Modulation question

[R. Fry SWL]

And R. Fry, the RF power output of an FM transmitter is constant no matter if it's 10% modulated or 100% modulated.

That is the point I was making, Fred, prompted by the statements in this thread about modulation peaks on a 7 kW FM transmitter.

And there are times the carrier actually disappears.

There is no "carrier" in FM as in AM.  The energy at the assigned FM center frequency can go to zero for some modulation conditions, and that energy is totally distributed in Bessel sidebands on each side of the center frequency.

This can be useful in calibrating an FM modulation monitor by selecting a center frequency null produced from FM deviation by a known modulating frequency.

[Steve - K4HX]

That doesn't match up with Part 97, which says:

(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.


It's power supplied to the antenna transmission line, not the antenna feedpoint. It would seem the FCC is not concerned with the feedline other than what is going into it.

...I suspect that the FCC would define the power limit in terms of the forward power - the reflected power (i.e. the power that is actually leaving the antenna, neglecting any resistive losses in the antenna itself).

If so, the only accurate means of doing that is to measure the power delivered to, and accepted by the feedpoint of the radiator.

[w1vtp]

Actually, I did the math and if one measures the carrier accurately and uses Steve, QIX's mod monitor, one can derive the PEP quite nicely.

I have done an Excel spread sheet on it and it works slick. One needs only to enter the carrier power and the positive % modulation. Gotta bounce it off Steve QIX some time.  So, while my previous doc is correct, given Stu's correct analysis of the inadequate response of the peak reading Bird 43, I think I will use Steve's excellent peak reading response characteristics of his mod monitor as a preferred method of measuring PEP.

Al

[AB2EZ]

Based on Steve's (K4HX) latest comment:

"That doesn't match up with Part 97, which says:

(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."

I've revised my opinion of how the FCC would interpret their rule in the context of reflected power coming back toward the transmitter from the antenna system. I think they would say something like the following:

"We are going to measure the power being supplied to the transmission line at the point where it connects to the output port of your transmitter. We are going to make this measurment using a directional coupler... and all we care about is the forward power traveling in the direction from the transmitter toward the antenna system. We recognize that there might be reflected power traveling back from the antenna system toward the transmitter. We recognize that there might be resistive losses in the antenna system. We recognize that there might be ground losses associated with the antenna system. We don't care about those effects. All we care about is that the peak envelope power supplied to the transmission line by your transmitter's output port, traveling in the direction from the transmitter toward the antenna, is less than 1500 watts."

In respsonse to that, I would argue that if I use an antenna tuner... consisting entirely of passive components... it should be considered as part of my antenna systems (along with the transmission line between the tuner and the antenna). Therefore, if I can adjust my antenna tuner to ensure essentially no reflected power flowing in the section of transmission line between the output port of my transmitter and the tuner... I can supply the full 1500 watts PEP into that transmission line ... even though there will be points along the other section of transmission line between the output side of the tuner and the antenna in which the forward power exceeds 1500 watts PEP.

I think the FCC would agree that the tuner (consisting of passive components) is just a part of my antenna system... and all they care about is the PEP supplied by the output port of my transmitter, into the transmission line connected to that port, traveling in the direction from that output port toward the antenna system.

Stu

[R. Fry SWL]

...I think the FCC would agree that the tuner is just a part of my antenna system... and all they care about is the PEP supplied by the output port of my transmitter, into the transmission line connected to that port,  in the direction from that output port toward the antenna system.

Maybe that is all the FCC cares about for amateur radio service, but my guess is that they are (or should be) more concerned about radiated power.

After all, it is radiated power that produces interference to other facilities.

[KM1H]

Why penalize those with high gain antenna systems?

If the FCC is using a Bird 43 they already know it is only accurate to +/- 5% of the full scale reading.

How accurate is the LP-100 peak reading meter? They also claim a true peak hold capability.
http://www.telepostinc.com/lp100.html

[KA2DZT]

Why not just use a dummy load (50ohm) and whatever meter the FCC thinks indicates the full PEP.  This way no need to consider reflected power or any other component of the antenna system.

Fred

[w1vtp]

...I think the FCC would agree that the tuner is just a part of my antenna system... and all they care about is the PEP supplied by the output port of my transmitter, into the transmission line connected to that port,  in the direction from that output port toward the antenna system.

Maybe that is all the FCC cares about for amateur radio service, but my guess is that they are (or should be) more concerned about radiated power.

After all, it is radiated power that produces interference to other facilities.

OH my!  Let's NOT go down that road.

