For those of you interested in the performance of Part 15 compliant low power AM transmitters HobbyBroadcaster.net had performed a competitive head-to-head shootout of these transmitters in the AM Transmitter Challenge.
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The AM Transmitter Challenge has arrived
- Thread starter RadioMan203
- Start date
The AMT-5000 is said to be a Class E transmitter, but the data given in the AM Transmitter Challenge demonstrates that this transmitter does not actually operate Class E; at least not when it is tuned according to the manufacturer's instructions. In a real Class E transmitter, after input power is set to 100 mW following the tuning of the tank circuit, maximum field strength does not occur after tuning the tank circuit for peak output power.
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Sorry: for some reason the comment in my previous post was cut short. What I wanted to add is that, since the AMT-5000 was not operating with the high efficency that the Class E mode can provide, it compared poorly compared to other transmitters tested in the AM Transmitter Challenge.
@Ermi: It had been mentioned to me that my use of a single ground rod for the challenge provided some disadvantage to the AMT5000. My thought was if there had been some malady with the earth ground that all test specimen would have been equally affected and not isolated to just one particular transmitter.
Since I had been unable to establish any contact with the designer / manufacturer of the AMT5000 all we had to go by was the provided instructions - the very same method a typical end user would both construct and operate this device. As an alternate comparison we also tried tuning the transmitter for peak signal strength by disregarding the supplied instructions and strictly using the Potomac FIM-41 for the measurement. Performance improved to some limited extent but not remarkably when compared to the other tested transmitters.
Since I had been unable to establish any contact with the designer / manufacturer of the AMT5000 all we had to go by was the provided instructions - the very same method a typical end user would both construct and operate this device. As an alternate comparison we also tried tuning the transmitter for peak signal strength by disregarding the supplied instructions and strictly using the Potomac FIM-41 for the measurement. Performance improved to some limited extent but not remarkably when compared to the other tested transmitters.
Bill,
You did everything correctly. You followed the manufacturer's instructions, which is all that you needed to do for your tests to be valid. Unfortunately, the manufacturer did not tell you (and the other users of his product) in the instruction manual anything about needing to select the drain-to-source capacitance of Q1. The manufacturer obviously knows about the need to change the value of drain-to-source capacitance for proper tuning because he has provided jumper-selectable options for setting this capacitance. The problem is that he did not tell you what to do to make the proper selection. Thus, the tuning was incorrect through no fault of yours.
If you really want to do the proper tuning, you would have to find a Class E tutorial on the Internet, or in a technical library.
In general, the higher the ground resistance of the transmitter-antenna setup, the lower the drain-to-surce capacitance must be. There is a limit to how low you can go in capacitance because Q1 has its own self capacitance in parallel with the external capacitors. If you cannot get the capacitance low enough, then ground radials have to be used to lower the ground resistance.
Since the transmitter will function to some extent even if it is not tuned properly, the manufacturer may have neglected to put the full explanation in the instruction manual because he thought it would just confuse his customers. I'm only guessing about that, of course.
You did everything correctly. You followed the manufacturer's instructions, which is all that you needed to do for your tests to be valid. Unfortunately, the manufacturer did not tell you (and the other users of his product) in the instruction manual anything about needing to select the drain-to-source capacitance of Q1. The manufacturer obviously knows about the need to change the value of drain-to-source capacitance for proper tuning because he has provided jumper-selectable options for setting this capacitance. The problem is that he did not tell you what to do to make the proper selection. Thus, the tuning was incorrect through no fault of yours.
If you really want to do the proper tuning, you would have to find a Class E tutorial on the Internet, or in a technical library.
In general, the higher the ground resistance of the transmitter-antenna setup, the lower the drain-to-surce capacitance must be. There is a limit to how low you can go in capacitance because Q1 has its own self capacitance in parallel with the external capacitors. If you cannot get the capacitance low enough, then ground radials have to be used to lower the ground resistance.
Since the transmitter will function to some extent even if it is not tuned properly, the manufacturer may have neglected to put the full explanation in the instruction manual because he thought it would just confuse his customers. I'm only guessing about that, of course.
