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On Wed, 20 Oct 2004 16:34:05 -0700, Roy Lewallen
wrote: I have some ideas, but that's all. I hope someone with more recent direct experience with AM broadcasting than mine who really knows the answer will comment. I will go out on a limb and speculate that the carrier isn't being reduced during modulation. If it were, simple envelope detectors would produce serious distortion. And if the carrier isn't being reduced, then the power has to be greater when modulation is present. But let's see if an expert will comment -- if I'm wrong I'll gladly eat my words. If *you* don't know, Roy, WTF does?? -- "What is now proved was once only imagin'd." - William Blake, 1793. |
Paul, why don't you simply say -
Total power = Carrier power + Power in the two sidebands. When no sidebands, just the carrier power. ---- Reg, G4FGQ |
On Thu, 21 Oct 2004 13:41:13 +0000 (UTC), "Reg Edwards"
wrote: Paul, why don't you simply say - Total power = Carrier power + Power in the two sidebands. When no sidebands, just the carrier power. "Total power"?? I dear... I feel another lengthy, definition spin-off thread coming on... ;-) -- "What is now proved was once only imagin'd." - William Blake, 1793. |
"Roy Lewallen" wrote in message ... Steve Nosko wrote: . . . I am not not trying to prolong the pain (or this thread), it is just that I was born with a bone in my head that makes it hard for me to give up explaining some basic concept like this. (yep, it can be a curse) . . . Egad, another person with the same genetic defect! Welcome! Roy Lewallen, W7EL So it's genetic!... Yea. And I do this in front of a class or 5-15 green students. It SURE is rewarding when someone says :"Oooh! NOW I get it!" 73, -- Steve N, K,9;d, c. i My email has no u's. |
Top post...
Well, friends, rather than speculate as we most certainly love to do, I just got off the phone with John, the KFI AM Engineer. (11:15am CDT Oct 21) He said that althought that meter is a pretty standard toroid type power sampler (very like the common ham units, but it has no reflected output), it is not designed to read PEP, but couldn't say what they did in the design. He went on to speculate that the meter ballistics(how fast the meter responds to changes) may be a factor. He also said that due to the level of compression used, you won't see much dynamic range (variation of power with modulation level changes) He said that "it *does* vary if you look closely." He advised that you *should have* looked at the final current meter. It *does* vary widely. He also said that Harris is doing "some pretty interesting things in that transmitter." He described the transmitter as a "50kw D to A converter" with hot redundancy (extra amps on line) and if one of those smaller amps dies, you can see a "notch" in the output waveform, but that it doesn't cause any high frequency sidebands--due in part to the filtering effect of the antenna bandwidth, but he couldn't elaborate if this was the only bandwidth limiting factor preventing splatter from said notches. Thinking about it I wonder if "notch" was not the best word, but that "flat spot" might be mbtter...but then, I didn't think to ask...Oh well. So there ya go... One comment I feel compelled to make here. It is all too common for us to ASSUME that EVERY measurement made is absolutely 100% correct. I see Engineers do this all the time (and though 99.99% of the time this probably is right, there are those times when some corrupting factor gives a strange reading and the experienced Engineer is the one who figures out that something is amiss IN THE SHORTEST TIME-- cuz' we all get confused by strange observations). It is sort of a "It says so on the HP digital meter, so it has to be 100%" assumption. Easy trap to fall into. Meter says "power, therefore it must be reading MY interpretation of power." Heep the faith AND keep asking... -- Steve N, K,9;d, c. i My email has no u's. "Roy Lewallen" wrote in message ... I have some ideas, but that's all. I hope someone with more recent direct experience with AM broadcasting than mine who really knows the answer will comment. I will go out on a limb and speculate that the carrier isn't being reduced during modulation. If it were, simple envelope detectors would produce serious distortion. And if the carrier isn't being reduced, then the power has to be greater when modulation is present. But let's see if an expert will comment -- if I'm wrong I'll gladly eat my words. Roy Lewallen, W7EL Bill Turner wrote: Last year I took a tour of the KFI transmitter site in Southern California and was fascinated by the 50kW transmitter. On the transmitter's front panel was a meter calibrated in output power. It read *steady* at 50kW, except when the operator dropped the power momentarily to 5kW, just to show he could. Ever since then, I've kicked myself for not asking why the power didn't rise with modulation. The transmitter was a Harris model DX50 (IIRC) which uses dozens of low power solid state modules which are switched on and off digitally to produce the RF output. Could it be that as they are switched on and off, they also are driven in such a way as to maintain constant power? In other words, when modulation is added the carrier power is reduced? It's the only thing that comes to mind, but there may be another reason. Ideas? -- Bill W6WRT |
On Wed, 20 Oct 2004 15:54:12 -0700, Bill Turner
