Got an old Harris tube transmitter here (FM5K). This transmitter has a PA tube, and an IPA tube. I've always gone by Plate Amps x Plate Current X Transmitter Efficiency = TPO. Have a situation right now, where, according to that formula, our TPO is 3600 watts, and where it should be. However, Power Output Meter, and Bird Wattmeter, are reading 5% power. Is that possible? Granted, IPA Grid current is down to 30 mA, and it should be around 140 mA, so I highly doubt the transmitter is putting out 3600 watts.
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FM Transmitter Power Output
- Thread starter Brian Bowers
- Start date
Believe the wattmeter. You don't have enough RF drive to IPA stage.
We usually run 9 watts out of the exciter. That also feeds the backup transmitter, which is now on the air with full power (4,000 watts). Exciter is reading 9 watts output. Could you mean not enough drive from the IPA to the PA? Also, isn't the FCC rule "to arrive at legal power, PA I x PA V x Efficiency?"
Replace the IPA tube. It could be bad. The FCC rule is for a properly operating transmitter. That's why a wattmeter is important.
Thanks. Already in the process of. Even though I've always had Bird Wattmeters around, and looked at them, I usually always went by the FCC rule, as the golden rule. I guess I won't be doing that anymore. Would the goal be to get the two meters (Bird and transmitter power output) as close, or right on, as possible, when tuning?
The Bird wattmeter should have calibrated slugs that match the meter. I'd trust the wattmeter, as it has no adjustment pot.
Common issue on the K series Harris - IPA socket will lose a bypass cap and power will drop off drastically. Make sure to check them while replacing the IPA tube.
RFB
Common issue on the K series Harris - IPA socket will lose a bypass cap and power will drop off drastically. Make sure to check them while replacing the IPA tube.
RFB
I'm also thinking about the exciter. 9 watts output and it's driving two transmitters?
How? Splitting the output of the exciter would not only create a mismatch for the exciter but it would also reduce the RF drive into both transmitters.
Most FM transmitters can't produce full power with so little RF drive (<4.5 watts) from the exciter.
Could you give us more information?
How? Splitting the output of the exciter would not only create a mismatch for the exciter but it would also reduce the RF drive into both transmitters.
Most FM transmitters can't produce full power with so little RF drive (<4.5 watts) from the exciter.
Could you give us more information?
Years ago, I had an RCA 10-watt exciter, feeding a pair of BTF-20 transmitters. It's not exactly what's being discussed here, but it would seem you can get a lot of horsepower out of 9 or 10 watts.
>>>Splitting the output of the exciter would not only create a mismatch for the exciter but it would also reduce the RF drive into both transmitters.<<<
Sorry, my screwup. The Continental 802B feeds the FM5K, and the Gates (FM5H) backup, still has a MS-15 in it, feeding it directly. I know, we are living in the stone ages with these transmitters.
>>>The Bird wattmeter should have calibrated slugs that match the meter. I'd trust the wattmeter, as it has no adjustment pot.<<<
We are used to calibrating the Power Output meter pot on the FM5K, after checking on the formula (PA I x PA V x efficiency). I guess I've always been taught not to use the Bird Wattmeters as references. Should I go back to school for that one?
>>>Common issue on the K series Harris - IPA socket will lose a bypass cap and power will drop off drastically. Make sure to check them while replacing the IPA tube.<<<
Are we talking a visual inspection here?
Sorry, my screwup. The Continental 802B feeds the FM5K, and the Gates (FM5H) backup, still has a MS-15 in it, feeding it directly. I know, we are living in the stone ages with these transmitters.
>>>The Bird wattmeter should have calibrated slugs that match the meter. I'd trust the wattmeter, as it has no adjustment pot.<<<
We are used to calibrating the Power Output meter pot on the FM5K, after checking on the formula (PA I x PA V x efficiency). I guess I've always been taught not to use the Bird Wattmeters as references. Should I go back to school for that one?
>>>Common issue on the K series Harris - IPA socket will lose a bypass cap and power will drop off drastically. Make sure to check them while replacing the IPA tube.<<<
Are we talking a visual inspection here?