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This website has a "hurry-up and type!" program that discourages long-winded post, forcing me to reply in two parts. Your tuning for peak output power was a good test for seeing if the AMT-5000 is really operating Class E. In a true Class E amplifier, peak efficiency does not occur when the tank circuit is tuned for peak output power. Since you got peak efficiency when output power was peaked, this is proof that the AMT-5000 was not operating class E during your test.
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There's a couple things I'm curious about ...
How does the performance of these compare to various $5-30 hobby kits, like the radio shack ones targeted at younger school-age children, or various online plans touting put these parts totaling $1-5 together and you have a transmitter, for example? I'm expecting the units tested in the challenge would blow away the budget ones, but by how much? Assuming you're using the same antenna, if the carrier from the kids' hobby kit is barely detectable on a PL-660 or ICF-2010 in SSB mode, would the better kits, like the SSTran or the Rangemaster be overloading a typical cheap shirt-pocket portable at the same distance?
And, how much of a difference does a ground system make? Some of us don't have room to install a full array of 120 1/4-wave radials at 540 kHz, or even 1700 kHz (not to mention both of those frequencies are occupied here). How much of a performance hit would I expect from, say, 8 radials each 6 feet long laid out on the ground, or even just no radials at all, just sitting on a table or something like that?
How does the performance of these compare to various $5-30 hobby kits, like the radio shack ones targeted at younger school-age children, or various online plans touting put these parts totaling $1-5 together and you have a transmitter, for example? I'm expecting the units tested in the challenge would blow away the budget ones, but by how much? Assuming you're using the same antenna, if the carrier from the kids' hobby kit is barely detectable on a PL-660 or ICF-2010 in SSB mode, would the better kits, like the SSTran or the Rangemaster be overloading a typical cheap shirt-pocket portable at the same distance?
And, how much of a difference does a ground system make? Some of us don't have room to install a full array of 120 1/4-wave radials at 540 kHz, or even 1700 kHz (not to mention both of those frequencies are occupied here). How much of a performance hit would I expect from, say, 8 radials each 6 feet long laid out on the ground, or even just no radials at all, just sitting on a table or something like that?
A lot.
RF current cannot flow along a monopole radiator if the transmit system has no path for that current to/from the r-f ground plane against which it is driven.
I actually detailed this same exact question and a followup elsewhere.
Would you mind posting here the URLs containing that same exact question, and your followup to it ?
Bill,
If your ground resistance was really poor in your Part 15 AM Transmitter Challenge test setup, as you suspect, the ground resistance would have been very high. Your reason for not reporting the field strength was that the high ground resistance due to low ground plane conductivity would have affected all of the tested transmitters proportionately, but that is not the case with the AMT-5000, because in a class E power amplifier, the load resistance is actually a component in the tuning of the transmitter. With the AMT-5000, the ground resistance is the load resistance of the power amplifier. If the actual load resistance is not the same as the design load resistance, the drain-to-source capacitance has to be changed to retune the transmitter. The instructions for the AMT-5000 do not tell the user how to select the proper drain-to-source capacitance.
If your ground resistance was really poor in your Part 15 AM Transmitter Challenge test setup, as you suspect, the ground resistance would have been very high. Your reason for not reporting the field strength was that the high ground resistance due to low ground plane conductivity would have affected all of the tested transmitters proportionately, but that is not the case with the AMT-5000, because in a class E power amplifier, the load resistance is actually a component in the tuning of the transmitter. With the AMT-5000, the ground resistance is the load resistance of the power amplifier. If the actual load resistance is not the same as the design load resistance, the drain-to-source capacitance has to be changed to retune the transmitter. The instructions for the AMT-5000 do not tell the user how to select the proper drain-to-source capacitance.
I had been thinking this for various reasons including the FCC chart data as from what another engineer had mentioned regarding their three tower directional array. This was the reason I opted not to report actual field strength but to use a relative scale from the transmitter with the highest field strength to the lowest.