 

And Fred.  Yup

[R. Fry SWL]

Why not just use a dummy load (50ohm) and whatever meter the FCC thinks indicates the full PEP.  This way no need to consider reflected power or any other component of the antenna system.

Because a dummy load doesn't radiate as much EM energy as most antenna systems, and may not have the same input Z as a given antenna system.

Probably the FCC would have no concern with anyone operating a ham-band transmitter of any power level into a non-radiating dummy load.

But substituting an antenna system for that dummy load may change the power delivered to the tx output connector when it is connected to the antenna system vs. the dummy load.

[W2PFY]

After all, it is radiated power that produces interference to other facilities.

What facilities? Perhaps all facilities??? Seems like your using a very broad brush here!

[KA2DZT]

R. Fry,

As you probably know, when the load matches the internal impedance of the generator you get maximum power out to the load.  If I'm using a xmtr that the specs say has a 50ohm output impedance, I would think that a load with a matching impedance is pulling full power from the xmtr.

Now having said this, you're right, maybe my set wasn't made exactly to the claimed spec, and some other load resistance will pull more power than the standard (or commonly used) 50ohm load.

I guess at that point, let the FCC do their testing with an adjustable load and figure out at what load my set delivers the most power out.

So, where does that brings us, back to your statement,  do the measuring with my antenna as the load.  I agree with your point.

If my xmtr has an internal impedance of 50 ohms and I connect a 75 ohm antenna system to it, I'm not going to be able to pull the full capable power from the xmtr.  And before everyone starts to jump up and down, if my antenna system is 75 ohm (cable, antenna, SWR meter) I will have no reflected power (1:1 SWR.resonant). I just won't be getting the maximum power from the xmtr.

In contrast to all of the above,  most of the xmtrs that us AMers are using have Pi network tank circuits which can be maximized for a limited range of loads.

Fred

[AB2EZ]

What do the folks at the FCC's Office of Technology Policy really care about?

I know from first hand experience... having served (several years ago) as a member of the FCC's Technological Advisory Council (If you serve on this Advisory Council, you are not an FCC employee, you don't get paid, and you don't get your travel and living expenses reimbursed) that what the folks in the FCC's Office of Technology Policy really care about is: the inefficient use of many portions of the frequency spectrum. Spectrum is very valuable... and the FCC is promoting (among other things) the increased use of spread spectrum techniques and things like "cognitive networking" to increase the number of users per unit of bandwidth and per unit volume of space.

http://www.fcc.gov/encyclopedia/technological-advisory-council
 

Fortunately, most of our HF bands are of no great interest to other prospective users or the FCC... but we need to "keep our heads down".

If the FCC were to revisit HF amateur radio (which I don't think they are likely to do), they would likely focus on how "empty" the bands are most of the time... and on how high power users with large transmitted bandwidths "unnecessarily"  interfere with adjacent users when the bands are not empty. Most AMers would stand out as sore thumbs with respect to "efficient" utilization of their allocated spectrum.

With respect to converting concerns into rules...

The FCC knows that rules have to be structured in terms of things that the FCC and radio operators can measure... even though what can be measured may not be 100% correlated with what the FCC is concerned about.

You can measure the peak envelope power traveling from the output port of a transmitter into the connected transmission line, toward the antenna system. At least until the present time, it has been very difficult/impossible to come up with a practical/enforceable way to measure of how much interference a particular transmitter is causing.

However, I know first hand, that the FCC and the industry are exploring concepts like "local radiation temperature" as a means of coordinating the use of the spectrum among "cognitive" spread spectrum devices... which will adjust their respective output powers and radiation patterns, in a coordinated way, to minimize interference among shared users of a specified band of frequencies in a specified volume of space.

Compared to those initiatives... our concerns about 1500 PEP limits for relatively short distance AM communication on HF bands would not likely fall on sympathetic ears at the FCC. If anything, the SSB folks would receive a more sympathetic hearing regarding the efficient use of the HF amateur bands... but, again, I doubt if the FCC would spend any time revisiting the use of the existing HF amateur bands.

Stu

 

[R. Fry SWL]

As you probably know, when the load matches the internal impedance of the generator you get maximum power out to the load.  If I'm using a xmtr that the specs say has a 50ohm output impedance, I would think that a load with a matching impedance is pulling full power from the xmtr. etc

Most transmitters are rated for the power they can safely deliver to a specified load Z, such as 50 ohms.  But that doesn't mean that the source impedance of the output stage of the transmitter is 50 ohms.  If it was, then the r-f power dissipation within the tx would be the same as it delivered to the load, and the tx never could have a PA circuit efficiency exceeding 50%.

