AM power VS SSB power

[R. Fry SWL]

With an AM signal modulated 100% equally in both the positive and negative directions, the average power as read on a meter should be equal to the carrier power,...

This depends on what the meter is measuring.

For +/- 100% DSB, full carrier AM by a sine wave, a thermocouple r-f ammeter in series with the antenna/load shows a current increase of SQRT(1.5) = 1.225 X (assuming the value of the load impedance is a constant).

AM broadcast stations commonly modulate to nearly 100% negative and 125% positive peaks

The jpg linked below shows peak values for various amount of asymmetric AM.

Of interest is that the peak power is a function of load SWR -- which introduces another dimension into this for ham  operators who need to observe a PEP limit.

[WD8BIL]

An old engineer at a radio station once showed me his power measurement system...the line voltage meter went down when it was putting out the right stuff...

That's right! When my desk lamp drops 1000 lumens I'm WorlWide!

[kb3ouk]

With an AM signal modulated 100% equally in both the positive and negative directions, the average power as read on a meter should be equal to the carrier power,...

This depends on what the meter is measuring.

For +/- 100% DSB, full carrier AM by a sine wave, a thermocouple r-f ammeter in series with the antenna/load shows a current increase of SQRT(1.5) = 1.225 X (assuming the value of the load impedance is a constant).

AM broadcast stations commonly modulate to nearly 100% negative and 125% positive peaks

The jpg linked below shows peak values for various amount of asymmetric AM.

Of interest is that the peak power is a function of load SWR -- which introduces another dimension into this for ham  operators who need to observe a PEP limit.

Ok, the only part I don't understand is at the top it says the unmodulated carrier is 187.5 watts, but in the chart gives the peak power as 375 watts at 0% mod, and 1500 watts for 100% and that's in a 50 ohm load with 1:1 VSWR. so going by that chart 187.5 watts carrier is 1500 watts PEP.

And second, going by the FCC's own rules, how could they cite someone for running over 1500 watts PEP if they don't have to measure it accurately?

[DMOD]

and on the receiving end - the formula applies there too against bandwidth vs noise and signal doesn't it?

so it is 4* more, * 4 times more. The real advantage seems to be 16*.

The rule of 4:1 should apply the other way around as well, IMHO. I think we have been brainwashed about the 375 Watt carrier limitation.

The previous set of math only showed the Total (average) power equivalency between an AM and SSB signal.

If an SSB signal is 1500 PEP (which is an "average" power for the peak of an RF waveform fully modulated), and which is only ONE sideband, then AMers should be allowed to generate an A3E signal which has the same sideband power. That is, in order to produce an equivalent signal which contains a sideband power in one of the sidebands of 1500 Watts, then working from bottom to top this time:

1500 watts sideband power in one sideband is 3kW total for two sidebands.

So for 100% modulation, I really need a transmitter with a total power output budget of 6kW.

power budget by category is:

Pc = carrier alone = 3kW

Psb = 3kW for both sidebands

Plsb = Pusb = 1.5kW sideband power.

Maybe this is to what Timtron was referring.  

P.S. I will be happy to accept the old 1kW power input, unmodulated, final stage power rating.  

Phil - AC0OB

[w1vtp]

In the final analysis, for the sake of this post I'd like to assume the legal limit of 1500 watts PEP, we should be able to use the modern tools at our disposal to satisfy FCC's power monitoring requirements.

There no longer are any power monitoring requirements in Part 97. It used to be a rule that if the DC input exceeded 900 watts, "accurate measuring instruments" were required to monitor input power.  When the p.e.p. bull-crap came out, the FCC, aware that the average Joe Bloe Hammy Hambone wouldn't  have a clue how to accurately measure p.e.p. output, let alone possess the necessary instrumentation, deleted the requirement altogether.  In their docket proceeding, they stated that amateurs could use other means besides accurate measurement, to determine power output.  Now try to figure that one out.

Hi Don.  Yup, that's the reason for my careful "...for the sake of this post..." wording.  Surely the FCC is aware of the implications of their changes in Part 97 and would not get into a complicated dialogue with the amateur community on the traditional ham bands regarding this question.  I’d like to limit my comments in this excellent thread the betterment of our state-of-the-art measurement techniques.

It’s my wish that we all seek good engineering in the amplitude modulation mode so that it is an enjoyable and uplifting experience.  We are the AM family and should always be looking for ways to improve our family. That would include, in my view, a well tutored family of AM’ers who know how to keep their stations in good operating status.

Al

[W7TFO]

It seems to me a lot of this 'concern' is misplaced, or even groundless (no pun intended).

