Often times adult men who befriend juveniles were outcasts when they themselves were young, and possibly bullied at the same time. Now, as a big person, such a man can join a chorus of young bullies and simply inject the radio hobby as an element of contention.
Completing a project like The Challenge resembles school assignments that may have been marked F and returned with constructive criticism, and today any criticism received for the Challenge is a big reminder of those unpleasant school days.
Learning handicaps are a large problem in society, but it's almost impossible to teach someone who resents instructive feedback.
Be that as it may, I give the Challenge a C, and suggest that as with any project, we can learn retrospectively how to improve any future attempt.
Posters on another website are discussing the next round of tests in the Challenge. The poor earth conductivity at that test site was acknowledged, and the fact that the r-f resistance of the ground path through a single ground rod driven into that earth would be high.
The reason for the single ground rod was given as a requirement of the property owner to use that site with minimal change to it. The single ground rod also was justified as useful in testing the transmitters under "worst case" conditions. That data set has been taken and published.
NEC4.2 shows that, for earth conductivity of 1 mS/m d.c.5 as might be predictable for that test site, r-f ground resistance would be greater than 200 ohms. A loading coil wound on a ferrite core would add 15-20 ohms to the resonant load resistance driven by the transmitter.
There is another configuration for this test that would reduce the resistive loss in the r-f ground path, and would not cause damage to the the test site. That lower ground loss would be more typical of what a Part 15 AM operator would have when using a single buried ground rod in "average" earth conductivity typical of much of the continental US.
For example, NEC4 shows that a set of six, evenly-spaced, 6-meter horizontal radial wires just lying on the surface of 1 mS/m d.c. 5 earth, and centered at the base of a 3-meter vertical monopole has an r-f resistance of about 40 ohms.* This is also about the r-f resistance of a single 3-meter ground rod driven into 8 mS/m, d.c. 13 earth (no radials).
This ~40 ohm r-f ground resistance plus the resistance of the loading coil would produce a resonant load resistance of ~57 ohms for the transmitter, which would be more typical of an average installation, and make for a more useful comparison of the transmitters in the Challenge.
*That same set of radials would have an r-f loss of about 5 ohms when lying on (or buried a few inches in) 8 mS/m, d.c. 13 earth.
All of these comments apply to the 1640 kHz frequency used in the Challenge.
The ground radials laid on the surface of the ground, as suggested by Rich(F), is an excellent suggestion for the possible second challenge at another web site. It is excellent by virtue of the increase in ground impedance efficiency as detailed by Rich(F), and the non-invasive protection of the earth surface at the location.
I very much would like to see this method adopted in any future transmitter comparison.
Yep...all anyone has to do take a gander at just about any lf/mf am transmitting tower (with the exception of roof or other high level mounting systems which share tower and ground) and find that most of them use a center-band 1/4 wave length active tower with, usually, 120 @ 1/4 wave buried copperstrap ground radials, often bonded to rods sunk to bedrock, and tunable loading coils.
Part 15 micro stations that work well are usually based on the same general model, but scaled down to meet Part 15 specs.
There's too much ego and not enough engineering and science involved with those guys doing the Challenge. There's a continuing discussion about the AMT5000 is that other (effectively) private Forum. When one individual who's been particularly critical of the 5000 and its tuning stated that he was glad that they were going to retest it under more favorable conditions (and hopefully tuned correctly), the other guy came back with (and I'm paraphrasing) - it's already been tested and the results published, I'll get around to it sometime, it's not a priority. He obviously can't admit that they messed it up the first time. Frankly, as far as I'm concerned that attitude taints everything they've done in the past, and everything they plan to do in the future.
For hobby fun I enjoy making a sociological study of comments and tactics over at the other place, reading the publicly available portion of what gets posted.
A lot of forum space has been devoted to negative attacks of two main targets: PhilB/AMT5000, and posters here at what they smirckingly call "the kiddie site" as we've written reviews of stuff said over there.
Running the AMT5000 into the ground, pun as it is, began in about April 2011, just after the transmitter was introduced, with Billy Boss and Ernie Ross being the jeer leaders and a chorus of loyal members supplying agreement.
Ernie Ross said something in July 2012 which would be of great interest to the AMT5000 community, when he declared that he had "designed and built his own 'efficiency meter'" for his own improvements to AMT5000 circuitry. This is notable for not actually having an AMT5000.
Too bad Ernie never shared the circuit for his improvement, but then he also said, "...but the true peak is hard to find," which is easy to comprehend since finding the peak of a transmitter you don't have can be difficult.
Apparently now, the leader of the "Part 15 AM Challenge" is considering using dummy loads for future tests in this Challenge instead of a system compliant with Part 15.219 using an intentional radiator. On his website that leader has asked for advice about doing so.
This is interesting in that this approach will require a completely different set of test equipment and operator comprehension/skills to accurately measure the r-f voltage or r-f current at the feedpoint of that load, before the modulation characteristics of that waveform can accurately be measured when driving that load -- by another set of calibrated instruments.
