AM power VS SSB power

[ND9B]

For legal limit output (1500W PEP), what would be the average output power of AM and SSB?

It must vary from voice to voice, but is there a rule of thumb for this?

Bobby Dipole ND9B

[Opcom]

I believe it varies greatly between individuals.

To be accurate, you might say "without processing".

For my voice and using a linear amplifier, the average reading RF power meter shows about 1/4 of the PEP and the needle stays put and hardly wiggles at all. This seems consistent whether it was the mobile 800W amp, where the carrier setting was about 180W, or the NCL-2000 amp in the station, where the carrier is 200W. In both cases the movement is very slightly upward but only 5% or so.

Whether that means I have a "perfect" voice is debatable!

I believe the answer is different when the meter is reading a plate modulated transmitter instead of a linear amplifier.  When I had the Viking I, the RF power meter would deflect upwards when I spoke.

In all cases on SSB, my speech seems to deflect the meter to about 1/3 of the amp's peak power number.

I expect the next reply will be totally different.

[Steve - K4HX]

The human male voice has a peak-to-average ratio (PAR) of about 14 dB on average (it will vary from this from one person to the next).

So for unprocessed (no compression, etc) audio, the "average" power output for SSB with 1500 watts PEP is about 60 watts PEP. Yes, I used average and PEP together.

For AM let's assume 1500 watts PEP is produced with 100% positive peak modulation. This means the carrier power would be 375 watts. So the average the average power is 375 watts when there is no modulation. This means to get to 1500 watts PEP, an additional 1125 watts of power comes into play but only at 100 percent modulation. Apply the 14 dB PAR for the modulation and the average power due to modulation is about 45 watts. So the total is 420 watts average.

[flintstone mop]

Now I know why I don't get out too well. My PEP's are low......I guess more processing and a D-104 mic might help.

[k4kyv]

What  kind of instrument are you using to measure average power?  To measure true average (aka "mean") power, you need a square-law, or RMS-reading instrument.  Nearly all commercial "wattmeters" such as the Bird 43 and the numerous Hammy Hambone wattmeters are in fact rf voltmeters that work by simple rectification of the rf sample, but with a scale calibrated in "watts" when working into a specified resistive load, usually 50 ohms.  This assumes a pure unmodulated sine wave, and only with a pure sine wave will the instrument accurately read average power.  With SSB, the average power reading will be significantly low.  With AM, the instrument will indicate carrier power.

The reason for this is that a rectifier type rf instrument reads average rf voltage. However, average power is not average voltage × average current, nor {average voltage}² ÷ load resistance , but RMS voltage × RMS current (or {RMS voltage}² ÷ load resistance). This has lead to a lot of confusion because of misuse and misunderstanding of the terms.  BTW, there is no such thing as "RMS power". The proper term is average, or mean power.

By definition, RMS (root-means-squared), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity, specifically the square root of the mean (average) of the squares of the values taken over a specified length of time.  The simplest and most familiar true rms reading rf measuring instrument is the thermocouple rf ammeter, which actually samples heat generating by the r.f. as it raises the temperature of a thermocouple.  Power is calculated by using the formula P=I² R.

True RMS-reading voltmeters have recently been developed commercially; therefore an average-reading wattmeter may be designed by calibrating an RMS voltmeter to a watts scale, using the formula P=V²/R.  Like the rectifier type "wattmeter",  these instruments are calibrated only to work into a specified resistive load. One such instrument, which is not cheap, is the Bird APM-16.  

The human male voice has a peak-to-average ratio (PAR) of about 14 dB on average (it will vary from this from one person to the next).

So for unprocessed (no compression, etc) audio, the "average" power output for SSB with 1500 watts PEP is about 60 watts PEP. Yes, I used average and PEP together.

For AM let's assume 1500 watts PEP is produced with 100% positive peak modulation. This means the carrier power would be 375 watts. So the average the average power is 375 watts when there is no modulation. This means to get to 1500 watts PEP, an additional 1125 watts of power comes into play but only at 100 percent modulation. Apply the 14 dB PAR for the modulation and the average power due to modulation is about 45 watts. So the total is 420 watts average.