My neighbor Roy had a 6-tower array hioldi9ng up a Steber curtain.  Fed it with OWL on 20m SSB, from a single 4-1000 linear.  His field strength was phenomenal, as one would expect. 

Suppose an FCC agent with one of their newfangled Tahoes set in the beam of that curtain and tried to estimate his TPO?  What could they possibly use as a benchmark?

Again, how would they make a determination without extensive testing on site with calorimeters, FS meters, input measurements, bigass budget, eTc?

Roy using a simple dipole would have a lot different result, with the same power input.

Boys, the bottom line is if nobody complains to them, they won't be at your QTH. 

That doesn't mean one has carte blanche to do as you wish, but decent use of the bands is your passport to good hamdom freedom.

A crappy 50-Watt splatter-box will get you a visit a lot quicker than a smooth 5kW into a solid antenna.

73DG

[DMOD]

Surely the FCC is aware of the implications of their changes in Part 97 and would not get into a complicated dialogue with the amateur community on the traditional ham bands regarding this question.  I’d like to limit my comments in this excellent thread the betterment of our state-of-the-art measurement techniques.

You're assuming the comm authority now has knowledgable people who have had radio communications engineering and mathematics somewhere in their training.

Back to measurement. From all of the reading I have done, measurement appears to be a "scope-and-calculate" methodology.

But then, do I measure on my voice or someone else's voice, or do I put in a two-tone signal (at what two frequencies?) and adjust my equipment so my scoped signal is 1500 Watts average power on RF peaks, and at what impedance? Theoretically, I could use any impedance since the definition for PEP = V^2rms/R(z).

I think this is why Part 97 is vague and maybe it should stay that way.

Phil - AC0OB

[k4kyv]

Surely the FCC is aware of the implications of their changes in Part 97 and would not get into a complicated dialogue with the amateur community on the traditional ham bands regarding this question.  I’d like to limit my comments in this excellent thread the betterment of our state-of-the-art measurement techniques.

It’s my wish that we all seek good engineering in the amplitude modulation mode so that it is an enjoyable and uplifting experience.  We are the AM family and should always be looking for ways to improve our family. That would include, in my view, a well tutored family of AM’ers who know how to keep their stations in good operating status.

To me, the number one consideration, regardless of mode and operating power used, is to maintain a CLEAN signal with (in the case of phone operation) good, undistorted audio quality.

Here's how the feecee shot themselves in the foot by adopting the p.e.p. garbage rather than using average power for their output standard, at least regarding SSB.  Under the old DC input rule, one could legally run as much peak power as one could generate, as  long as the DC input remained under 1 kw.  Although there are other variables involved, average output power is at least an indirect function of DC input power, with a given transmitter under given operating conditions.  That means one could run as much peak output power as one wanted, as long as the average power doesn't exceed a certain limit - and it is average power, not the power developed on occasional voice peaks, that determines the loudness and interference-causing potential of a transmitted signal. 

Under the original rule, one could run a peak SSB power as much as 14 dB (using Steve's figure) above the average output power limit and remain legal (this would apply equally well to DC input as to average output power). The signal would attain high peaks, but the average power level would still fall within the limits and therefore the signal would not produce undue interference due to its loudness, and the signal would be clean and undistorted if good engineering practice were used in generating the SSB signal, including operating the linear within its undistorted output capability. OTOH, trying to stay within a p.e.p. limit encourages heavy processing or simply overdriving the linear to get the average power (and thus the loudness of the signal) up high, while keeping the peaks under a  certain limit.

Under the old rule, a clean unprocessed signal could be maintained within the legal limit by running as high a plate voltage on the final amplifier tube as practicable, to allow plenty of head room, but the peak-to-average ratio of the human voice would keep the average DC input, and thus the average power output, within the specified limit despite the high amplitude that would occur on instantaneous voice peaks. Run the signal level required to hit the maximum DC input or average power output on your loudest voice syllables, let the peaks go where they may, and make sure the amplifier and exciter don't flat-top up to and slightly beyond the point of the highest voice peak.

The limitation on p.e.p. encourages broad, crappy-sounding SSB signals. If the signal is kept clean with little processing, the p.e.p. rule actually reduced SSB power as much as, if not more than, the alleged AM power reduction.

I think this is why Part 97 is vague and maybe it should stay that way.

The same goes for bandwidth limitations.  Riley said as much a few years ago at one of the Dayton FCC forums.

[WD8BIL]

97.3, (b)(6) PEP (peak envelope power). The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions.

Yes P.E.P. Is an AVERAGE measurement by Part 97 definition.

It basically says the highest average power measured at the peak of one cycle of the carrier during modulation.
"Normal operating conditions" means this highest power being transmitted while YOU are operating in YOUR normal manner.

