(looks like this post disappeared after an hour or so... so I'll try again)
Hello all. Been working w/ part 15 AM for a couple months now. Bought a Procaster, installed it and it is currently broadcasting roughly a mile radius.
Anyway, I've got the part 15 bug now and I am currently testing a talking house transmitter and would like to measure it's output power.
I set the selector switch to External antenna and soldered a 1 watt 50 ohm resistor to the end of a short coax cable for a dummy load and hooked a scope to it.
I am getting a 15 volt peak-to-peak voltage across the 50 ohm load (+-7.5 volts).
To calculate power, I convert to RMS i.e.: 7.5 x 0.707 = 5.3 Vrms
Current through the dummy load is: I = V/R = 5.3/50 = 0.106 amps rms
Power = I**2 x R = 0.106**2 x 50 = 0.56 watts!!! What am I doing wrong? or is the TH actually putting out 560 mW?
I checked my scope calibration by measuring a known voltage.
Also, when I modulate it at the edge of distortion, the peak-peak voltage seen on the scope is +-10 volts (even more than 0.56 watts).
Any thoughts would be welcomed.
Thanks
Recently someone posted that the Talking House they measured had an unexpectedly high reading, knowing that it's certified for part 15.
Another thing to be sure about is to use a NON-inductive 50-ohm dummy load resistor. Evidently there are many high-Wattage resistors that are "wire-wound", or inductive, and this will not provide a proper dummy load.
Recently someone posted that the Talking House they measured had an unexpectedly high reading, knowing that it's certified for part 15.
Another thing to be sure about is to use a NON-inductive 50-ohm dummy load resistor. Evidently there are many high-Wattage resistors that are "wire-wound", or inductive, and this will not provide a proper dummy load.
Yes, it is not a wire wound resistor.
I also read a previous thread about the TH and higher than 100 mW power, but it wasn't that high.
Yes, it is not a wire wound resistor.
I also read a previous thread about the TH and higher than 100 mW power, but it wasn't that high.
Hello Perry and welcome to the forum.
From your description it appears that you did the measurement correctly. There is always a chance of instrumentation error when working with RF but a factor of five error would be unlikely with a scope.
Regarding the higher voltage peaks during modulation, this is to be expected. At 100% modulation on an AM signal one third of the total power (one half the unmodulated carrier power) is contained in the sidebands and this power adds to the unmodulated carrier power. Remember that using 0.707 x Vpeak only works with a sine wave.
Neil
Hello Perry and welcome to the forum.
From your description it appears that you did the measurement correctly. There is always a chance of instrumentation error when working with RF but a factor of five error would be unlikely with a scope.
Regarding the higher voltage peaks during modulation, this is to be expected. At 100% modulation on an AM signal one third of the total power (one half the unmodulated carrier power) is contained in the sidebands and this power adds to the unmodulated carrier power. Remember that using 0.707 x Vpeak only works with a sine wave.
Neil
So if I'm putting out 560 mW with no modulation, then it would be even more power when I'm at 100% modulation. It sure makes me think I'm doing something wrong.
One problem could be that I think the T.H. has a 75 ohm impedance, so I should probably be using a 75 ohm dummy load, but that doesn't seem like enough to make a big difference.
For the record, this is the 2nd TH I've been playing with recently and this one's range seems poorer than the TH 5 I had last week. (This one is an older one and says MODEL NO TH II on the bottom.)
So if I'm putting out 560 mW with no modulation, then it would be even more power when I'm at 100% modulation. It sure makes me think I'm doing something wrong.
One problem could be that I think the T.H. has a 75 ohm impedance, so I should probably be using a 75 ohm dummy load, but that doesn't seem like enough to make a big difference.
For the record, this is the 2nd TH I've been playing with recently and this one's range seems poorer than the TH 5 I had last week. (This one is an older one and says MODEL NO TH II on the bottom.)
Just to note that the unlicensed power permitted by the FCC for a compliant transmitter certified under §15.219 refers to the d-c input power supplied to the final r-f amplifier of that transmitter, without modulation.
The legal, unmodulated OUTPUT power for those same conditions could not exceed the legal, unmodulated INPUT power -- as that would require a circuit efficiency greater than 100%.
Just to note that the unlicensed power permitted by the FCC for a compliant transmitter certified under §15.219 refers to the d-c input power supplied to the final r-f amplifier of that transmitter, without modulation.
The legal, unmodulated OUTPUT power for those same conditions could not exceed the legal, unmodulated INPUT power -- as that would require a circuit efficiency greater than 100%.
The p-p reading across the 50-ohm resistor appears to be too high by more than a factor of 2. I would do a very rough check of of the scope input amplifier and probe calibration by measuring a known DC voltage (maybe a DC power supply voltage checked with a DMM) with the same probe and scope input amp setting.
I see you say you checked the scope calibration. Please check again because the measurement is very suspicious. Let's face it: If the true reading is 15 v p-p, your transmiter is really putting out more than a half watt--something that is not very likely.
The p-p reading across the 50-ohm resistor appears to be too high by more than a factor of 2. I would do a very rough check of of the scope input amplifier and probe calibration by measuring a known DC voltage (maybe a DC power supply voltage checked with a DMM) with the same probe and scope input amp setting.
I see you say you checked the scope calibration. Please check again because the measurement is very suspicious. Let's face it: If the true reading is 15 v p-p, your transmiter is really putting out more than a half watt--something that is not very likely.
Dummy load & short coax run checked with Mmeter at 52.1 ohms
AA alkaline battery measured at 1.60 volts with Mmeter
Measure my DC power supply open circuit voltage at 15.52 vdc with MMeter
Measure Alkiline AA battery w/ scope @ 1.5+-
Measure my Dc Power SUpply at 15 volts
I'm now reading about 17 Volts peak-peak with a modulated signal over a 50 ohm dummy load.
Dummy load & short coax run checked with Mmeter at 52.1 ohms
AA alkaline battery measured at 1.60 volts with Mmeter
Measure my DC power supply open circuit voltage at 15.52 vdc with MMeter
Measure Alkiline AA battery w/ scope @ 1.5+-
Measure my Dc Power SUpply at 15 volts
I'm now reading about 17 Volts peak-peak with a modulated signal over a 50 ohm dummy load.