The source Z of most transmitters usually is much lower than the load impedance they are specified to drive.  This permits Class C amplifiers (for example) to deliver 75% or more of their d.c. input power as r-f power to the load.

[AB2EZ]

As SWL points out:

The concept of a "matched load" does not apply (as just one example of when it does not apply) to a complex, non-linear situation like this where:

a. The source is essentially a current source producing a periodic sequence of pulses of (plate) current at some repetition frequency... i.e. a tetrode or a pentode with an RF bypass from screen to ground. Note that a tetrode or a pentode acts like a high impedance source because it delivers essentially a fixed plate current, independent of the plate voltage, provided the plate voltage is positive. In the special case of class A operation, the periodic sequence of pulses of plate current takes the form of an average current plus a sinusoidal current. In the special case of class C operation, the periodic sequence of pulses of plate current consists of pulses whose duration is less than half a cycle.

b. The load (through which the current flows) is required to be a tuned circuit that has a much higher impedance at the fundamental frequency of the current source than its impedance at DC or at any of the harmonic frequencies. I.e. we are trying to maximize the power delivered to the load at the fundamental frequency

c. Across this current source, there is a fixed voltage source in series with the load

d. There is a switch in the circuit that turns off the current source if the voltage across the current source goes below zero volts


I.e. in the case of an RF output stage, the fact that the plate current goes to zero (turns off) if the plate voltage is not positive leads to the inapplicability of the matched load concept. If, hypothetically, the plate current were independent of the plate voltage... whether the plate voltage was positive or negative... then the matched load concept would apply... and the value of that matched load would be very high (because the output impedance of a tetrode or a pentode is very high). Of course, the tube doesn't work that way... it "turns off" (i.e. no plate current) when the plate voltage is not positive.

However, it is true that there is a value of load impedance that causes the voltage on the plate of the RF output tube to drop to zero at one instant during each RF cycle... and that is the value of load resistance that is close to the optimal value for maximum RF power output.

Note that if the tank circuit is tuned to resonance, the time at which the plate voltage drops to its minimum value in each RF cycle coincides to the time when the plate current (i.e. each of the pulses of current that make up the plate current) is at its maximum value (provided the plate voltage is greater than zero volts). Therefore, if the plate voltage drops to a value below zero volts for too long in each cycle, the current source (that represents the tube's plate current) will be able to deliver less and less energy to the tank circuit during each cycle. That is why the value of load impedance (at the fundamental frequency) that causes the plate voltage to drop to zero for one instant in each cycle is approximately the optimal value.

The current passing through the RF load (i.e. into the input side of the tank circuit) is not a sine wave... but it contains a component at the fundamental frequency. The impedance of RF the load (at resonance) is much higher at the fundamental frequency than the impedance of the RF load (at resonance) is at multiples of the fundamental frequency. Therefore the voltage across the RF load is approximately a sine wave at the fundamental frequency. The voltage on the plate of the RF output tube is approximately the sum of a DC (the B+) and a sine wave at the fundamental frequency. The optimal value of the load impedance (at the fundamental frequency) is approximately equal to the DC plate voltage divided by the amplitude of the plate current component at the fundamental frequency.

In class C operation, the plate current is a periodic sequence of narrow pulses. It contains an average value, a component at the fundamental frequency, and components at harmonics of the fundamental frequency. From Fourier analysis, the amplitude of component of the plate current that is at the fundamental frequency is approximately twice the average plate current. Therefore, the optimal load impedance (looking into the tank circuit) in class C operation is approximately: the DC plate voltage / [2 x the average plate current]

Stu  

[WD5JKO]

So how did the topic of "Modulation question" morph into FCC enforcement of power rules, and whether Hams have the right to keep our HF spectrum? 

So since the topic is all over the place, what about transmitters like the big Wilcox (96D?) that has OWL output at 600 ohms? Go a step further, and use resonant feeders.

So as hams, are we in any way obligated to have on our transmitters a 50 ohm output port that makes it convenient for an FCC enforcement engineer to connect their 50 ohm peak reading wattmeter?

Jim
WD5JKO

[Steve - K4HX]

Given that the FCC has never put any limitation on antenna gain in an amateur station, it's clear that they don't care about radiated power. And there are good reasons for this.

Being concerned with interference and thus radiated power makes sense in broadcasting since there are strict limits on cochannel and adjacent channel interference. The public is being impacted by the interference.

