This is probably a crazy or impractical idea, but.. A post that made a while back got me to thinking. He can pick up his station audibly for a given range, but can pick up the carrier from the station considerably farther.
As I understand it, the audio information is actually on the sidebands rather than the carrier? So, assuming his signal is typical in terms of the carrier being detectable further than the audio is actually intelligible/present, would this be because the carrier is more of the total %age of signal than the sidebands? So that if the carrier travels some particular distance at a useful level, the sidebands would travel less far because their actual power is lower than that of the carrier that they were created by mixing the audio with in the modulator?
If so, then here's my thought. Would it be feasible to *partially* surpress the carrier down towards the point where it is equal in power to the sidebands before the final rf amplifier? Not sure how it would actually be done, maybe a balanced modulator that was adjustable or use a balanced modulator to make a DSB signal from the audio and the output from the oscillator and then inject a little of the signal off the oscillator back into the DSB signal to give it just enough carrier to sound right without a BFO or etc on the standard AM receiver.
But what I'm wondering is if a measure like this could be used to improve the reception of a usable signal out in the direction of the fringe range without actually increasing the power output from the transmitter/antenna. If it's possible, it seems like it would be a reasonable enough tradeoff since if the carrier usually keeps going past the range the sidebands go and that's part of what accounts for the loss of signal quality over distance.. well, the carrier going off into the distance without having usable audio on it is "range", I suppose but not in the sense of being usuable range. So I was thinking if the 100 milliwatt input final amplifier stage could use some of its 100 mw for the sidebands to make the signal usable for more of it's actual receivable range rather than shooting a "dead carrier" out past some point, it would be a more efficient use of the existing signal and provide better coverage in the range where the signal strength is actually high enough to be of some use. The loss of some of the distance that the carrier continues to go after it ceases to actually carry the audio information to a usable degree (or when it's too weak to be received by typical listeners) shouldn't matter anyway, I think?
Like I said, it is probably crazy or impractical. I'm just trying to figure out how some of this stuff works and why things are done the way they are.
Daniel
As I understand it, the audio information is actually on the sidebands rather than the carrier?
True. At 100% sine wave AM, the upper and lower sidebands each have 1/4 of the average power of the unmodulated carrier, and the total output power of the tx is then 1.5X that of the unmodulated carrier
Would it be feasible to *partially* surpress the carrier down towards the point where it is equal in power to the sidebands before the final rf amplifier?
That can be done, but not to any useful purpose because of the audio distortion that creates in the diode detector of the typical AM broadcast receiver.
An AM carrier can be reduced or even eliminated only if the receiver has a way to properly demodulate the spectrum that is left.
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"That can be done, but not to any useful purpose because of the audio distortion that creates in the diode detector of the typical AM broadcast receiver."
Okay, so no practical value in this application. Thanks Rich!
Daniel
I've experienced that issue (having the carrier quiet the receiver with no content after a certain distance) with my old AM 100 and with my new Talking House. When the transmitter has the capability to modulate a stronger audio signal, I just CRANK up the audio and try to modulate more of the carrier.
In both the AM 100 and the Talking House (which is an awesome machine, more on that later) the increased input levels helped the range considerably. With my hand built unit, and also with my Gizmo, the modulation broke down and all I got was distortion.
Experimental broadcasting for a better tomorrow!
scwis wrote: When the transmitter has the capability to modulate a stronger audio signal, I just CRANK up the audio and try to modulate more of the carrier.
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No AM transmitter can be modulated more than 100 % on negative-going peaks of modulation without creating audio distortion in receivers tuned to that modulated signal, as well as interference to stations on adjacent channels.
Also kindly note that the more you CRANK your modulation beyond that which the limits of physics and the capabilities of your tx design and installation can support, the more distortion and interference you will create.
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Driving the poor little transmitter past it's capability will also cause splatter into other frequencies, so don't do that.
However, it's amazing how much additional input transmitters of certain designs can handle, so I always give it a try.
But be careful 🙂
Experimental broadcasting for a better tomorrow!
Daniel,
Rich adequately answered your proposal, but I wanted to add that you are right in your thinking on the topic of reassigning the energy from the carrier to the sidebands. SSB (single sideband) does what you proposed. It not only concentrates the power wasted in the carrier and the other redundant sideband into the sideband carrying the mail, it also cuts the bandwidth required in half. That is why it is so popular for ham and military uses. The problem on the AM bands, as Rich mentioned, is that unless the receiver is designed to process these modified signals the distortion makes it useless.
Keep thinking as you do and it will be my pleasure to say "I knew him when" when you hit on your really innovative idea which works. Many here might be surprised that the Austrian actress Hedy Lamarr was co-inventor for the patent for spread spectrum modulation which you use every time you place a cell call. I have to wonder what she was thinking.
Also remember that when an idea is shown not to work it is just another step to build on to finding one which does. Believe me, I've been there, done that.
Neil
I have read where some communciation modes use a carrier but only one side band. The way I understand it, the intelligence in both sidebands are identical. So, why do we still use both sidebands? It seems like more radio stations could fit on the AM band that way and 1/4 of the power would not be wasted. A really "wild idea" might be to add a sideband where the first was removed and thus add another channel, maybe even a crude form of AM stereo. I'm sure it has been tried but just wondered why it didn't take off in the broadcast world.
AM radio legend, Leonard Kahn, researched these assumptions for several decades with some success. Visit this site for radio engineers comments on this and other Kahn inventions.
http://www.sbe36.org/1999/0408_letters.html
Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.
Rhema Radio - The Word In Worship
AM 1660 - FM 93.5
http://www.rhemaradio.org
So, why do we still use both sidebands? It seems like more radio stations could fit on the AM band that way and 1/4 of the power would not be wasted.
The power in the opposing (upper or lower) sideband of a full-carrier, DSB AM signal is not really "wasted." For consumer-type AM receivers that are center-tuned to the carrier frequency, and with other things equal -- that duplicate sideband improves the detected signal-to-noise ratio from the receiver.
And for the envelope (simple diode) detectors used in consumer-level AM receivers, the presence of both of the sidebands produced by conventional, full-carrier DSB AM reduces the audio distortion ultimately heard by the listener.
A really "wild idea" might be to add a sideband where the first was removed and thus add another channel, maybe even a crude form of AM stereo. I'm sure it has been tried but just wondered why it didn't take off in the broadcast world.
Good out-of-the-box thinking, but this idea is not so wild or novel as might be first thought. This approach has been/is used in some professional voice communications systems where receivers with the required selectivity and demodulator designs are utilized. But even then, "broadcast quality" is not readily achievable (especially for the lower audio frequencies).
And certainly, most of the consumer-level AM broadcast receivers in use today could not deal with independent programs on the upper and lower sidebands of an AM station, no matter at what level the carrier was present.
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