Steve, you are correct about the SSB peak-to-average ratio with the human voice. But I believe you made a typo.  It should have simply read the "average" power output for SSB with 1500 watts PEP is about 60 watts.

Slopbucketeers run average power much greater than that either by using heavy processing, or by simply driving their leen-yars well into the flat-topping region, resulting in rf clipping and broad splatter.
 

[Steve - K4HX]

Thanks for the correction Don. Yes, it's average not PEP.

[kb3ouk]

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:

375 watts- the ARRL and everyone's typical 4 times the carrier is PEP

1000 watts- 1000 watt carrier modulated 100% is 1500 watt PEP because one half the carrier power for audio added to the carrier, and finally...

3000 watts 
I did not hear the whole explaination on this one, but it had to do with the fact that the sidebands are some number of dB down from the carrier, and if one were to produce a 1500 watt PEP DSB signal, they would have to mix in a 3000 watt carrier in order for it to produce a AM signal that was the equivalent of a 1500 watt PEP SSB signal.

[w1vtp]

I'm not crazy by some generalizations that are popular in amateur radio circles - for example, an AM carrier is 1/4th the peak envelop power.  It serves as a good "get a handle on it" concept but doesn't do a good job of the complex modulation component of the human voice.

I like the paper put out by Agilent:

http://cp.literature.agilent.com/litweb/pdf/5965-6630E.pdf

There is some good technology coming out for peak power measurements.  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 requiements. Modern pulsed radar can no longer be satisfied with measurement techniques performed just a few decades ago and the industry is rising to the occasion.

My approach is fairly simple.  First, I measure my 1500 watt power output developed across a precision 1500 watt load using, yes, the venerable Bird 43 watt meter.  At the same time I'm using a variable coupler to establish a 8 cm envelope display on my  scope ( 50 ohm input on the scope to accomodate the coupler).  Next, I adjust my transmitter (AM or SSB) so that my instantaneous peaks do not exceed the 8 cm level.  The carrier falls where it will - the final requirement from FCC (in my view) is that I never exceed the instantaneous 1500 watt level as established by my fairly simple setup.  At the same time, I can be reasonably assured that I am not flat-topping by observing the scope.

If an AM transmitter is properly adjusted, there will be no observable change in average power during modulation

Al

[DMOD]

Hmmm, doesn't add up if they are talking about total sideband power and total AM power equivalencies.

If you're talking about replacing a 1.5kW PEP SSB with an equivalent AM station:

Let's say you're going to replace a 1500 PEP sideband signal with an equivalent AM signal at 100% modulation.

Pt = Pssb, total power is 1500W

Modulation percentage = 100% so modulation Index in formula = 1

Pt = total power in SSB

Pc = carrier power(for AM)

Ps is total AM sideband power

Pt = Pc + Ps; Pc + Ps = carrier plus sideband power for AM

Pt = Total Power = 1.5kW(total power in SSB) =  Pc + 2(1^2)/4)) = Pc + 0.5Pc

Pt = 1.5 Pc

solving for Pc, Pc = Pt/1.5Pc, so Pc = 1.5kW(total power in SSB)/1.5 = 1kW carrier power for AM.

Ps = is Total Power of AM sidebands = Pt - Pc = 1.5kW - 1kW = 500 Watts

Ps = 500 Watts, or 1/3 total Power of combined  Ps + Pc

[kb3ouk]

Hmmm, doesn't add up if they are talking about total sideband power and total AM power equivalencies.

If you're talking about replacing a 1.5kW PEP SSB with an equivalent AM station:

Let's say you're going to replace a 1500 PEP sideband signal with an equivalent AM signal at 100% modulation.

Pt = Pssb, total power is 1500W

Modulation percentage = 100% so modulation Index in formula = 1

Pt = total power in SSB

Pc = carrier power(for AM)

Ps is total AM sideband power

Pt = Pc + Ps; Pc + Ps = carrier plus sideband power for AM

Pt = Total Power = 1.5kW(total power in SSB) =  Pc + 2(1^2)/4)) = Pc + 0.5Pc

solving for Pc, Pc = Pt/1.5Pc, so Pc = 1.5kW(total power in SSB)/1.5 = 1kW carrier power for AM.

