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AM power VS SSB power




 
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Author Topic: AM power VS SSB power  (Read 59442 times)
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« Reply #75 on: April 26, 2012, 01:17:56 PM »

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...where an AMer's station was inspected by the FCC, and was running something like 750 watts carrier, but the agent didn't do anything since his power meter showed 750 watts, and that's under 1500, so the ham was good to go, the thing was the agent took the readings with a dead carrier, no modulation...

Works for me.   Grin

I think if you run a clean ship and are courteous to other opereators, the FCC doesn't care about your AM intrepretation of the Part 97 power subparagraph.

Phil - AC0OB
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« Reply #76 on: April 26, 2012, 01:30:48 PM »

Or just let the final run at a KW DC input, and let the power outpoot peak where it may.

And if the FCC ever checks you and complains, tell them you thought your peak was 1500 Watts because you believe in the quote: "talk softly, but carry a big stick"

I prefer to paraphrase it to "Strap softly and turn up the wick".

But that doesn't translate to the bogus practice I have heard a few mention, of running 750w of carrier and keeping the modulation low enough so that their Hammy Hambone meter doesn't kick above 1500.  Although, with most voices, phasing the audio the wrong way at 1 KW DC input will likely cause that meter to peak right at 1500 when the negative modulation hits 100%

Here's the latest suggestion someone sent me to add to the Phony Operator list:

You might be a phony operator if you obsess about running precisely 375 watts, or ask if anyone manufactures a 375 watt rig.

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« Reply #77 on: April 26, 2012, 11:00:16 PM »

Or just let the final run at a KW DC input, and let the power outpoot peak where it may.

And if the FCC ever checks you and complains, tell them you thought your peak was 1500 Watts because you believe in the quote: "talk softly, but carry a big stick"

I prefer to paraphrase it to "Strap softly and turn up the wick".

But that doesn't translate to the bogus practice I have heard a few mention, of running 750w of carrier and keeping the modulation low enough so that their Hammy Hambone meter doesn't kick above 1500.  Although, with most voices, phasing the audio the wrong way at 1 KW DC input will likely cause that meter to peak right at 1500 when the negative modulation hits 100%

Here's the latest suggestion someone sent me to add to the Phony Operator list:

You might be a phony operator if you obsess about running precisely 375 watts, or ask if anyone manufactures a 375 watt rig.



There's a big difference between guys saying they run a 375W carrier for the sake of discussion of general settings and those who believe that is a requirement or actually do it. I'd say cut them some slack because they are recognizing the rules as they themselves understand it.

Your phony operator who thinks he is getting a real 375w/1500W amplifier will probably be disappointed when he cracks the book and finds out how he's been robbed in the duty cycle department. hehe 10 minutes plate time, 30 minutes standby..

High-carrier with downward modulation looks on paper like it makes the exact same output waveform result in the diode detector as running a low carrier and using ultramodulation to hit 1500W peaks. Except that the receiver AGC does not go as crazy. The speech wave is inverted but that is only speaker phase and does not matter for listening.

Before the high carrier is dismissed, check the transmitting tube manuals for the words "modulation essentially negative". No one has really answered what/why that was done. Military reasons? Everyone knows the 833. It was brought up before in a tube discussion but IIRC no one remembers why it was in the handbooks.


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« Reply #78 on: April 27, 2012, 10:26:32 AM »

How would one build a transmitter that modulated only downward? Besides inverting the speech, probably have to have some kind of positive peak limiting?

Here's what I was thinking about this morning. Going by the sidebands are 4 times the carrier idea, that would mean that the sidebands are +6dB to to the carrier, when in reality, the sidebands together are -3dB to the carrier, making a single sideband -6dB. If the sidebands were really stronger, then when listening to a really weak station, you should be able to hear the sidebands but no carrier. But, it is more likely that you can detect a carrier than you can the sidebands. That's part of the reasoning behind running more audio power so that your modulation is over 100%. For example, say we have an upside down tube transmitter. We feed 100 watts of audio in and get 75 watts DSB out (not applying plate voltage yet). Now, we apply the plate voltage for 25 watts input and get 18.75 watts carrier out. Sidebands are 4 times the carrier, right? Now we compare this to a regular plate modulated transmitter that is also running 18.75 watts of output. Sidebands should also be 75 watts PEP. But the actual audio power in the sidebands will only be 9.375 watts. Now which one sounds better on a receiver. Sure, the carrier to sideband ratio is supposedly the same, but the plate modulated transmitter is gonna sound a lot better than the upside down tube transmitter when received on a conventional AM receiver. The upside down tube rig is gonna sound way overmodulated, because it would have more audio power.
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« Reply #79 on: April 27, 2012, 12:16:27 PM »

You don't have to  do anything to the audio, if the speech amplifier and modulator are reasonably distortion free and have good phase-shift characteristics, particularly towards the low end of the audio spectrum.  Most voices are naturally asymmetrical.  Just make sure the polarity of the audio line feeding the modulator is phased the direction you want it.

As far as interference-generating potential of the signal is concerned, I can see no reason why a 750 watt carrier modulated 100% negative and 50% positive would cause any less interference than one modulated 100% positive and 50% negative for full peak output; you still have the same sideband power.  The advantage of phasing the dominant peaks to the positive direction is that you can modulate 100% negative and well above 100% positive, if the transmitter has the head-room.  This increases the total sideband power for a given carrier power. 