Use both to determine power output. In this case, the Bird is most accurate. After you repair the transmitter, you can use the FCC-approved method I x E x efficiency to determine the power output. The formula is accurate only when the transmitter is operating properly and meeting the original manufacturer specs.
Read 73.267 of the Rules. If your wattmeter section is connected to the output of the LP filter, and has a current calibration, you have the option of using it for direct power measurement. You can also use it to calibrate the transmitter's output meter.
If you would rather continue using the indirect method, you can also use it to determine the current efficiency factor of your transmitter (keep a record of the determination) once it is back in working order. If you discover the transmitter has become more efficient than it was at the factory (different tubes can cause this), then use the factory F number and use the extra watts legally. There is no requirement when using indirect measurement that the actual power output be that which is licensed. I had one transmitter that, when properly tuned for best efficiency and lowest incidental AM, would put out about 15% more than one would expect. Sure, I could have adjusted the tuning and loading to match the factor F number at licensed output, but why? That only uses more electricity with no more output.
If you would rather continue using the indirect method, you can also use it to determine the current efficiency factor of your transmitter (keep a record of the determination) once it is back in working order. If you discover the transmitter has become more efficient than it was at the factory (different tubes can cause this), then use the factory F number and use the extra watts legally. There is no requirement when using indirect measurement that the actual power output be that which is licensed. I had one transmitter that, when properly tuned for best efficiency and lowest incidental AM, would put out about 15% more than one would expect. Sure, I could have adjusted the tuning and loading to match the factor F number at licensed output, but why? That only uses more electricity with no more output.
>>>If your wattmeter section is connected to the output of the LP filter, and has a current calibration, you have the option of using it for direct power measurement.<<<
Can you refresh my memory as to what qualifies for current calibration?
>>>If you would rather continue using the indirect method, you can also use it to determine the current efficiency factor of your transmitter (keep a record of the determination) once it is back in working order. If you discover the transmitter has become more efficient than it was at the factory (different tubes can cause this), then use the factory F number and use the extra watts legally. There is no requirement when using indirect measurement that the actual power output be that which is licensed. I had one transmitter that, when properly tuned for best efficiency and lowest incidental AM, would put out about 15% more than one would expect. Sure, I could have adjusted the tuning and loading to match the factor F number at licensed output, but why? That only uses more electricity with no more output.<<<
Hmmm. I did not know that. Is it safe to assume "the factory F number" is the factory spec'd efficiency number? Could you elaborate on the process, or give me an example?
Can you refresh my memory as to what qualifies for current calibration?
>>>If you would rather continue using the indirect method, you can also use it to determine the current efficiency factor of your transmitter (keep a record of the determination) once it is back in working order. If you discover the transmitter has become more efficient than it was at the factory (different tubes can cause this), then use the factory F number and use the extra watts legally. There is no requirement when using indirect measurement that the actual power output be that which is licensed. I had one transmitter that, when properly tuned for best efficiency and lowest incidental AM, would put out about 15% more than one would expect. Sure, I could have adjusted the tuning and loading to match the factor F number at licensed output, but why? That only uses more electricity with no more output.<<<
Hmmm. I did not know that. Is it safe to assume "the factory F number" is the factory spec'd efficiency number? Could you elaborate on the process, or give me an example?
I'm guessing your real efficiency has went to heck on you for some reason. How how is the stack temp out of it? What's normal? Have you tried to put it into a dummy load (if you have one) to see what that does? Keep in mind that Bird wattmeters are only somewhat accurate into 50 ohm loads. If your antenna system isn't right at 50, then there's plenty of room for the Bird to "lie". Also, I've seen Coaxial Dynamics slugs heat up and read differently. Bird doesn't seem to be as bad about that, but who knows. I think you need to dig a bit deeper on this one.
99.8% of the time, this transmitter operates the way it is supposed to, or at least, I think the way it is supposed to. Yeah, there are some funky relays that do act up sometimes in this thing. Stack temperature runs about 142 degrees. I do watch for that when I tune the transmitter. We just opened it up, and the final tube looks to be in very good shape (heat fins look to be almost the same copper color as when we put it in, with the obvious microdust). Someone taught me along the way to pay attention to the final. If the heat fins are dark to very dark, that is a sign of mis-tuning, or being starved for air.
