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Author Topic: 1500 pep output max  (Read 94908 times)
NO2A
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Posts: 1400




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« Reply #15 on: December 23, 2015, 10:26:20 AM »

The F.C.C. already gave us a break. The power limitation used to be for input power, not output. In some countries the Novice License equivalent might be 10 watts. (No offense to the QRP crowd. )
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SM0AOM
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Posts: 261




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« Reply #16 on: December 23, 2015, 10:46:06 AM »

Putting RMS vs. peak power into perspective....

A continuous tone on SSB with a PEP of 1500 watts will provide an RMS output of 750 watts, which is "3 db"  "less", so it would take 3 KW PEP to achieve 1500 watts RMS with a single tone..

This is only 1/2 s-unit improvement (relative to a single tone) with a major increase in amplifier mass and cost and major improvements needed to most antenna systems.  And the near-field RF danger will increase as will stray RF problems, so shack grounding and stand-off distances become even more critical.

Considering that some countries have a 400 watt limit (UK, Australia) and our close neighbor (Canada) at 2.25 KW - only 1/4 of an S-unit higher - we are doing OK.

Brian K6BRN

This contains two misconceptions;
first there is no such thing as "RMS Power",
and second a continuous single AF tone modulating an SSB transmitter results in a
constant-envelope waveform with a peak to average power ratio of unity, or average power = PEP.

This is known by every RF engineer.
 
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WB2WIK
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Posts: 21837




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« Reply #17 on: December 23, 2015, 11:18:27 AM »

The F.C.C. already gave us a break. The power limitation used to be for input power, not output. In some countries the Novice License equivalent might be 10 watts. (No offense to the QRP crowd. )

I agree.  Our output power limitation here in the U.S. is very liberal and only a few places in the world allow any more; many places allow quite a lot less.

It would be cool if we could run more power on 30m, though.  Some places can.

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AA4PB
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Posts: 15066




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« Reply #18 on: December 23, 2015, 11:27:09 AM »

RMS power is sometimes used in the rating of amplifiers in the audio industry. It is the resistance of the load times the square of the RMS current.
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Bob  AA4PB
Garrisonville, VA
KB8E
Member

Posts: 50




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« Reply #19 on: December 23, 2015, 12:18:04 PM »

There is such a thing as rms power, it just doesn't have any significance in the real world. Consider a sinusoidal voltage of V = 170 sin (2*pi*60*t). The average voltage is zero (no DC component). The peak voltage is 170 V. The rms voltage is found by squaring the voltage waveform, averaging over an integer number of complete cycles, and then taking the square root. The result for the above waveform is 120 V. The waveform for instantaneous power is found by taking the voltage waveform, squaring it and dividing by the load resistance. For a load of 144 ohms for example, we get a waveform of P = 200 sin(2*pi*60*t)*sin(2*pi*60*t). The average is 100 W. The peak is 200 W. The rms is 158 W.

Sam
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K6UJ
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Posts: 1377




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« Reply #20 on: December 23, 2015, 08:07:01 PM »

It's all about ERP.
That would be Wyatt ERP.  Wink


I love it !!    Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy
Good one Bryan !!

73,
Bob
K6UJ
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N0GW
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Posts: 47




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« Reply #21 on: December 23, 2015, 08:27:24 PM »

"There is such a thing as rms power, it just doesn't have any significance in the real world. Consider a sinusoidal voltage of V = 170 sin (2*pi*60*t). The average voltage is zero (no DC component). The peak voltage is 170 V. The rms voltage is found by squaring the voltage waveform, averaging over an integer number of complete cycles, and then taking the square root. The result for the above waveform is 120 V. The waveform for instantaneous power is found by taking the voltage waveform, squaring it and dividing by the load resistance. For a load of 144 ohms for example, we get a waveform of P = 200 sin(2*pi*60*t)*sin(2*pi*60*t). The average is 100 W. The peak is 200 W. The rms is 158 W. "

Sam, you seem to be confusing two different concepts in your presentation.  When measuring the power of an RF signal with an oscilliscope you do, in fact, see peak voltage, not RMS voltage.  Of course, to calculate the power of the RF signal, you do have to multiply the voltage by 0.707 (1 over the square root of 2) to get the RMS voltage of that sine wave.  That is what we use to calculate our 1500 watts of RF.  The FCC 1500 watt output power is true watts.   By true, I mean that 1500 watts has the same heating effect on a resistor as 1500 watts of DC power.

