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Best low power, Part 15 compliant AM transmitter?

Many of our visitors are interested in experimenting with low power radio while staying within the limits of the law.  It's fun to broadcast to your neighborhood without having to look over your shoulder for that tinted-windowed FCC SUV heading down your block.
     As we have shared in previous posts, it's generally agreed that low power AM broadcasting under Part 15 has the best opportunity for useful legal range, due in part to the opportunities for experimentation under the "100 mW power input and 3 meter antenna" subpart in the regulations.
     Happily, there is a nice selection of certified Part 15 AM transmitters and Part 15 compliant (meaning the transmitter complies with the letter and spirit of Part 15 without being certified by the FCC), AM transmitter kits on the market today.
     Of course, quality and compliance can be expensive and kits require a significant investment in time and a certain skill level to complete, so, which AM transmitter is best?
     I think we can all agree that "best" means range - so, of the popular AM transmitters on the market today, which has the best range?  Sure, you can research online but all I've ever been able to find is a wide range of opinions and hyperbole and very few facts.
     Wouldn't it be great if an unbiased report were created, where sensitive measuring equipment was used by skilled operators to test a collection of transmitters in a straight up, head to head, identical conditions comparison where we could learn once and for all which transmitter would deliver the best bang for the bucks (or effort)?
     Enter the Hobbybroadcaster.net AM Transmitter Challenge, a Herculean effort undertaken by the talented and dedicated Bill DeFelice and a cadre of capable assistants who put in the time and, from the sounds of the report, no small amount of work in designing, coordinating and delivering the final word on legal low power Part 15 compliant AM transmitter range.
     So, which one won?  You'll need to visit Bill Defelice's most excellent web site dedicated to legal, low power radio experimentation, www.hobbybroadcaster.net, and find out for yourself.
     Check out The AM Transmitter Challenge on Hobbybroadcaster.net and start planning your Part 15 compliant operation today!
     A huge lowpowerradio bog "Thank You!" to Bill and his team for their selfless efforts in fianlly settling this contentious topic with a repeatable, fact based approach to range measurement.


David Chamberlain said...

There were problems with the AM transmitter challenge that Hobbybroadcaster refuses to admit, particularly with the testing of the AMT5000.

The tests were supposed to show which transmitter was the best, as delivered to a novice broadcaster. However, the AMT5000 is delivered as a kit, while all the others are assembled. The AMT5000 tested basically came from an unknown source, and had unknown build quality, while all the other transmitters came from either the manufacturer or distributor.

The operational documentation supplied with the AMT5000 is oriented towards the more technical user (after all, you supposedly had to build the kit to get to the point where you are installing it). The testers had difficulty with this documentation and they claimed that they could not get hold of the manufacturer for clarification. But instead of halting the testing until they DID get the answers, they forged ahead into the darkness, and the results are suspect, to say the least.

The AMT5000 really should not have been included in the shootout. While it did quite poorly in the final results (most likely because of the described testing issues), there is plenty of ancedotal evidence from others who have used it to suggest that it ranks right up there with the Rangemaster or ProCaster, if not better.

Hobbybroadcaster is supposedly doing a followup complete review of the transmitter (although it has been coming for quite a while now). It will be interesting to see what they say about it in that review.

Low Power Radio Guy said...

Hi David,

Thank you very much for stopping by and sharing your observations! If you have any performance data or other information you'd like to share, please let us know at low power radio at gmail dot com. We'd be happy to pass it along to our readers.

David Chamberlain said...

I have been involved with low power broadcasting for coming up to 10 years now - I launched my BETS-1 (the Canadian equivalent of Part 15) FM station in 2006 on Bowen Island, BC.

During that time, I have experimented with and used numerous AM and FM transmitters.

The folks at Hobbybroadcaster have to be commended for generally doing a good job in the AM Transmitter challenge, except for the problematic AMT5000 results. I have never used an AMT5000 myself, so I have no particular axe to grind, one way or another; but as I stated, others have reported exceptional results with proper installation.

Generally, my experiences with the other transmitters have matched up with the results of the Challenge. Unfortunately, the equipment required to actually measure field strength is very expensive, so I have had to rely on physical measurements for range, and qualitative measurements (i.e., my somewhat subjective ear) for sound quality.

I achieved the best range (and the best sound) from a Rangemaster, followed closely by the ProCaster, with the Talking House lagging considerably behind, both in terms of range and sound.

One transmitter that I have had great results with, and that was not considered in the Challenge, was the Talking Sign (likely because it is not currently in production) - this was the precursor to the ProCaster and made by the same folks. With an installation similar to the ProCaster, I was actually able to achieve better range (over a mile), although the sound quality was not as good.

I eventually ended up using FM, as Canadian unlicensed regulations are somewhat more liberal than those in the U.S. (AM regulations are almost identical). In Canada, the maximum field strength for a BETS-1 transmitter can be 100uv per meter at 30 meters. If you can get your FM antenna up high enough to negate the influence of ground reflection interfering with your signal, you can get significant range to a sensitive receiver; in my case on Bowen Island, the target audience was a ferry lineup (car radios can have sensitivity of under 1uv and they have great selectivity as well). My transmitter was located in a building at the top of a hill overlooking said ferry lineup, and I was able to achieve a range of up to 1km (just over half a mile).

That was a very specific application, however, and in general use, with much less sensitive receivers, AM gives you much more usable range.

HobbyBroadcaster said...

It stymies me that someone has has simply dabbled with low power broadcasting for a mere 10 years can somehow discern that we somehow “had difficulty with this documentation” or “forged ahead into the darkness“ as he was neither on-site nor one of several professional broadcast engineers in attendance during the testing of these AM transmitters - many of these engineers who have multiple decades under their belts engineering for both non-commercial as well as commercial FM and AM broadcast facilities.