wrote: On Wed, 20 Oct 2004 13:17:49 -0700, Roy Lewallen wrote: The average power of a 100% modulated 4 watt carrier is 6 watts, not 4. (If you want to look at it in the frequency domain, where the total power has to be the same as in the time domain, you've now got the original carrier plus two sidebands. The power in the two sidebands totals 2 watts.) _________________________________________________ ________ I have a question about AM sideband power. I have always believed what Roy says above, that the sidebands add to the total radiated power. You can see this when watching an S-meter. However. Last year I took a tour of the KFI transmitter site in Southern California and was fascinated by the 50kW transmitter. On the transmitter's front panel was a meter calibrated in output power. It read *steady* at 50kW, except when the operator dropped the power momentarily to 5kW, just to show he could. Ever since then, I've kicked myself for not asking why the power didn't rise with modulation. The transmitter was a Harris model DX50 (IIRC) which uses dozens of low power solid state modules which are switched on and off digitally to produce the RF output. Could it be that as they are switched on and off, they also are driven in such a way as to maintain constant power? In other words, when modulation is added the carrier power is reduced? It's the only thing that comes to mind, but there may be another reason. Ideas? I'll take a stab at this. It is probably because of how the power is being measured. An actual sample of output power is rarely used for monitoring. They may have just a simple rf voltmeter measuring the antenna line voltage, calibrated in watts. Many watt meters work in this fashion. When the carrier is modulated, the composite signal, carrier and side band voltage swings up to twice the voltage and down to zero volts with 100% positive and negative modulation. So the average voltage is the voltage that the carrier produces itself. The meter can't follow the swings fast enough so it stays at the average which is equal to the carrier. The plate current meter is in the same situation. It stands still also with modulation for the same reason even though the plate voltage may swing between 2 times and zero. If the modulation is not symmetrical, positive peaks greater than negative peaks, then you will see a slight upward kick with modulation as the average is no longer equal to the carrier voltage. An antenna current meter on the other hand will show a definite upward kick with modulation. The current meter is usually the thermal couple type of meter that actually requires power for it to operate. Heating of a resistor in it is what makes it work. Heating that thermal couple requires power. Since power output only increases with modulation, it never goes below carrier level, the antenna ammeter sees an average power increase with modulation. 73 Gary K4FMX |
"Gary Schafer" wrote in message ... On Wed, 20 Oct 2004 15:54:12 -0700, Bill Turner wrote: On Wed, 20 Oct 2004 13:17:49 -0700, Roy Lewallen wrote: The average power of a 100% modulated 4 watt carrier is 6 watts, not 4. (If you want to look at it in the frequency domain, where the total power has to be the same as in the time domain, you've now got the original carrier plus two sidebands. The power in the two sidebands totals 2 watts.) _________________________________________________ ________ I have a question about AM sideband power. I have always believed what Roy says above, that the sidebands add to the total radiated power. You can see this when watching an S-meter. However. Last year I took a tour of the KFI transmitter site in Southern California and was fascinated by the 50kW transmitter. On the transmitter's front panel was a meter calibrated in output power. It read *steady* at 50kW, except when the operator dropped the power momentarily to 5kW, just to show he could. Ever since then, I've kicked myself for not asking why the power didn't rise with modulation. The transmitter was a Harris model DX50 (IIRC) which uses dozens of low power solid state modules which are switched on and off digitally to produce the RF output. Could it be that as they are switched on and off, they also are driven in such a way as to maintain constant power? In other words, when modulation is added the carrier power is reduced? It's the only thing that comes to mind, but there may be another reason. Ideas? I'll take a stab at this. It is probably because of how the power is being measured. An actual sample of output power is rarely used for monitoring. They may have just a simple rf voltmeter measuring the antenna line voltage, calibrated in watts. Many watt meters work in this fashion. When the carrier is modulated, the composite signal, carrier and side band voltage swings up to twice the voltage and down to zero volts with 100% positive and negative modulation. So the average voltage is the voltage that the carrier produces itself. The meter can't follow the swings fast enough so it stays at the average which is equal to the carrier. The plate current meter is in the same situation. It stands still also with modulation for the same reason even though the plate voltage may swing between 2 times and zero. If the modulation is not symmetrical, positive peaks greater than negative peaks, then you will see a slight upward kick with modulation as the average is no longer equal to the carrier voltage. An antenna current meter on the other hand will show a definite upward kick with modulation. The current meter is usually the thermal couple type of meter that actually requires power for it to operate. Heating of a resistor in it is what makes it work. Heating that thermal couple requires power. Since power output only increases with modulation, it never goes below carrier level, the antenna ammeter sees an average power increase with modulation. ============== Yet 'total' RF power can be neatly measured by means of a calibrated oscilloscope. Frank GM0CSZ / KN6WH |
Gary Schafer wrote in message . ..