When we apply the term Peak Envelope Power, we are talking about the true RF power at the peak of our modulated RF signal.  Our SSB signals vary between zero power and some maximum depending upon our applied audio waveforms.  Whatever that maximum is we call the Peak Envelope Power.  That is to say that if we look at the signal waveform with an oscilloscope, we would see a voltage peak 1.414 times the RMS or DC equivalent of that power.  We are already taking all that into account.

Is that making sense?

Gary
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KB8E
Member

Posts: 50




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« Reply #22 on: December 24, 2015, 02:30:56 AM »

Gary,

My understanding is that we're talking about a regulatory limit of 1500 watts PEP. I interpret that to mean an instantaneous maximum power of 1500 watts. When observing the voltage waveform on an oscilloscope, this would occur at the peak of the RF envelope and would be equal to the peak voltage squared divided by the load resistance. This would also be the peak 'heating' power. The troughs of the envelope where the RF waveform is zero represent zero power. If one is able to determine the rms value of the RF voltage waveform, that value can be squared and divided by thr load resistance to get average power output. Due to time constants in a real load, this heat is what would be observed as heat in a dummy load.

Sam

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K6BRN
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Posts: 1348




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« Reply #23 on: December 24, 2015, 06:54:30 AM »

To SM0AOM:

(Sigh!)  Really?  Did you read the OP and understand the context of my reply?  He asked about Peak vs. RMS power limits, and I replied in kind.

RMS power can be easily calculated for any single tone (by definition a sinusoidal waveform), from 50 Hz AC to microwave and beyond and is a valid metric for power comparisons.

A single tone is defined as a pure sinusoid at any frequency, mechanical or electrical, however it is derived.  It will appear as a single spectral line on an analyzer - no side lobes, no width.   How a reasonable facsimile might be produced - i.e. single constant amplitude audio tone injected into an SSB modulator to produce CW, is not really important, except that the OP was asking the question in terms of SSB.

The OP asked about moving from a 1500 watt peak SSB power output limit to a 1500 watts RMS limit.  Did you really expect a long lecture on "Average Power"?  My answer, qualified in terms of single tones, a context under which the OPs question is easily understood and answered, was simply that the real world performance difference is not that great and the relative cost is high - a lot of effort for a small gain.   That was my point.

So... what's the problem?

Brian  K6BRN
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K7EXJ
Member

Posts: 875




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« Reply #24 on: December 24, 2015, 07:40:22 AM »

It's all about ERP.
That would be Wyatt ERP.  Wink


I love it !!    Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy Cheesy
Good one Bryan !!

73,
Bob
K6UJ
Urp! ('Scuse) Cheesy
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73s de K7EXJ
Craig Smiley
K6BRN
Member

Posts: 1348




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« Reply #25 on: December 24, 2015, 10:45:20 AM »

Craig:  Prilosec works wonders for indigestion.

Gary:  Agree with your comments ...  exept that RMS voltage is the mathematically handy fiction - you'll never see it on an oscilloscope - unless its a flat DC trace.

RMS power, calculated from the scaled peak voltage of a sinusoid and the load resistance, will measure as true power with an RF bolometer. 

BTW - RMS power was used as the sole legal definition of audio amplifier power output during the 70's, when false amplifier power claims based on arbitrary metrics were out of control and the government stepped in.

Brian K6BRN
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N0GW
Member

Posts: 47




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« Reply #26 on: December 24, 2015, 01:24:46 PM »

"Gary:  Agree with your comments ...  exept that RMS voltage is the mathematically handy fiction - you'll never see it on an oscilloscope - unless its a flat DC trace.

RMS power, calculated from the scaled peak voltage of a sinusoid and the load resistance, will measure as true power with an RF bolometer. "

No argument from me.  If my words did not indicate that, I apologize.  My intent was to emphasize that with sine wave power measurement, we do use the concept of RMS.  This term has a very specific meaning in engineering that allows us to describe what true heating value is of any arbitrary voltage waveform.  For sine waves, the RMS (Root Mean Square) value works out nicely to have a square root of two relationship between peak voltage and the RMS heating (DC) equivalent value.  Other waveforms have different relationships.

