(Continued from this thread: www.part15.us/node/574 )
Here's an update on my experiments with the Ramsey AM-25. I have documented what I did for this test.
Purpose: To determine the resistive load at which maximum power output occurs with a Ramsey AM-25 transmitter.
Result: The maximum power output from the Ramsey AM-25 transmitter occurs with a 24 ohm resistive load when resistance is the only parameter considered.
(Continued from this thread: www.part15.us/node/574 )
Here's an update on my experiments with the Ramsey AM-25. I have documented what I did for this test.
Purpose: To determine the resistive load at which maximum power output occurs with a Ramsey AM-25 transmitter.
Result: The maximum power output from the Ramsey AM-25 transmitter occurs with a 24 ohm resistive load when resistance is the only parameter considered.
Method: A Ramsey AM-25 transmitter assembled according to the instructions for operation below 1710 kHz. and with bias adjusted according to the manual was used in this experiment. A composition 100 ohm potentiometer was soldered to a BNC connector, wiper to center, end to ground using short leads to reduce reactive components at the test frequency. Though these reactive components were not measured it is assumed for this test that they are negligible. A BNC tee connector allowed connection with the pot in parallel with the scope input (Fluke 123) and the transmitter output. A BNC connector was installed at the transmitter antenna output connection and was attached to the scope and pot by a 3 foot RG58/U cable. With the transmitter disconnected, various R values were set with the pot as indicated on the scope. For each R, with the transmitter connected, the RMS voltage was measured with the scope at a frequency of 1520 kHz. It was noted qualitatively that the output voltage waveform was a sine wave over the range of load R used.
The power delivered to the resistor was calculated for each test value of R and a peak was identified. The resistance was adjusted up and down by approximate 2 ohm increments (2.3 max) from the resistance which produced the peak power and the optimum load resistance was thus determined within 2.3 ohms.
Discussion: The result does not necessarily represent the optimum impedance expected by the transmitter since no reactive component was inserted with the resistive load for this test. Proper interpretation of the experiment result is that the maximum power delivered to a resistive load with this particular transmitter occurs at R = 24 ohms +/- 2.3 ohms.
The load power expressed as a percentage of the maximum at 24 ohms is given below for various load resistances.
R(ohms) %of maximum power
10.6...........81
24.3.........100
50.4...........85
79.8...........67
101.5.........57
This leads to the conclusion that since a resonant base coil loaded antenna with buried radials as discussed previously on this board presents a resistive load in the range of 20 to 100 ohms, the Ramsey AM-25 will effectively apply power to such an antenna. The Ramsey AM-25 will tolerate a rather wide resistive load variation from 24 ohms.
Thanks to Rich for supplying the antenna data and for his other helpful comments.
Neil
Neil,
I think I understand your transmitter test configuration. How did you measure or calculate power output? A scope measures voltage, how did you determine the milliamperes arriving at the load? I understand by using ohms law, one could calculate the power being delivered to the circuit. And in a completely resistive load (no reactance), "E x I = P". Did you extrapulate "I" by dividing "E" by "R"? If so, then your test makes sense. Just trying to understand the test process completely. And BTW, good work.
Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
Hi Marshall,
The oscilloscope I used (Fluke 123) displays the RMS voltage directly on the screen. Since I knew the load resistance, I calculated the power dissipated in the resistor using P = (E^2)/R. Implicit in this equation is a calculation of the current. It goes:
I = E/R and P = E x I so P = E(the voltage) * (E/R)(the current) = E*(E/R) = (E^2)/R.
If you know the voltage and resistance this equation is easier to use since you don't have to calculate the current.
Hope this helps,
Neil
by MRAM 1500 kHz
I'm curious to know if you took a look at the modulating characteristics when the transmitter was loaded at 24.3 ohms. I've noticed that some transmitters don't modulate well when loaded for maximum RF output. They tend to have little or no upward modulation when loaded for max RF output.
Metzo is one of them. It would seem that the "long-tail" output stage runs out of gas when it comes to modulating if it's cranking out all the RF it can.
Hi,
**edited** Oops, the specified voltage of 7.1 volts is measured at R34's lead closest to C23, not at TP1 as I wrote. ****
No, I did not look at this, but from previous experience I know this could be an issue. I applied a 1 kHz. sine wave from a HP audio generator and observed the modulated waveform on the scope. I was on the air with a coupled antenna which I have previously described and noted the best looking waveform when R23 was adjusted for a TP1 voltage of 4.7 volts rather than the specified 7.1 volts. This adjustment changes the input power so proceed carefully if you try this.
I did not observe the modulation with a changing load but it would not surprise me if this would affect the quality of the audio as well since it changes the load line for the amplifier. Again, this is just speculation and if I can I will do an experiment.
Neil