In any case, the carrier should not shift upwards or downwards during modulation.  When running AM linear, particularly if you are handicapped by marginal plate dissipation and/or power supply capability, phasing the dominant peaks in the downward direction to 100% might actually be advantageous, since the resting carrier efficiency could be substantially increased without driving the amplifier to flat-topping on modulation peaks, allowing more carrier output for a given DC input and tube plate dissipation.  This would allow for greater total sideband power, plus more carrier power to suppress the background noise.
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« Reply #80 on: April 27, 2012, 12:54:49 PM »

Forum readers may be interested in the paper linked below, relating to power-saving modulation techniques available on newer AM broadcast transmitters.

It is not the "controlled carrier" approach that was used in the Heathkit DX-35, etc.

http://radioworld.com/article/mendenhall-expands-on-mdcl-discussion/211748
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« Reply #81 on: April 27, 2012, 12:58:40 PM »

So all you would have to do is determine which direction your voice is asymmetrical towards (positive or negative), then adjust the phase so that it goes more negative than positive. With a plate modulated rig, this can be done pretty easily just by reversing the leads on the mod iron's primary (if side A connects to tube A, and side B connects to tube B, then switch it so that side A goes to tube B, and side B goes to tube A). But the problem with running high carrier with high percentages of negative modulation would be you would need a tube with higher power handling capabilities to be able to produce the amount of carrier you want, but the tube would actually probably run about as cool as the same tube would in a linear amplifier, since the power would actually be dropping on peaks, instead of increasing, you wouldn't have tubes with glowing plates like you would with a normal plate modulated transmitter.
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« Reply #82 on: April 27, 2012, 04:23:30 PM »

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Forum readers may be interested in the paper linked below, relating to power-saving modulation techniques available on newer AM broadcast transmitters.


IMHO, this is best done in software such as SDR. SDR processing is usually part of the AM Companding system.

For example, analog audio or digital data comes in and is converted or stored for pickup respectively by the Digital Processing System. DPS subsystem examines the present waveform it has just outputted, and waveform just stored, and makes a feedforward prediction based on an algorithm.

The output might be a data word previously stored in a look-up table or the output of the algorithm, which then controls Carrier Power and Modulation Index.

Phil - AC0OB

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« Reply #83 on: April 27, 2012, 08:07:02 PM »

How would one build a transmitter that modulated only downward?

That's the $25 question.
CBers do it by overdriving everything but that's a poor answer.



As far as interference-generating potential of the signal is concerned, I can see no reason why a 750 watt carrier modulated 100% negative and 50% positive would cause any less interference than one modulated 100% positive and 50% negative for full peak output; you still have the same sideband power.  The advantage of phasing the dominant peaks to the positive direction is that you can modulate 100% negative and well above 100% positive, if the transmitter has the head-room.  This increases the total sideband power for a given carrier power.  



My comment took into account all possible methods of making that signal including high level or whatever other weirdness could be imagined. so as with splatter, there is danger of interference if the amplifier is clipping.
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« Reply #84 on: April 27, 2012, 08:24:02 PM »

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Or just let the final run at a KW DC input, and let the power outpoot peak where it may.

Exactly!

Ive no interest in LOSING rights that I had as far back as the late 50's and run what I damn well please within those regs that I personally consider grandfathered.

Now, let some snot nose whiner on here send THAT to Charlie.

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« Reply #85 on: April 27, 2012, 08:32:54 PM »

The idea would be to rig the transmitter to act kinda like a backwards balanced modulator. A normal balanced modulator would cancel out the carrier and leave just the positive peaks (double sideband suppressed carrier). The "backwards" balanced modulator would leave the carrier, but transmit negative peaks. Or another possible way of doing it that I can think of is to somehow build a modulator that runs on a negative voltage, but run the final on a positive voltage. I could see this possibly being done with a class D/E rig and PWM modulator, but the modulator would be running on negative voltage. So you would run the final at the normal positive voltage. As the modulator drove the final more negative, it would reduce the positive voltage to the final, which would increase the negative peaks. If the modulator had a positive peak limiter, then you adjust that to limit the positive peaks to whatever you like.
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« Reply #86 on: April 28, 2012, 11:20:16 AM »

Ive no interest in LOSING rights that I had as far back as the late 50's and run what I damn well please within those regs that I personally consider grandfathered.

Now, let some snot nose whiner on here send THAT to Charlie.

My sentiments exactly.

So now your "Maserati" that you once could legally floor is now stuck in third gear if one is to abide by the revised rules that would allegedly reduce the maximum output for AM. While I don't condone illegal amateur radio operation, if the rules you have lived with all your life are fraudulently changed on you to your disadvantage, how apt would you be to respect them?
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Don, K4KYV                                       AMI#5
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« Reply #87 on: April 28, 2012, 04:01:46 PM »

It can be frustrating, do what you think is right. For my friend in the tiny Republic of Vinco, there was never any real question as their privileges were (and are) a little freer.It's a shame we don't have reciprocal privileges with them but the Canadians have a pretty good thing.

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