While I don't have the manual that accompanied the loaned AMT5000 in front of me I do have a paper copy of the Sept 2011 version of the manual given to me by somebody who disposed of theirs. I had perused the manual a couple of times and unless it's been updated in a subsequent revision the only settings I see referenced is in the tables for "toroid only inductor" operation spanning pages 37 and 38. There is also only a brief mention of C7 / C8 in the RF Output portion of the Theory of Operation with no details on ascertaining the proper settings for their implementation. The person performing the tuning of all transmitters in the transmitter challenge is a highly experienced commercial RF engineer and ham radio operator so I believe he was familiar with the requirements for the circuit as defined by the manufacturer. I do recall this particular transmitter requiring the most amount of time to tune properly of all the specimen tested that day.
This particular manufacturer chose to ignore both email and postal mail communications in spite of my extending the olive branch and requesting their participation. Their involvement in the challenge may have addressed unanswered questions for prospective buyers in addition to providing improved clarity to their supplied instructions. Besides, all configuration was performed by using the documentation supplied by each manufacturer and to operate in compliance with the maximum allow input power as defined under Part 15.219.
There are notes in the instructions about selecting "Enable Capacitor C8" which is the default capacitor and has 470 pF, and "Enable Capacitor C7" which has 1000 pF: both would be connected from drain to source at Q1; and there is the bland note appended to both saying that they are "for optimizing Q1 voltage and current waveforms for Class E mode." The operative term is "waveforms," and how can you see a waveform? Not with a DMM, but with a Scope! The manufacturer is, in effect, telling his hapless customer, who probably does not have advanced test equipment, "get a scope." If the customer ony has a DMM, that's his tough luck. Also, even if he has a scope, the manufacturer does not tell his customer how to use it to tune the transmitter.
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I have seen spectrum analyzers mentioned as instruments that can be used for tuning the AMT-5000. Actually, a spectrum analyzer would not be useful for tuning a Class E amplifier, but it can be very valuable once the tuning is complete. It can find the amplitudes of the second and higher order harmonics to avoid interference to other services above the AM Broadcast Band.
Other than that brief mention there's no methodology offered on how to approach advanced tuning by altering these values for a given situation. Somebody unfamiliar with the product will take the manual as gospel and set things according to it. Some may actually venture down the less traveled highway and try experimenting but little concrete information is offered in regards to the switch settings for C7 and C8. Granted, my documentation may be older and it could have possibly been added to a subsequent revision of the manual but what about the other owners of said product? Will they ever find out or will they be kept in the dark?
I can't figure out why somebody would feel a spectrum analyzer would be beneficial in tuning the AMT5000. Being able to analyze the over-the-air signal once tuned is one thing, but for the actual tuning I would think an o'scope and DMM would be much more practical.
I would have liked to review the manual. Most manufacturers have a downloadable "read before you buy." Sometimes that's the clincher for me.
@pascoradio: I believe the only manuals not available to download are those from SSTran. The other four companies, Chezradio, Hamilton, Radio Systems and Wayne Miller Associates (Grain Industries) allow you to download the manual directly from their sites.
On another site, the manufacturer posted to double down on previous claims, and makes a new claim that, even if the output resistance is increased from what he says is the ideal load resistance (30 ohms) to 100 ohms, the so-called "transistor efficiency" is reduced from the previously claimed 98% to a still high 88%. If that were the case, the AMT-5000 would have beat the Rangemaster in the AM Transmitter Challenge without a problem. The manufacturer made this claim in friendly territory where a number of posters on the site were inclined to believe that the results of the Challenge test were falsified, or just plain wrong because of experimental error.
The post, complete with a graph, and and a misrepresented quote, is an example of what I call a "paper blizzard." It dazzled or confused the reader, but did not enlighten him, because, IMHO, impressing the reader, not providing information, was its purpose.
The post, complete with a graph, and and a misrepresented quote, is an example of what I call a "paper blizzard." It dazzled or confused the reader, but did not enlighten him, because, IMHO, impressing the reader, not providing information, was its purpose.
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Please post here the proof you have to support your conclusions, Mr. Roos.
Probably a lot of readers here would like to form their own opinions about the assertions you have posted in this thread.
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