That's not hard to understand.

[KE6DF]

97.3, (b)(6) PEP (peak envelope power). The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions.

Yes P.E.P. Is an AVERAGE measurement by Part 97 definition.

It basically says the highest average power measured at the peak of one cycle of the carrier during modulation.
"Normal operating conditions" means this highest power being transmitted while YOU are operating in YOUR normal manner.

That's not hard to understand.

Not hard to understand?

Well, saying "highest average power measured at the peak of one cycle"
is hard for me to understand.

The highest "average" measured at the "peak" sound like double talk to me.

[DMOD]

The highest "average" measured at the "peak" sound like double talk to me.

Peak of what?  Or do they mean "crest? of an RF cycle? A single crest of an RF cycle would not allow enough time for heating of a thermocouple to accurately measure power, since a thermocouple has a thermodynamic lag or thermal "time constant."

This is why I say it's a scope measurement-calculate thingy.

Phil - AC0OB

[WD8BIL]

Hey, it's the definition we have to work with.

If you have a decent peak reading wattmeter you'll probably be fine ifn the G men come knocking!

[K5IIA]

I would like to see when the last time the FCC actualy took action against a ham for excessive power output. That was interfearing with people or causing problems.

[Opcom]

"The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions."

Get out the 5Gs/S scope and capture that one cycle!

There are all kinds of ways to present math and graphical arguments to express opinions about the "average" of the area under the one cycle curve. It was even 'proved' the power was zero at all times if there was no modulation.

The discussion about that rule and measurements has been disemboweled and beaten to death about 2 years ago. Now the dry bones will be ground.

http://amfone.net/Amforum/index.php?topic=25835.0
http://amfone.net/Amforum/index.php?topic=20087.0

http://amfone.net/Amforum/index.php?topic=3670.0 was good too

[DMOD]

People new to AMing (and it seems more are coming on board to it) will be wanting answers/opinions to this question for some time to come.

Don and Timtron's comments are right on according to the math in this thread:

http://amfone.net/Amforum/index.php?topic=20087.0

Phil - AC0OB

[Steve - K4HX]

The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions.

It is simple. Find the peak of the RF envelope (or the crest is you prefer that terminology), take one cycle of the RF portion and calculate the average power of that cycle. Using a scope is a good way to make the measurement and it allows you to easily visualize what is going on. In the attached scope image, you would make the measurement of the one RF cycle at the point marked by the red arrow. This would be the maximum PEP for the time shown on the scope. Most PEP meters have a time constant, so really the PEP shown is for a given time.

[k4kyv]

I was listening to a converstaion between timtron and someone else friday night on the subject of what carrier level would give 1500 watts pep on AM. I heard three different answers:

(Per Timtron) ...1000 watts- 1000 watt carrier modulated 100% is 1500 watt PEP because one half the carrier power for audio added to the carrier...

What he is saying, is that when 1 kw of carrier is modulated 100%, the additional power in the sidebands is 500w peak power.  Peak power, because it occurs only at 100% modulation.  Most of the time, with the average human voice, the modulation percentage is more like 30%.  So 1000 watts carrier + 500 watts peak sideband power = 1500 watts total peak power.

97.3, (b)(6) PEP (peak envelope power). The average power supplied to the antenna transmission line by a transmitter during one RF cycle at the crest of the modulation envelope taken under normal operating conditions.

That would mean that an AM signal consists of a carrier that varies in amplitude with the modulation.  We have known for decades that this is not the case. An AM signal consists of a steady, unvarying carrier, plus the upper sideband and lower sideband off to the sides of the carrier.  This is clearly visible on a spectrum analyser. You see the carrier which extends way up to the highest point on the display.  To each side, you see the sidebands, each of whose peak amplitude extends up to a maximum of 50% of the carrier voltage, which equals 25% of the carrier power. There is no "rf cycle" that has an average power greater than that of the dead carrier over any portion of the modulation cycle. What the scope's envelope pattern shows is the total power, the sum of the independent carrier plus two independent sidebands. If the total sum of the power of each sideband plus the carrier is the limiting factor, then W1AW is violating the power rule with their bulletin simulcast on multiple bands, since the sum of the total power they are running on all of the multiple frequencies added together exceeds the so-called "legal limit". This goes back to the 1920s debate over the physical reality of sidebands.

That sounds like the logic behind Timtron's determination of what the legal AM carrier limit is. I read a book once that said the peak power of an am signal was 4 times the carrier on one page, then on another said that the peak power of a 1 kw carrier at 100% sine wave modulation was 1.5 kw (carrier plus the audio power from the modulator).

