable.
Non sequitur.
First you say it is an instant half the bandwidth, then you say there is nothing inherently narrow about it.
You can't have it both ways.
The early days were a half century ago and crystal filters were mostly replaced with DSP decades ago.
Non sequitur.
You've already said with the right detector you don't need the carrier at all.
-- Jim Pennino Remove .spam.sux to reply.
Poulsen arcs typically worked in the 50-100 KHz range, up to a megawatt or so. The arc chamber dwarfed the operators and had massive air and water cooling bits.
Around 1927, Alexanderson alternators (rotating AC generators) were generating 400KW at 24 KHz and 1 KW at 200 KHz. Goldschmidt alternators hit numbers like 200KW at 50 KHz.
John
The inverse square law applies to anisotropic radiators, too.
-- Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1
I know this is a Radium thread so I hesitate....but you guys are REALLY in need of a radio engineer!
Yes, the inverse square law applies to anisotropic radiators as well IN THE FAR FIELD REGION!
In the "near field" or what some here have suggested is the "faster than light" region it doesn't apply.
VLF transmissions tend to bend around the earth which is one reason they are useful for long distances and found exclusive use in the early days of radio. Higher frequencies (so-called "short wave" ) were discovered to bounce off the ionosphere (more or less) and thus achieved popularity later for long distance transmission using that bounce. Higher frequencies are not reflected back so longer distances are harder to achieve. But this does now mean that over the horizon transmissions are not possible. The high frequency waves tend to be scattered by diffraction sending energy down below the horizon. This is the way that certain over the horizon radars (DEW line) work. But since most of the energy is NOT scattered, HUGE amounts of power are needed.
So.... What if we had just ONE photon at a frequency of ONE Hz, how much carrier power would be needed? Would it be less than the DEW radar? What if that one photon were single sideband?
So you are saying a perfectly collimated beam follows the inverse square law?
-- Jim Pennino Remove .spam.sux to reply.
Yes, he is, Jim! And the reason for that is because a "perfectly collimated" beam simply does not exist!
Even in the case of a single mode TEM 00 Gausian beam laser, the radiation spreads out in the far field. One can arrange things so that the narrow "waist" of the output beam occurs at a distance from the laser, and that beam SEEMS to not follow the inverse square relationship, but the fact is as I pointed out above, the seeming failure is due to being in what is essential the "near field" of the beam. At a great enough distance the beam expands.
You are REALLY in need of a physicist!
1 photon per second at 1 Hz is a power level of 6e-34 watts. Somewhat less than the DEW line transmitters.Single-photon SSB is meaningless.
John
It certainly does mathematically.
If you want to talk practical, it is practical to generate a beam that over the distances of interest is collimated well enough that the inverse square law does not strictly apply.
Most real microwave links are that way.
-- Jim Pennino Remove .spam.sux to reply.
Not for a beam made of waves.
Never heard of the Radar Equation? Or done a microwave link budget?
John
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