In the amateur world, there were never any interference limits within the amateur bands other than things like "good amateur practice." This is because the public and other radio services were not being impacted by any amateur radio interference.

Coverage area is also a concern in broadcasting, again requiring the measurement of radiated power. Coverage is not a concern in amateur radio (at least not in the regulations) other than to the individual stations desires (local, DX, long path, etc) and is often dictated by propagation more than radiated power.

On the other hand, the FCC has historically been concerned with out of band operation, harmonic and spurious radiation from amateur stations because these could impact other radio services and the public.

...I think the FCC would agree that the tuner is just a part of my antenna system... and all they care about is the PEP supplied by the output port of my transmitter, into the transmission line connected to that port,  in the direction from that output port toward the antenna system.

Maybe that is all the FCC cares about for amateur radio service, but my guess is that they are (or should be) more concerned about radiated power.

After all, it is radiated power that produces interference to other facilities.

[Steve - K4HX]

What does Part 97 say?

So as hams, are we in any way obligated to have on our transmitters a 50 ohm output port that makes it convenient for an FCC enforcement engineer to connect their 50 ohm peak reading wattmeter?

[Steve - K4HX]

Maybe you didn't mean it this way, but you seem to make a contradiction. If the bands are so empty, how could they ever be crowded and how could interference be a problem? Seems there would be plenty of room to spread out and avoid interference.

Interference has always been a part of amateur radio. Part 97 says nothing that would indicate that one should expect interference free operation at any time (bands empty or crowded). Riley Hollingsworth stated something similar in the past. So, I'm not sure the FCC cares about interference, unless it is malicious or due to an improperly functioning transmitter.

I don't think all the HF bands are empty or under used (recognizing that these are fairly nonspecific and ambiguous terms). Over the past 10 years I've noticed a decrease in the density of stations on 75 meters. I think there is less activity, but there is also more room to spread out with the increased phone allocation. In that same period, I've seen an increase in activity on 160 meters. The portion between 7.1 and 7.2 MHz most certainly has more activity than it did before 2009 due the most of the broadcasters departing.

If protecting amateur spectrum is of concern, we should worry about our allocations in the VHF, UHF and SHF ranges. Those are the ones useful to the modern wireless/broadband services that are currently pushing the demand for more spectrum.

they would likely focus on how "empty" the bands are most of the time... and on how high power users with large transmitted bandwidths "unnecessarily"  interfere with adjacent users when the bands are not empty.

It seems to me that comparing single channel, fixed frequency, simplex radio systems using low efficiency modulation schemes (all amateur comms to a greater or lesser extent) to frequency agile, cognitive radio systems using high efficiency modulation schemes will always result in the former looking poor when using spectrum utilization as a metric. In other words, all amateur comms are in the same boat in this regard, AM or otherwise.

That said, given the nature and purpose of the amateur service, spectrum efficiency shouldn't be a metric or most certainly not the only one. See Part 97.1. You could argue that the purpose in paragraph (a) could require improving spectrum efficiency and to some extent paragraph (b) too (although there are many ways to advance the state of the art in radio outside of spectrum efficiency.) I don't think the rest argue explicitly for spectrum efficiency, but I suppose one could always find a way to make these fit if they were really pushing spectrum efficiency.


§ 97.1 Basis and purpose.

The rules and regulations in this part are designed to provide an amateur radio service having a fundamental purpose as expressed in the following principles:

(a) Recognition and enhancement of the value of the amateur service to the public as a voluntary noncommercial communication service, particularly with respect to providing emergency communications.

(b) Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.

(c) Encouragement and improvement of the amateur service through rules which provide for advancing skills in both the communication and technical phases of the art.

(d) Expansion of the existing reservoir within the amateur radio service of trained operators, technicians, and electronics experts.

(e) Continuation and extension of the amateur's unique ability to enhance international goodwill.

[K1JJ]

"(e) Continuation and extension of the amateur's unique ability to enhance international goodwill."


Time to do our part again, Steve - hold court on 75M and 40M into Eu.    Tell the Pascals of the whirl what a good job they're doing... 

T

[Steve - K4HX]

Yes. Very inefficient use of the spectrum but very fun!

[KA2DZT]

R. Fry,

Thanks for your comments.

I did give some thought that the usual stated 50 ohm output impedance may not be the source impedance.  You correctly and simply point out the usual greater than 50% efficiency of class C finals.  I guess I failed to connect the dots on that one.

Thanks again

Stu,

Thanks for your comments.  As usual, one finds that things are quite as simple as one thinks.

I guess that's why some folks are students and some are professors.