As plainly disclosed in The AM Transmitter Challenge article, every tested transmitter was installed and aligned per the instructions supplied by their respective manufacturers - the very same instructional documentation a typical end-user and/or purchaser would receive and use in their own installations!

It was quite evident that the AMT5000’s marketing hype of touting “Ultra-high 98% Class E RF output transistor efficiency” does not necessarily translate into effectively being able to couple the resulting RF energy into an intentional radiator which is compliant with FCC Part 15.219(b)

Though commenter David freely admitted that he had “… never used an AMT5000 myself” it’s interesting in how he implied that these test\s as being flawed. There had been numerous years prior to both the creation of HobbyBroadcaster.net and our AM Transmitter Challenge where anyone could have acquired test samples in addition to the use of the $14,000 Potomac Instruments FIM-41 field intensity meter - the very same meter broadcast engineers as well as the FCC used to perform measurements of licensed broadcast stations.

HobbyBroadcaster.net made the investment of not only purchasing this meter but also bore the cost of having the meter factory serviced and calibrated. The transmitter challenge also employed the services of several other professional broadcast engineers from ML Media and ESPN Radio Networks to perform checks and balances to ensure procedural steps were uniform across all tested systems. An engineer from the Cox Media Group also assisted with some background info and another broadcaster assisted un in obtaining the open field utilized for the testing.

Bill DeFelice

David Chamberlain said...

It was not my intention to question the competency of Mr. DeFelice & his helpers (although he does a good job of it himself by focusing on qualifications as opposed to his testing mechanics).

What Mr. DeFelice conveniently doesn't mention in his reply is the subsequent discussion in his own Forum which analyzed the possible reasons behind the relatively poor performance of the AMT5000. Several respected professional engineers (if you care about that) led the analysis.

It was obvious that Mr. DeFelice really didn't understand the Class E operation that the AMT5000 utilizes (at least at the time of the testing), based on his questions.

The conclusions were that it was likely that the AMT5000 in the Shootout wasn't tuned correctly for the ground that was used, resulting in it not operating in Class E mode. Hence it's relatively poor performance.

The AMT5000 documentation which the testers used is pretty skimpy in its tuning instructions, and not very precise (rotate a control for maximum output and back it off slightly). It works in the majority of cases. However, it turns out that the ONLY way you can determine if you are really operating in Class E mode is to use an oscilloscope.

It's interesting that, for all the tester's vaunted experience, and their highly touted expensive test equipment, they didn't see fit to attach an inexpensive oscilloscope to the AMT5000 to investigate results that were unusual and unexpected.

Any scientist (or professional engineer) worth their salt would not have blindly accepted these results verbatim, and then published them without comment, in a deliberately misleading manner.

Other than comparing qualifications (I'm great, my testers were great, you're not), Mr. DeFelice has blamed SSTran for the poor tuning documentation (not wanting to leave a blame stone unturned). So did the discussions over at the Hobbybroadcaster website, actually. I happen to concur. The documentation doesn't take into consideration the wide variation of grounds that may be used with a Part 15 transmitter.

That is definitely a CON for the AMT5000. So you mention it in the write up. But that shouldn't be the reason for not doing everything within your power to determine how good the transmitter CAN be. Publish both good and bad results. Nothing should be hidden.

I find it rather sad that Mr. DeFelice refuses to learn anything from this experience. Even the best of us make mistakes, but the trick is to not make the same ones again.

HobbyBroadcaster said...

While Mr. Chamberlain continues ad nauseum with back handed comments directed toward me in addition to allegation about so-called “testing mechanics” his limited understanding of the entire purpose of The AM Transmitter Challenge is most apparent.

The AM Transmitter Challenge was orchestrated not only as a performance comparison of each transmitter installed in an identical Part 15.219 compliant manner, but to demonstrate the operating characteristics that a typical end user would achieve by following each manufacturer’s supplied documentation.

It has been witnessed that the vast majority of Part 15 radio enthusiasts are merely appliance operators, having limited to no comprehension of basic RF principals - let alone any background of Class E amplification in a micropower RF circuit. Perhaps this is relevant to David’s own limited understanding.

It is doubtful a Part 15 radio appliance operator would have an oscilloscope at hand to adjust this transmitter. In addition, the manufacturer’s supplied documentation makes no assertion that an oscilloscope is required to achieve the promised performance benefit, rather that only a screwdriver and a voltmeter as the only prerequisites outlined by the manufacturer.

As mentioned in the dedicated product review for this transmitter appearing on my site the promise of Class E operation was debunked by the author of “Class E Amplifiers and their Modulation Behaviour,” Dr. David P. Kimber. Kimber stated in no uncertain terms that it was doubtful the device in question actually operated in genuine Class E mode.

The AM Transmitter Challenge was well received, not only by my broadcast engineering peers but also by the amateur radio community at large. I received a great deal of positive feedback received from many licensed ham radio operators after the publication of the Challenge in amateur radio publication CQ Extra. I haven’t received any negative feedback from the amateur radio community, whose proficiency in RF is multiple times beyond that of Mr. Chamberlain.

I also find it entertaining that Mr. Chamberlain is somehow privy to my background in spite of his own lack of experience of ever having been employed in the broadcast engineering industry. His additional admission of never operated or owning the transmitter in question makes his own statements purely speculative since he is neither familiar with me, the additional engineering talent, the transmitter or the conditions that the field testing was performed under.

If Mr. Chamberlain feels he is vastly more qualified than my group of broadcast engineers I’d welcome him to provide his comprehensive test platform, companion measurement data and a procedural outline for comparison to that which was undertaken by our team nearly four years ago.

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