Ps = is Total Power of AM sidebands = Pt - Pc = 1.5kW - 1kW = 500 Watts

Ps = 500 Watts, or 1/3 total Power of combined  Ps + Pt

Each sideband then contains 1/4 of Ps + Pt or 250 Watts each.

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).

[Steve - K4HX]

If you read that in a book then the book was wrong. The peak power of an AM (DSB with full carrier) is 4x the carrier power at 100 percent modulation. The average power of an AM signal modulated with a sinewave to 100 percent is 1.5x the carrier power. Some AMers want to improperly mix PEP and average powers to claim they can legally run 1 kW carrier and modulate 100%.

Al has the correct approach. Measure 1500 watts carrier with an accurate meter and then set your scope for that level. If your positive modulation peaks never exceed the level on the scope, you will never be running more than 1500 watts peak, no matter the carrier level.

[Opcom]

yup.

[WD5JKO]

Al has the correct approach. Measure 1500 watts carrier with an accurate meter and then set your scope for that level. If your positive modulation peaks never exceed the level on the scope, you will never be running more than 1500 watts peak, no matter the carrier level.

   I am jealous that Al has that kind of power capability to run a dead 1500 watt carrier. Not many of us can do that. That said E^2/R = W, so it is just a mathematical relationship to run say 375 watts carrier as measured on the watt meter along with X CM of deflection on the scope, and from that calculate where 1500 watts would be on the scope.

Jim
WD5JKO

Now I understand what happened to Al's power transformer.

[kb3ouk]

If you read that in a book then the book was wrong. The peak power of an AM (DSB with full carrier) is 4x the carrier power at 100 percent modulation. The average power of an AM signal modulated with a sinewave to 100 percent is 1.5x the carrier power. Some AMers want to improperly mix PEP and average powers to claim they can legally run 1 kW carrier and modulate 100%.

Al has the correct approach. Measure 1500 watts carrier with an accurate meter and then set your scope for that level. If your positive modulation peaks never exceed the level on the scope, you will never be running more than 1500 watts peak, no matter the carrier level.

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.

[w1vtp]

Al has the correct approach. Measure 1500 watts carrier with an accurate meter and then set your scope for that level. If your positive modulation peaks never exceed the level on the scope, you will never be running more than 1500 watts peak, no matter the carrier level.

   I am jealous that Al has that kind of power capability to run a dead 1500 watt carrier. Not many of us can do that. That said E^2/R = W, so it is just a mathematical relationship to run say 375 watts carrier as measured on the watt meter along with X CM of deflection on the scope, and from that calculate where 1500 watts would be on the scope.

Jim
WD5JKO

Now I understand what happened to Al's power transformer.

At no time did I ever run the AL82 such that it exceeded specification. The transformer should have not failed.

If my comments were carefully read, it would be seen that my 8 cm scope setting was based on a measured 1500 watt level using a Bird 43 and terminated in a precision load - not based on any other assumptions

Power is ultimately measured based on the scientific fact that both DC and RF when properly dissipated, generate the same amount of heat.  When this heat is accurately measured the RF (or DC) power can be accurately determined.  I have used this calorimetric technique during high power radar transmissions.  In that application, the actual rise in water temperature is measured against a measured flow rate / volume. NIST (National Institute of Standards and Technology) uses this method to establish a national standard for accurate power measurements.  I offer one page from Agilent's paper (not copyrighted) that shows how NIST establishes these standards.

If one is running much less power than legal limit, the requirement for my previously posted technique or its equivalent is not necessary, in my view.  It is then only necessary to have some sort of power measuring equipment that will give a general idea that the transmitter is "healthy."  It is, however, necessary to know that the transmitter is properly adjusted so that excessive  bandwidth does not occur.  I think a scope is essential for that monitoring.

Actually, I think we may be a bit off topic.  As I understand the question, how does the average power compare of AM and SSB stations outputting 1500 watts PEP?  

[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, only when you start to get some assymetrical modulation would the meter start to move upward or downward.

[WD5JKO]

My approach is fairly simple.  First, I measure my 1500 watt power output developed across a precision 1500 watt load using, yes, the venerable Bird 43 watt meter.  At the same time I'm using a variable coupler to establish a 8 cm envelope display on my  scope ( 50 ohm input on the scope to accomodate the coupler).

