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Radio Professionals,
I have two Allied Radio broadcasters, both versions, one early with open chassis one with transformer (audio) mounted on the chassis.
Recently on the open chassis one, I replaced the power supply capacitor and also replaced the selenium rectifier with a solid state diode. These improvements work great.
The question is this and possibly someone with one of these broadcasters can answer my question about the RF power output.
Both transmitters are running the correct input power supply voltage and the DC on the parts is approximately 140 V at the main tap. On the plate of the 50C5s about 100 Volts
For testing the power output at the antenna terminal, (not DC input to the oscillator) I use a 50 ohm carbon resistor across the output and ground. The resistor on the hot side has a 1N34A diode which is on the test probe side. There is also a small capacitor that charges to peak on the test probe side. The diode rectifies the RF across the resistor, which I read with Hi Z DC volt meter.
I take the reading from the volt meter and use the formula E squared over R to get the value in watts.
The system works well for getting a relative indication of power, I’m not saying it is the best way or only way or accurate way but it does provide an indication of power which I use as comparison for all my transmitter projects.
The question I have is this, both of these Allied Radio broadcasters work really good, have good low end on the audio and overall audio quality is very good. When I went to check the power output with the method mentioned above, I was a bit surprised to find that these transmitters are only putting out about 20 Mw which is 1 volt across 50 ohms.
I checked the RCA tube manual and it says that the 50C5 has a max rating of 7 watts output. OK I realize this may be possible under audio situations or some power application, but the 50C5 in this case is configured as an oscillator running about 100 Volts on the plate.
So the question is: since both of these transmitters are running about the same RF power out, and voltage wise and all tube condition are normal, just what is the deal with the power output? I had one of these transmitters 50 years ago and transmitted a few blocks with no antenna tuning… doesn’t seem possible that you can do that on 20 Mw
I have to admit I was using a vertical made from a cane pole from fishing mounted on top of a flat roof garage with the transmitter sitting right there but I thought these transmitters would have more power than 20 Mw.
Can anyone give me some information on what the actual MEASURED power output of the Allied Radio broadcaster is? Has anyone actually tested the RF output power using a system as I have or other means? I would think these older units would produce more like 80-100 Mw.
The Talking house transmitters I have easily test at 100 Mw with the same method I have used to test RF power out on the Allied Radio broadcaster. I am just curious if this is a normal power output for this device or should I be looking further for faulty parts to improve power output
Thanks for your help on this
Radio Joe
You made the assumption that the characteristic impedance of the RF output is 50 Ohms which is incorrect. The plate of the 50C5 Oscillator from which the RF is capacitively coupled to the antenna terminal via a 100pf ceramic capacitor has a much higher impedance. The coupling capacitor alone puts the impedance somewhere north of 1K Ohms at the high end of the band and about 3K Ohms at the bottom of the AM band. While I didn’t take the time to crunch the numbers on the impedance at the 50C5 plate I’m certain it is significantly greater than zero meaning that the transmitter is looking into a load well below its characteristic output impedance.
Swamping the RF output with a 50 Ohm resistor that is well below the impedance of the transmitter’s antenna output would cause inefficient power transfer to the restive load and therefore an incorrect measurement result. You may want to repeat your measurements with a resistive load that is closer to the characteristic impedance and see what you measure.
Rather than using a DC voltmeter I’d recommend using an AC coupled high impedance oscilloscope probe minus the diode detector and looking at the Peak to Peak RF waveform voltage across the load. Even the cheapest probes and scopes have plenty of bandwidth to very accurately measure RF voltage below 2 MHz. You might even want to determine the characteristic impedance experimentally by connecting a non inductive 10K potentiometer as a load and varying the resistance until you see a peak in the RF voltage Amplitude across the load.
That would be a best case power output to an impedance matched antenna, The efficiency of a 10′ wire might be somewhat less as its impedance at the top of the AM band is about 2800 Ohms almost purely capacitive reactance which means the antenna looks like a 33pf coupling capacitor connected in series with a 10′ piece of wire. As you can see, measuring the RF voltage across a 50 Ohm load in this case is not representative of the real impedance therefore the readings you get will not make a lot of sense.
Hi Paul, you make a very good point, I was assuming it had a 50 ohm output but i can see how I was wrong ! so easy to see it too when i gave it a look and reading your detailed explanation. Thank you so very much for letting me see my mistake. Never too old to learn! thank you for the excellent explanation, i do have a good scope, so ill see what i come up with. Thank you again, some time a person cant see forest for the trees!
Radio Joe
RadioJoe,
Paul nailed it with his comments. You have to consider the antenna impedance when optimizing the transmitted power and the impedance of a 3 meter wire antenna is high and reactive.
I built a tube transmitter based on the KnightKit design and it compares equally in performance (except for DC input power) to my KK using the wire antenna. I have a base coil loaded 3 meter antenna suspended from the ceiling in my lab and at resonance it provides a 30 ohm resistive load to the transmitter. This is a huge mismatch for the KK circuit.
Just for fun I experimented by winding an output link around the oscillator coil of my home made transmitter attempting to match the 30 ohm base coil antenna and it worked. The link winding which was just a few turns of wire formed a transformer which made the low antenna load impedance appear to the transmitter as a high Z load. Keep in mind that this is not useful for just a wire antenna without a loading coil.
What Paul and I are talking about is the reason tube amplifiers for audio use a transformer between the plate circuit and the speaker. Tube amps generally have an impedance at the plate on the order of 5 to 10 kOhms which has to be matched to the load appropriately. The same goes for RF applications.
Below is a calculation of the performance of a “typical” Part 15 AM transmit system, from a spreadsheet using the appropriate equations appearing in antenna engineering textbooks.
The impedance of the antenna system “seen” by the transmitter there is the sum of the radiation resistance with the r-f resistances of the loading coil and the path to r-f ground. At system resonance and in this case, that impedance is 0.1104 Ω + 15 Ω + 15 Ω = 30.1104 Ω.
Radiated field intensities are shown for radius distances of 1 km and 1.609 km (1 mile) from the radiator. A field intensity of 100 µV/m or more may produce a usable signal to a good AM receive setup in a receive location with acceptably low interference from co- and adjacent-channel AM stations, and sufficiently low ambient r-f noise levels (from SCR dimmers, etc).
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