Knowing the r-f power dissipated in that (resonant) load also requires accurate knowledge of its feedpoint resistance at the operating frequency. Still different instrumentation is needed to determine that.
Nevertheless, scientific protocol requires that all measurements for a given system are best taken and reported for the same set of operating conditions.
One good suggestion has come out of the discussion 'over there' - that the AMT5000 include some sort of auto-tuning capability, to eliminate the unknowns involved in tuning a Class E amplifier (such as ground resistance). I have no idea how that would be done, or even if it would be possible. But it would go a long way to eliminating most of the criticism (right or wrong) thrown at the transmitter.
And I still don't understand what the problem is with the Challenge team to get the best possible results out of all the transmitters. The test wasn't called The Best Transmitter For Newbies, although that's what it ended up being (or, more accurately, the best transmitter for those who think they know how newbie's minds work, i.e. they won't do their homework and investigate further if they have problems with the documentation).
There have also been comments about the feasibility of using the AMT5000 if, in some cases, you need to use a scope to tune it properly (i.e., where there is poor ground conductivity, and/or poor grounds in general).
I can't speak for anyone else, but I would be willing to go out of my way to tune this transmitter, even if it was difficult and required a scope, if I could get a significant increase in field strength (and therefore, range), i.e. a TRUE Challenge, best against best. But we're obviously not going to get any answers to this, at least from that direction.
So the question is: What is the feedoint impedance of a 3 meter monipole fed at its base with a radial ground system? Rich: You may have already answered this in other posts.
What is the feedoint impedance of a 3 meter monipole fed at its base with a radial ground system?
From post #183 of this thread...
For example, NEC4 shows that a set of six, evenly-spaced, 6-meter horizontal radial wires just lying on the surface of 1 mS/m d.c. 5 earth, and centered at the base of a 3-meter vertical monopole has an r-f resistance of about 40 ohms. This is also about the r-f resistance of a single 3-meter ground rod driven into 8 mS/m, d.c. 13 earth (no radials).
This ~40 ohm r-f ground resistance plus the resistance of the loading coil would produce a resonant load resistance of ~57 ohms for the transmitter, which would be more typical of an average installation, and make for a more useful comparison of the transmitters in the Challenge.
All of these comments apply to the 1640 kHz frequency used in the Challenge.
Artisan, I think you may inadvertantly be spreading some of the myths that have infected the anti-AMT5000 postings that challengers have placed on numerous forums.
A serious AMT5000 owner is well served by the manual as it is, but the real problem is that the publication contains a lot of information and requires patience to read and understand, because it is technically very thorough.
The section on RF tuning alone extends for several pages and explains many variables which must be explored and understood by the user. The board jumper options allow adjustment for a range of preferences, the lengthy section on Ground covers a whole palette of methods with emphasis on achieving a good, low impedance, ground, and the Theory of Operation gives an informative portrait of the RF Power Output and everything involved with achieving Class-E Operation.
An oscilloscope is not necessary to properly tune the transmitter to a 3-meter antenna, but is simply an option some users might elect to employ.
I would agree that the AMT5000 may not be the best choice for the "newbie," unless he/she is the type of person interested in learning more about radio technology.
Actually knowing the feedpoint impedance at the antenna is a superflous distraction brought in to confound the potential user, because "knowing" this fact is only of academic interest and does not matter so long as that number is in the low impedance range as per the manual.
Rich: So are you saying that a 50 ohm non-inductive resistor might be an appropriate load?
... a 50 ohm non-inductive resistor might be an appropriate load?
No, because it would not duplicate the capacitance that a 3-meter vertical monopole has to the earth.
Therefore no loading coil would be needed or could be used in bench-testing the transmitters, which is not the configuration in which they will be used when operating under §15.219.
I'm certainly not anti-AMT5000. But there is no doubt that the Challenge guys got 'caught', so to speak, with their likely high impedance ground.
I also have no doubt that if you have a good ground, with a relatively low impedance, then the 'trick' described in the documentation and by Phil here works fine. Obviously, that was not the case in the Challenge testing with the 5000.
The biggest mistake the Challenge made was in not pursuing the issues that they had further, and instead blaming their poor results on the manufacturer, when it was their problem to begin with. Egos and excuses, a poor combination.
However, and this is a big however, I also do believe that while the AMT5000 holds great promise to improve Part 15 AM range, you really do need to tune it with an oscilloscope in SOME cases, if nothing else to prove to yourself that it is operating in Class E mode.
I don't think that this invalidates it in any way shape or form - and like I said previously, if I can improve my range significantly by doing it, then I (and I'm sure lots others) will.
The Radio Talk Administrator, after watching the blaming and naming taking place on his website, posted this generous and responsive message:
"Guys, if it will help, I'd be willing to set up and compare transmitters as you seem to need done. I do have a calibrated FIM as well as several places to mount and test the transmitters."
The one and only responder was Fob Martin, who dismissed the offer by saying, "You could do the testing, but it won't be the same cause of the location ground differences."
Albert Einstein said, "Imagination is more important than knowledge." Now we know what he really meant.