So from this statement I said:

  I am jealous that Al has that kind of power capability to run a dead 1500 watt carrier. Not many of us can do that. That said E^2/R = W, so it is just a mathematical relationship to run say 375 watts carrier as measured on the watt meter along with X CM of deflection on the scope, and from that calculate where 1500 watts would be on the scope.
Now I understand what happened to Al's power transformer.

 I didn't suggest Al that you were running over 1500 watts; just stating that many of us cannot make 1500 watts carrier with our stations. I can do 1500 PEP, but no more than 1100 watts carrier due to poor line voltage regulation.

 The comment I said about the power transformer was uncalled for, and I'm sorry if that offended you. It was a joke in poor taste.

At no time did I ever run the AL82 such that it exceeded specification. The transformer should have not failed.

Noted, but those Birds are good for +/- 5% accuracy usually and that is full scale. So to measure 1500 watts with a Bird 43 don't you need a 3KW slug (or is it 2500 watts?). Point here is the Bird could have been more than 5% off and still be within specification for a near mid scale reading. Most dummy loads see a shift in the 50 ohm resistance from a huge thermal load. I see that at work with the big 5KW Bird load with the cooling fans. The measured power drops about 5% after 60 minutes at 3KW (13.56 Mhz). The point here is getting an accurate power measurement to 5% or better is tricky for a carrier, and even trickier for repeatable PEP measurements.


Still if we run AM or SSB and we exceed 1500 watts PEP occasionally, and for brief intervals, who would know, or who would care? Someone else running half the PEP could sound louder when using audio processing.

Jim
WD5JKO

[Steve - K4HX]

If the FCC shows up at your door, it's pretty likely they will use a Bird too.    No worries.

[k4kyv]

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.

[DMOD]

An SSB transmitter sends no carrier (or at least a 50 dB down carrier), so the carrier power is essentially zero.

A given SSB transmitter is said to have the same communication effectiveness as a conventional AM unit running much more power. For example, a 100-W SSB transmitter supposedly offers the performance capabilities of an AM transmitter running a total of 400 W, since they both show 100 W of power in one sideband. The supposed power advantage then, of SSB over AM is 4:1.

The only other advantages stated for SSB over AM is occupied spectrum and lower noise inder the same conditions.

But then again, SSB ain't 1/4 the fun as A3E.   

Phil - AC0OB

[Opcom]

If an AM transmitter is properly adjusted, there will be no observable change in average power during modulation

Al

The carrier 1/4 peak convention comes about from the practice of using a sine or other symmetrical wave for tests. If it swings equally both ways, that's what it will do. My unprocessed voice is on average fairly symmetrical. It is different for every person and voice and for each lash-up. It is even different depending on what is said, such as vowels etc.

It's possible the plate modulated TX was mis-adjusted or malfunctioning during those observations. Could have been running a slightly low carrier, leaving the modulator headroom to go above 100% positive.

I don't like the conventions/old wifes tales etc either but a test with voice or a defined wave that others can duplicate is a good place to start.

[Opcom]

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.

If that is correct, then I think the expectation is that an operator would make an honest effort such as using a scope to compare peak voltage to a known voltage. Many scopes in ham hands are definitely in the "other means besides accurate measurement" category so it fits the rule.

What do you think they really want us to do?

[Opcom]

An SSB transmitter sends no carrier (or at least a 50 dB down carrier), so the carrier power is essentially zero.

A given SSB transmitter is said to have the same communication effectiveness as a conventional AM unit running much more power. For example, a 100-W SSB transmitter supposedly offers the performance capabilities of an AM transmitter running a total of 400 W, since they both show 100 W of power in one sideband. The supposed power advantage then, of SSB over AM is 4:1.

The only other advantages stated for SSB over AM is occupied spectrum and lower noise inder the same conditions.

But then again, SSB ain't 1/4 the fun as A3E.   

Phil - AC0OB

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*.

[KA2DZT]

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.

Maybe just putting your hand on the tank coil would be enough of a "other means" of measurement.

[W7TFO]

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...

73DG