On Wed, 20 Oct 2004 15:54:12 -0700, Bill Turner wrote: I have a question about AM sideband power. I have always believed what Roy says above, that the sidebands add to the total radiated power. You can see this when watching an S-meter. This is true. Consider a classic plate modulated Class C AM transmitter running 200 watts input. It requires 100 watts of audio for modulation. Assume it has plate efficiency of 75%. With no input to the modulator, the transmitter produces 150 watts of RF from the 200 watts input. With sine-wave audio modulation, the input rises to 300 watts (200 watts DC plus 100 watts audio) and the total RF output rises to 225 watts. That's not a lot - only 75 watts more (75% of 100). However. Last year I took a tour of the KFI transmitter site in Southern California and was fascinated by the 50kW transmitter. On the transmitter's front panel was a meter calibrated in output power. It read *steady* at 50kW, except when the operator dropped the power momentarily to 5kW, just to show he could. Ever since then, I've kicked myself for not asking why the power didn't rise with modulation. The transmitter was a Harris model DX50 (IIRC) which uses dozens of low power solid state modules which are switched on and off digitally to produce the RF output. Could it be that as they are switched on and off, they also are driven in such a way as to maintain constant power? In other words, when modulation is added the carrier power is reduced? It's the only thing that comes to mind, but there may be another reason. Ideas? I'll take a stab at this. It is probably because of how the power is being measured. True - but not for the reasons you state. An actual sample of output power is rarely used for monitoring. They may have just a simple rf voltmeter measuring the antenna line voltage, calibrated in watts. Many watt meters work in this fashion. When the carrier is modulated, the composite signal, carrier and side band voltage swings up to twice the voltage and down to zero volts with 100% positive and negative modulation. So the average voltage is the voltage that the carrier produces itself. No, it isn't. The meter can't follow the swings fast enough so it stays at the average which is equal to the carrier. The plate current meter is in the same situation. It stands still also with modulation for the same reason even though the plate voltage may swing between 2 times and zero. True, but that's not the problem. If the modulation is not symmetrical, positive peaks greater than negative peaks, then you will see a slight upward kick with modulation as the average is no longer equal to the carrier voltage. An antenna current meter on the other hand will show a definite upward kick with modulation. The current meter is usually the thermal couple type of meter that actually requires power for it to operate. Heating of a resistor in it is what makes it work. Heating that thermal couple requires power. Since power output only increases with modulation, it never goes below carrier level, the antenna ammeter sees an average power increase with modulation. Not the issue. Both an RF voltmeter and a thermocouple ammeter require some power to work. If the antenna is a linear load, the voltage and current must retain a linear relationship, so that increasing one increases the other. IOW, voltmeter vs. ammeter makes no difference. Sorry. The reason the power meter didn't move was probably due to averaging. The measeurement system probably averages the power over several seconds, rather than trying to follow the audio. If they had transmitted just an unmodulated carrier for a long-enough time, you'd see the power output drop. But BC stations don't do that intentionally - "dead air" is a real no-no. 73 de Jim, N2EY |
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In article , Gary Schafer
writes: I set the carrier level out at 20 watts. Modulated the transmitter with a 1000 hz tone up to 100% modulation as seen on the scope monitoring the output of the transmitter. What sort of rig? Using a drake w4 watt meter (same as a bird) the watt meter held steady at 20 watts. It read the same at full 100% modulation as it did with no modulation, just the carrier. No ALC involved here either. Watching the scope the rf output voltage did double with 100% modulation as would be expected. Now had I put a thermocouple rf ammeter in the coax line I would have seen an increase in the line current with modulation. But you didn't put an RF ammeter in the line, nor an RF voltmeter. What did I do wrong? I don't think you did anything wrong. What sort of rig was it? How modulated? 73 de Jim, N2EY |
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