Of course, the transfer efficiency to the antenna (overall transmitter efficiency), does vary widely with ground resistance because it is a function of the ratio of the fixed coil loss resistance and the load resistance, which are in series. But, this is independent of the class of the amplifier.
Other factors affect the performance of Part 15 AM, too. The link below develops this for various radiator ODs and system r-f bandwidths.
Knowing how any particular system exactly fits in this table likely will be impossible, but the trends shown there are still important.
You know something, your feeble efforts here to discredit someone with far more knowledge and experience than you ever will have will not work.
Tell me something, why have you yourself never tested that thing on an outdoor system and publish those results? Why have you been hiding for months at a time and pop in with the same BS routine over the last 2 years?
Are you that desperate for sales or something?
I suggest taking on a new hobby.
I'm done with you.
RFB
carl try and use your SA to tune up amt5k by F/S rather than current.
i id mine this way and found as the F/S went up the current went down.
also the course tap on toroid that gives highest f/s reading is not the correct tap it is one or two before that as you wont obtain a peak using the cap when you have the highest f/s tap on toroid selected. don't worry after you find correct tap and tune for peak on the cap you will find the f/s much higher than the highest f/s tap on toroid and much higher than the current measurement method.
after all that then you tune the input power as you normally would do according to instructions.
this is a very good transmitter design and i am very happy with mine and would recommend this unit to others. i am in no way downing this transmitter, just documenting it's quirks.
For those who are interested in the deep technical details of a high efficiency transmitter I offer some information.
When I designed the "high efficiency" transmitter which I use here I did both simulations and bench testing. The results support Phil's statement regarding the stability of the efficiency with varying antenna resistance. The simulation, for example, predicted an efficiency of 96% and the measured efficiency was 86%. The discrepancy is due to the output filter I used (not used in the AMT5000) and when the loss in the filter was included in the simulation the predicted efficiency agreed with that observed.
The simulation of the final RF current and voltage waveforms agreed precisely with what was observed using an oscilloscope which gives credibility to the simulation which was done with LTSpice (free for download from Linear Technologies). Both the simulation and lab bench testing showed the same variation of efficiency with load R which is a change from 86% at 29 ohms load to 82% at 109 ohms load.
What is observed when the load R is changed is that going from 29 ohms to 109 ohms lowers the input power to the final and thus lowers the output power. It could appear that this decrease in output is due to lower efficiency but this is not the case since if the input power is reset to 100 mW the output power remains constant withing a few mW. There is a slight change in efficiency but it is small.
This, with the information I posted earlier about a "real world" outdoor installation, should support the conclusion that stable operation in terms of efficiency is possible and should be expected from these types of transmitters.
Antenna resonance and ground resistance are the driving factors for maintaining field strength and experience here has shown resonance is stable and ground resistance varies with soil moisture. Change in soil moisture is the dominant influence on field strength and range at my installation.-
Neil
Hi PhilB, your personal retort to RFB is unprofessional and detracts from your informative post above. Please refrain. Let your information stand for itself.
one thing that might have helped the design of the 5000 is to isolate the different RF stages and the final rf stage from the antenna tuning network.
you lose some over all efficiency but gain stability. also the toroid should be somewhat low loss design but also be a slightly lower Q.
just some improvement ideas for Rev.2 of the AMT5000.
this is meant to be constructive criticism to help phil b improve upon his design. yes it will likely raise the kit cost by 50-100 but i would pay more for a better design.
i would also like to see phil take his profits and reinvest in a better tx design and after a while maybe make a "FCC Certified Kit" version of the amt5000.
the THII ver5 offers a very good building block design where phil could combine his Class E design with the THII design.
again phill b take this as constructive and meant to help you improve on an already great transmitter.
i have had this thing compare with the range on the hamilton am1k for the same installation scenarios.
you also may want to try a entirely balanced push pull design as well from modulator stage to final rf stage in addition to above ideas.
maybe design it to load a 10ft wire loop as well for a no ground installation.
Robert,
Improvement suggestions are always welcome here and I take your suggestions as constructive. Let me comment on them individually:
"isolate the different RF stages and the final RF stage from the antenna tuning network" I don't have a clear picture of what you mean here. Can you clarify a bit and tell me more about what the gaol would be?
"the toroid should be somewhat low loss design but also be a slightly lower Q". The Q of the toroid isn't "super-high" or anything out of the ordinary. I is probably similar to the AM1000 and is similar to the 3" external loading coil used for the AMT3000, which is similar to the one Neil, Radio8z has been reporting about lately. I do think it is higher than the ferrite slug-tuned coils in the TH just from general coil characteristics, but I have no data to substantiate that. Regardless, the sensitivity to body capacitance in close proximity to the antenna is a characteristic common to all part 15 AM transmitters. Now, if a super high Q coil were used, it may not be desirable due to extreme sensitivity to antenna capacitance variations and a drastic bandwidth reduction. I have not seen any part 15 transmitters with such a coil.
"try a entirely balanced push pull design as well from modulator stage to final rf stage". That's a configuration used in the TH and in at least one other older transmitter now out of production. It's a decent enough design, but not quite as good as a Class E design. I think that would be a step backward.
"maybe design it to load a 10ft wire loop as well for a no ground installation". Loop antennas are excellent for receiving due to noise immunity and directionality, but they are not very effective for transmitting. There are some hams that have demonstrated contacts with loop transmitter antennas, but they are running higher power at higher frequencies. I don't think 100 mW would go too far on a 10 ft loop in the BC band. I would like to hear of any successes though. The AMT5000 has an on board jumper selection to bypass the internal tuning circuit, so experimentation with external tuning circuits is possible.
"FCC Certified Kit version". Although the FCC does allow for certifying kits, none have ever been certified. The certification requirement only applies to manufacturers of ready to use transmitters. They must be certified or the manufacturer is liable for a fine. Kits are not required to be certified. As far as the user is concerned, transmitter certification isn't a requirement. The FCC mandates that the user is responsible for compliance with rules regardless of what transmitter is used. I believe the great WWW has shown that all well known AM kit transmitters are compliant. It would be an unnecessary price adder to get a kit certified. Certification costs about $15,000 to $20,000 plus a lot of engineering time to babysit the certification lab. This is a niche market, so the price adder would be more than just a few bucks for each unit.
KC8GPD, I recall that you have AMT5000 Serial Number 1, and have often spoken very highly about the AMT5000.
But every once in awhile you seem to criticize the AMT5000.
This is very confusing.