Your input here on the forum is a great benefit to all us students.

I'll re-read and study your comments further later today.

I'm sure, given ample time, I'll be able to come up with more brilliant concepts.

Fred

Stu,

I read through your detailed answer to my earlier statement.  Had to read it a few times.  I can see the relationship of the plate current, voltage and impedance is complex.  I was aware that there are only short duration pulses into the tank circuit.  One thing I finally got an answer to is why we have to divide the plate voltage by twice the plate current to find the plate load.  Something I've done many times calculating Pi Network circuits but never knew exactly why.

Thank you for all the time you give to accurately answer the questions we know-it-alls come up with.

[W4RFM]

The FCC makes it simple for broadcast folk and their rated power. 

AM's are licensed for their carrier power, with no mention of anything else. 

FM's are rated at their radiated (ERP) power. MUCH more simple.

[KA2DZT]

Well Bob,  as you can see, the threat you started is now on page 4 and growing.  I hope by this time we have answered your question.

BTW Bob, what was the question   I seem to have forgot.

Fred

[Steve - K4HX]

True, as far as it goes. The BC stations must also make measurements on their radiated power or coverage patterns. These are far more involved and difficult than the simple power measurement in the amateur radio realm. Hooking a PEP reading wattmeter to the output of most amateur transmitters, transceivers or amplifiers is about as simple as it gets. I can read my power with nothing more than a glance at the meter. Performing a proof of an AM directional station requires a few orders of magnitude more time and effort.

The FCC makes it simple for broadcast folk and their rated power. 

AM's are licensed for their carrier power, with no mention of anything else. 

FM's are rated at their radiated (ERP) power. MUCH more simple.

[steve_qix]

WAY back when I was in broadcasting as chief engineer, we measured AM station power using an RF current meter at the common point.  The common point was either the input to the phasing equipment (in the case of a multi-tower array), or the antenna matching network.  The meter was often directly at the base of the tower for a single tower system.

For directional stations, we also had to measure the base current of EACH tower in the array.  In the case of a 4 tower directional array, that was a lot of measuring, as it had to be done in not-so-infrequent intervals during the broadcast day (which, in some cases was a 24/7 operation).  This was called the "direct method".  

If your common point RF current meter failed, or if the common point impedance was off for some reason (there are reasons), you were allowed to measure your power using the "indirect method".  This used the power input to the transmitter RF final amplifier(s).

Actual field strength measurements of critical points in the radiation pattern were also required on (as I am trying to recall from so far back) a weekly basis.  These measurement points were specified in the station license.

All measurements had to be logged, and the FCC would inspect these logs, and the entire operation, from time to time (I had a number of FCC inspections during my career in broadcasting).

We also had to perform a "skeleton proof" of directional arrays YEARLY, where you would take field strength measurements at  various compass points (I forget how many - it has been many decades)  and also what is called a "partial proof" at less frequent intervals (years), which required checking the field strength of all of the compass points.  A full proof would involve taking measurements at every compass point, and going out many miles along each compass radial.  That was quite a process and could take a week or more.  This was usually only done if the station was looking to increase power or change the radiation pattern.  It was also required to measure the field strength of the co-channel, 1st adjacent and sometimes the 2nd adjacent stations as well if a change in power or radiation pattern was to be requested.

It was also required that stations measure their common point impedance, and also sweep the common point and note the impedance change at various frequencies near to the station's frequency of license.  This was to verify that the common point impedance was reasonably stable over the audio range.  Many directional arrays are quite sharp, and the common point impedance can vary significantly over the audio range if steps are not taken to compensate.

From that I understand, many of the logging and measurement requirements have been significantly relaxed over the decades.

At the time I was in radio, there had to be a First Class FCC licensed engineer on duty at all times if the station was a directional station, or ran over a certain amount of power (a number which I do not remember).  

Because of this requirement, there were many "6 week wonders" employed at various medium-market directional stations - these were typically DJs, who took a course and studied like crazy to pass the First Class Radiotelephone Operator's exam, and promptly forgot everything once the license arrived.  But, this allowed the station to meet the requirement of a First Class licensed "engineer" on duty at all times.  Of course, if ANYTHING at all went wrong, these guys were absolutely clueless, which is when I would get the 2 AM call that a big storm had put the station off the air!

Oh well, a little history.  We have it easy on the Amateur side as compared to broadcast, although it was easier (and certainly less expensive) when Amateur power was measured as power INPUT to the final RF amplifier - and you used your plate voltage and current meters - rather than PEP output, which requires more sophisticated and more expensive equipment.