The confusion in this discussion is Sam's assumption that the FCC 1500 watt PEP power limit is based upon the peak voltage of the RF sine wave waveform and not the RMS heating value of the RF.  Of course, all the FCC cares about it the equivalent heating value in watts.  Of course, I'd use something a little beefier than my bolometer element for this measurement.

There may be another confusion in this.  We have all seen diagrams of the Christmas Tree oscilloscope trace of our voice modulated SSB transmitted signal.  I hope everyone understands that the term Peak Envelope Power (PEP) refers to the RMS RF power at the highest points in the trace.  With our typical 100 watt output SSB transceivers, the highest points are at the 100 watt RMS power level.  That is defined as 100 watts PEP.  With voice modulation, the average power output of that transceiver with voice is typically only 20% or 30% of that 100 watt PEP signal.  While there were arguments in the 1980s when the change from final amplifier input power to the present output power limit about using average power versus PEP, the PEP won on technical points.  (When measuring average power, what time constant do you use for the averaging? 100 milliseconds?  1 second? 1 day?  PEP avoids that issue.)

Gary - N0GW

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KM1H
Member

Posts: 5541




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« Reply #27 on: December 24, 2015, 06:36:21 PM »

RMS power is an invention of the audiophools and now some technically challenged hams have picked up the new buzzword.

Even JI agrees Roll Eyes
http://www.w8ji.com/amplitude_modulation.htm

Sort of reminds me of Sears, Harbor Freight, and other frauds when rating compressor HP, generator power, etc.
Or the BHP rating of the bad old muscle car days.

Someone in marketing will always find a new gimmick to skin the consumer.

Here is some interesting reading to digest, I havent gone thru it all yet.
http://www.audioholics.com/audio-amplifier/amplifier-power-ratings
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N0GW
Member

Posts: 47




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« Reply #28 on: December 24, 2015, 07:19:27 PM »

"RMS power is an invention of the audiophools and now some technically challenged hams have picked up the new buzzword."

OK, I guess I was getting carried away with my terminology.  I was trying to get across that the PEP power we are discussing is based upon RMS voltage and RMS current.  I apologize for the confusion on that detail.

I'll say it definitively: The term RMS power is incorrect.  There is RMS voltage and RMS current but when we discuss power, it is simply power.  We can talk about peak power, average power, or even Peak Envelope Power.  Power is exactly true heating power when we are discussing RF power.

Gary - N0GW
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SM0AOM
Member

Posts: 261




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« Reply #29 on: December 24, 2015, 11:45:24 PM »

The concept of RF power is considered sufficiently important by the international community to warrant its own definitions in the ITU Radio Regulations, which usually have been carried over verbatim into national rules;

"1.156     power: Whenever the power of a radio transmitter, etc. is referred to it shall be expressed in one of the following forms, according to the class of emission, using the arbitrary symbols indicated:

- peak envelope power (PX or pX);

- mean power (PY or pY);

- carrier power (PZ or pZ).

For different classes of emission, the relationships between peak envelope power, mean power and carrier power, under the conditions of normal operation and of no modulation, are contained in ITU-R Recommendations which may be used as a guide.

For use in formulae, the symbol p denotes power expressed in watts and the symbol P denotes power expressed in decibels relative to a reference level.

1.157     peak envelope power (of a radio transmitter): The average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions.

1.158     mean power (of a radio transmitter): The average power supplied to the antenna transmission line by a transmitter during an interval of time sufficiently long compared with the lowest frequency encountered in the modulation taken under normal operating conditions.

1.159     carrier power (of a radio transmitter): The average power supplied to the antenna transmission line by a transmitter during
one radio frequency cycle taken under the condition of no modulation."


Note that there are no references to "RMS Power" here, nor in any radio engineering texts.

If the power limits should be expressed in mean power the question of the proper time constant to use would arise. In the previous regime of specifying maximum indicated input power, the FCC (and other Administrations) prescribed an allowed maximum time constant of 0,25 s for the plate meter when expressing current readings in SSB transmitters.

Up to two years ago, the Swedish Power limits were expressed as an undefined "maximum power" of 1000W.
There was no guidance in the rules how this was defined or should be measured, in contrast to other Administrations that used the ITU definitions.


« Last Edit: December 25, 2015, 08:00:45 PM by SM0AOM » Logged
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