Just in the past 2 or 3 weeks, we've been keeping an eye on the transmitter, as we have seen the Bird fall a little bit each day, but Plate Amps X Plate Voltage x Efficiency was still claiming it was making full power (the Bird and transmitter power meter were not). I also noticed that the filament voltage was cranked wide open, but the meter was reading 7.2 volts (the voltage area we usually keep it in, I think). Keep in mind, I am only at this facility on a very part-time basis. So I am thinking we will replace both the IPA and PA tube when they arrive. Normally we have them on the shelf ready to go, but not this time. Don't all jump on me at once, but we have a dummy sitting on the side, but it has never been connected to the coax switch. Yeah, this facility is a bit behind the times.
Just in the past 2 or 3 weeks, we've been keeping an eye on the transmitter, as we have seen the Bird fall a little bit each day, but Plate Amps X Plate Voltage x Efficiency was still claiming it was making full power (the Bird and transmitter power meter were not). I also noticed that the filament voltage was cranked wide open, but the meter was reading 7.2 volts (the voltage area we usually keep it in, I think). Keep in mind, I am only at this facility on a very part-time basis. So I am thinking we will replace both the IPA and PA tube when they arrive. Normally we have them on the shelf ready to go, but not this time. Don't all jump on me at once, but we have a dummy sitting on the side, but it has never been connected to the coax switch. Yeah, this facility is a bit behind the times.
>>>I also noticed that the filament voltage was cranked wide open<<<
PA Filament Voltage
PA Filament Voltage
Current calibration:
Within the last year or so.
There are two numbers for F. One is the one on the final test data for that particular transmitter. That's usually only valid if the transmitter is on the frequency it was in the factory, and operating at the power level that was considered 100% in the factory. The efficiency will change if either of those are different. If the same frequency and a different power, that difference will normally be small enough to disregard. The other F is the one buried somewhere in the manual and is representative of what can be expected for the transmitter, as accepted by the Commission. That one is usually used only if the factory test data is not available. If you don't have the test data, a call to the factory with the serial number of your box may yield results.
When adjusting the transmitter you kind of work backwards. Figure what the plate I should be for the current plate E at the desired output power. I.E., for 5000 watts, with a plate E of 3200 V and test data sheet F of 78%, plate I should be 2.0 amps. Then you adjust everything for that number. Remember when you adjust the screen, the loading will change, so you will have to go through a few iterations to get the actual efficiency peaked. If, when you are done, your power measured by the wattmeter is 5300 watts, you get to keep the extra power.
Within the last year or so.
There are two numbers for F. One is the one on the final test data for that particular transmitter. That's usually only valid if the transmitter is on the frequency it was in the factory, and operating at the power level that was considered 100% in the factory. The efficiency will change if either of those are different. If the same frequency and a different power, that difference will normally be small enough to disregard. The other F is the one buried somewhere in the manual and is representative of what can be expected for the transmitter, as accepted by the Commission. That one is usually used only if the factory test data is not available. If you don't have the test data, a call to the factory with the serial number of your box may yield results.
When adjusting the transmitter you kind of work backwards. Figure what the plate I should be for the current plate E at the desired output power. I.E., for 5000 watts, with a plate E of 3200 V and test data sheet F of 78%, plate I should be 2.0 amps. Then you adjust everything for that number. Remember when you adjust the screen, the loading will change, so you will have to go through a few iterations to get the actual efficiency peaked. If, when you are done, your power measured by the wattmeter is 5300 watts, you get to keep the extra power.
Thanks. Factory data is for the frequency we are on, so I'll continue using that F number. Bird is old, so the direct method won't be used as the reference. I've always adjusted for peak efficiency, but I never knew I could keep that extra wattage, so thanks for the tip. As always, thanks everyone for the advice. I'm glad to see the Radio Discussions board back. I hope it's for good.
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