It's been often reported in these forums that the Ramsey AM25 suffers from inferior radiation when used with a 3-meter antenna and compared to an SStran AMT 3000 under the same conditions.
It's been often reported in these forums that the Ramsey AM25 suffers from inferior radiation when used with a 3-meter antenna and compared to an SStran AMT 3000 under the same conditions.
In it's manual to AMT 3000 tells us on page 22 that a 3-meter short-wire antenna presents a high impedance and therefore the published circuit is deliberately matched to this higher impedance. Accompanying literature describes circuit changes which may be made to lower the AMT3000's output impedance to match a coil loaded grounded low impedance antenna.
On the other hand the AM25 is designed with a low impedance output of nominally 50 ohms, and Neil radio8z has measured it at actually 84 ohms, and by the way a professional standard in broadcasting is precisely around 50 ohms, but of course that assumes a matched 1/4 wavelength antenna or better, which is not available for Part 15. Indeed the Ramsey manual describes winding one's own matching coil to bring the antenna impedance closer to a match, and I've found this very effective.
But here's the point I'm headed toward. What about a circuit modification to raise the output impedance of the AM25? This would be the inverse of the SStran approach. Before I try this idea I'll hope to hear from anyone who has either tried it already or has an opinion about its rational.
Whether the antenna of a Part 15 transmitter has high or low impedance depends upon whether you use a series or parallel equivalent circuit to describe it. Both equivalent circuits give the same results, but one or the other is more convenient to use for a particular transmitter.
As a series equivalent circuit, the antenna impedance can be represented as a capacitive reactance of about 3000 ohms in series with the sum of the ground loss resistance and the loading coil loss resistance, which may be around 50 ohms. There is also a radiation resistance of about .1 ohms, but that is a small part of the series antenna resistance, and can be ignored (nearly all of the output of a Part 15 AM transmitter is lost in the ground and the loading coil, and very little is radiated).
As a parallel-equivalent circuit, the antenna of the previous paragraph would be about 3000 ohms of capacitive reactance in parallel with 180 k ohms. The parallel equivalent of the radiation resistance is 90 M ohms.
Whether to use the series or parallel equivalent circuit in the analysis is a matter of personal preference. For the SSTRAN, I prefer to use the series-equivalent circuit. The collector of the RF output transistor of the SSTRAN has about a 500 ohm to 1000 ohm optimum load resistance. In the SSTRAN design, a capacitor-input "L"-type impedance-matching network converts the 50-or-so ohm sum of the loading coil loss resistance and the ground resistance to the optimum load resistance of the final transmitter stage. The loading coil cancels out the 3000 ohm capacitive reactance of the antenna.
I prefer to use the parallel-equivalent circuit to analyze circuits like the Rangemaster, that use an RF stepup transformer at the output. The secondary winding of the stepup transformer acts like a loading coil, which cancels out the parallel capacitive reactance of the antenna. The transmitter then applies a high voltage to the high parallel-equivalent load resistance.
The series-equivalent circuit represents a "low" impedance to the transmitter, and the parallel-equivalent circuit represents a high impedance. Both of the equivalent circuits represents the same antenna; and so, whether you consider the electrically short antenna to have a high or a low impedance is entirely arbitrary, and depends upon how you are going to do the analysis.
50 ohms has no significance in Part 15 because transmission lines are not used, because their length is included in the 3 m length budget. Do not try to raise or lower the output impedance of your transmitter without thoroughly analyzing what effect the change will have.
Thank you ermi roos for your detailed comment on my impedance question. You convince me it would be inadvisable to start juggling impedances in the way I imagined doing.
There is a wishful psychology that creeps in with part 15: trying to squeeze an extra drop out of a tea spoon.
I'll be back with other fanciful ideas.
Folks,
I have read the previous posts here and it prompts a few comments.
First, for Carl, I did not claim an output impedance for the AM-25. I only stated that the maximum output power was obtained when the load resistance was 24 ohms resistive on my bench. The maximum power transfer theorem predicts that at "maximum power transfer" to the load the efficiency will be 50 %. I measured 32% but the source in this case was highly non-linear and conventional circuit theory goes out the window under these conditions.
Secondly, I do not understand what prompted Ermi to interject what he did. There is nothing technically wrong with his presentation but I fail to see the relevance. The transmitter will see what it sees regardless of the analysis tricks used; admittance vs. reactance, series equivalent vs. parallel equvalent and so on. One's view for analysis for calculation convenience doesn't change the physical reality.
Carl, according to my measurements, the AM-25 will find a satisfactory load with a resonant base coil loaded antenna regardless of whether it is a parallel or series equivalent circuit. As was claimed, a series resonant circuit is low Z and a parallel resonant circuit is high Z is true but the physical reality does not change simply due to the model the analyzer uses.
Here's my advice based on my experience with the AM-25. It will work very well with a base coil loaded antenna as it is built and very little if anything will be gained by trying to redesign the circuit. The circuit is almost insensitive to variations in antenna Z over a wide range which can be expected in practice. Carl, you mentioned that it worked well with the loaded antenna vs. a wire. Can you elaborate?
Carl, you ask thought provoking questions which usually elicit good discussions. Rock on!
Neil
There are claims that Part 15 AM transmitters have high impedance or low impedance loads. I tried to show that both claims are correct. Whether the transmitter has a high impedance or a low impedance load does not depend upon the physical facts, but on the method of analysis.
This is because of the complex impedance characteristics of electically-short antennas. Resonant antennas do not have this problem because they have a particular load resistance at resonance.
Neil, thank you for putting in (inputting) and here's what little elaboration I can make about Ramsey AM25 with A.) plain 3-meter vertical, or, B.) coil-loaded 3-meter vertical.
I count this as my "indoor" transmitter and use SStrans to reach outdoor area.
With plain 3-meter vertical the signal had background noise creeping through in the rooms farthest from the signal, and outdoors the signal was almost unreadable.
Adding the coil, there aren't that many windings for my 1550 KHz frequency, I'll have to count them, but now the signal is solid everywhere indoors and covers well outdoors to about 100' in all directions from the (brick) house.
For the first time today I grounded to the power ground and had no improvement whatsoever.
Now a question. If it's so irrelevant what the output impedance of the transmitter is, why does the SStran call for lowering the output impedance for use with an outdoor coil-loaded antenna?
Ermi,
Thanks, and I understand your point. I think what follows is an example.
Carl, as I understand this we are actually talking about two electrically different antenna systems when we compare the SSTRAN with the AM-25. Though the antennas and coils look identical they are operated differently.
Since we know (if my measurements are correct) that the AM-25 will drive a 20 to 100 ohm resistive load we tune the antenna/coil to resonance. This removes all but the resistive component of the load the transmitter sees which is typically in this resistance range.
According to the designer of the SSTRAN (PhilB) per his post elsewhere on this board, the collector of the ouput transistor of the SSTRAN needs about a 750 ohm resistive load. To get this, the output network has a variable cap. to ground and if the proper inductance is placed in series with the antenna then the antenna resistance is transformed up to 750 ohms. This inductance is provided by the loading coil when the antenna is operated slightly off resonance toward the inductive reactance side. Same antenna/coil, just slightly different tuning.
Neil