If the carrier is modulated 100% on voice peaks, then at each peak the sideband power is exactly 1/2 the carrier power. That means the total power is 1.5 times the carrier power at voice peaks.

oops, I found a mistake in that post. The book was quoting input power, not output. It said the modulator causes the plate voltage and plate current to double, which would make the power go up 4 times (2000v .5A under modulation would be 4000v 1A). On the other page it said the total power input to the final in a kw input transmitter would be 1.5 kw, 1 kw of DC and 500w audio. But the same book also says that the input may not really be 4 times exactly at 100% modulation, because the plate voltage and current are not both going to double. It says the plate voltage may double, but the current might not quite double.  

What they are saying is that the final amplifier may not have perfect modulation linearity.  With perfect linearity, if the voltage is doubled, the current is also doubled.  This is true with both input power and output power.  The question is, is that 4X power all in one rf cycle, or is it the sum of one cycle of carrier, plus the sum of one cycle of all the signal components of each sideband added together.

[kb3ouk]

The comment about W1AW reminded me of something I thought of the other day. what if you took a transmitter, split the ouput in two, then fed the two signals into two seperate linears, which fed two seperate antennas in a way so that both where in phase. say both linears put out the legal limit by themselves. so in theory, the effective power being put out would be twice the legal limit, but technically should be legal, since no where that I have found does it say that you can't have more than one transmitter in operate at once. it would kinda work like the synchronous AM stations that are in some places.

[Steve - K4HX]

This is clearly visible on a spectrum analyser.

Not really. The spectrum analyzer displays average power or voltage. Use a realtime spectrum analyzer and you will see the carrier vary in amplitude.

What the scope's envelope pattern shows is the total power, the sum of the independent carrier plus two independent sidebands.

Partially true. A scope displays voltage, not power. And if you change the scope's time base and you will see a single sinewave at the carrier frequency and the amplitude will vary.

[k4kyv]

So, does the single-frequency carrier vary in amplitude with the modulation, or does the carrier remain steady, while the modulation produces adjacent sideband energy components along side the carrier frequency?

[DMOD]

Spectrum Analyzer wise (frequency domain wise), as modulation percentage increases, does the carrier start to dissapear in order to put power into the sidebands, or is power placed into the sidebands and the carrier remains steady:

http://www.williamson-labs.com/480_am.htm

Update: consider the basic total AM power expression where Pt = Pc + 0.5*m^2*Pc; The only part that varies is the sideband power(second term), and not the carrier power, carrier power is constant.

Phil - AC0OB

[k4kyv]

Looks like the carrier remains steady regardless of modulation, and its amplitude is greater than that of any sideband component at any time.

The carrier amplitude appears to increase and decrease with modulation only when the display lacks the necessary selectivity to discriminate between the carrier and sidebands. A monitor scope with the rf sample directly feeding the deflection plates , and a Bird 43, lack that selectivity.

So what's the legal power limit at a multi-transmitter field day site?  Is the output power of each transmitter treated as a separate entity, or is the transmitter power the sum total of the outputs of all the transmitters operating simultaneously, added together?

If each of the transmitters is to be treated separately, then the Timtron method of AM power measurement is correct.

If the vector sum of the output power of all the transmitters added together is used for the power calculation, then the ARRL method of AM power measurement prevails.

[KE6DF]

I suspect if the FCC ever came down on a rich HAM and the operator decided to take the FCC to court, a smart lawyer could make the FCC look stupid for such a confusing reguation.

There seems to be as many interpretations of the rules as there are people responding to this thread.

I'm more confused that I was before I read this thread.

Since I can't put up a visible antenna in my CC&R-bound subdivision, none of my neighbors even need to know I'm a ham, so I figure no complaints are likely.

[K5UJ]

There seems to be as many interpretations of the rules as there are people responding to this thread.

I'm more confused that I was before I read this thread.

Jackpot!  Or to put it another way, this is why I just fire up the rig, get a carrier going around 300+ watts, modulate it and don't worry.  What I do worry about is where the audio rolls off on the high end, the leen-yar being linear, and the carrier not getting clipped off. 

 

[w1vtp]

<snip>

Since I can't put up a visible antenna in my CC&R-bound subdivision, none of my neighbors even need to know I'm a ham, so I figure no complaints are likely.

New game "Find the Ant!"     Finding the tower doesn't count.    Now if we could come up with an invisible sky hook. . . . .

What about that tree line to the west of your QTH.  Might there be a possibility?  Or you could set up a remote using a Flex + linear and you would be a tall ship.  Looks like a back yard to me    Under the cover of darkness (COD) you could run a coax to the base of a tree and go from there

Al