The following pictures show the voltage waveforms at the MOSFET drain terminal (test point T4 on the circuit board) for various settings of the tuning trimmer capacitor. Freq = 1550kHz, Antenna Capacitance=30pF, Rrad+Rgndloss=33ohms. "PWR IN ADJUST" control unchanged. It was set for 100mW for the 3rd picture below, but was left unchanged from that setting for all the others.
I am hoping that this will help convince everyone that an oscilloscope is NOT required for tuning. Instead, simply adjust the coil tap jumper and trimmer for maximum input current, then adjust the trimmer 1 full turn clockwise, and then adjust the "PWR IN ADJUST" control for 100mW input power. This simple trimmer "trick" works very well over the 1350kHz - 1700kHz frequency range of the toroid coil and an antenna ground loss resistance range of 10ohms - 75ohms.
Notice particularly that the "Power In" is maximum when tuned for max power in (first picture), but the efficiency is only 78%. When tuned to maximum efficiency, the Power In drops, but that drop is compensated for by the final tuning step where you set the power in back to 100mW. The radiated signal power is maximum at the highest efficiency tuning point and 100mW input power. All other tunings with legal Power In will result in lower radiated signal power.
If you want to use an oscilloscope, connect the probe to test point T4 (MOSFET drain) and connect the scope probe ground to test point T1 (ground). Maintain the scope trigger for a stable display. Adjust the coil tap jumper and trimmer to get maximum input power on your voltmeter. The scope should show the first waveform below. Then adjust the trimmer to approximate the third picture below. Then adjust the "PWR INPUT ADJ" control for 100mW.
Tuning trimmer set for maximum input current meter display
Power In = 0.140W
MOSFET drain efficiency = 78%
Tuning trimmer set .42 turn clockwise (+.3pF)
Power In = 0.131W
MOSFET drain efficiency = 93%
Tuning trimmer set 1 turn clockwise (+.7pF)
Power In = 0.100W
MOSFET drain efficiency = 99%
Tuning trimmer set 1.4 turns clockwise (+1.0pF)
Power In = 0.058W
MOSFET drain efficiency = 99%
Tuning trimmer set 2 turns clockwise (+1.4pF)
Power In = 0.037W
MOSFET drain efficiency = 94%
Tuning trimmer set 3 turns clockwise (+2.1pF)
Power In = 0.028W
MOSFET drain efficiency = 60%
Some may expect that the full 99 mW of output power from adjustment per waveform 3 of PhilB's post above will be radiated by the antenna system, but that isn't the case. There are losses in the loading coil and r-f ground connection that consume a high percentage of the available power, no matter what it is.
Below is a spreadsheet adjusted to duplicate the "1550kHz Frequency, Antenna Capacitance=30pF, Rrad+Rgndloss=33ohm" conditions described by PhilB. The r-f loss resistance of the internal loading coil in the AMT5000 on this frequency was estimated (by me) to be 16 ohms.
Given these conditions, the radiation efficiency of the antenna system (including the loading coil) is 0.2%, and it radiates about 0.2 mW of the 99 mW output power of the MOSFET final r-f amplifier.
Note that the OD of a 3-m monopole antenna needs to be about 0.33" in order for its capacitance to earth to be 30 pF on 1550 kHz. This OD is more typical of an external "whip" antenna than a flexible wire antenna (stretched out straight, and in the vertical plane). This suggests that the internal loading coil in the AMT5000 is compatible with driving a ~3-m whip antenna at its base, without bypassing the AMT5000 coil.
If an external coil is available for the base of the 3-m whip that will resonate it at the operating frequency with less loss than the coil in the AMT5000, the AMT5000 coil could be bypassed and system efficiency would improve.
A resistance of 5 ohms in an external coil would increase the radiated power from the system in this spreadsheet from 0.2016 mW to 0.26 mW = 1.29 times, approx, and field intensity at every distance would increase by SQRT(1.29), or a bit over 13%.
A larger image of the spreadsheet is available by right-clicking on that image, and selecting "View Image," (at least in Firefox).
Rich,
I started this thread to focus on the subject of Class E tuning, the object of which is to minimize power dissipated by the RF transistor. The transistor dissipates only a couple mW allowing the remaining portion of the 100mW input to flow to the loading coil and antenna. Losses in the loading coil and antenna are a fact of life that are common to all Part 15 transmitters.
Neil and I had a short discussion about losses recently in the following posts in another thread:
Class E Efficiency, Class E Efficiency, and Class E Efficiency Measurement
Phil, what was the test setup that produced these pictures?
Phil has posted very good information regarding tuning the Class E transmitter. The "magic" happens because the current through the final transistor is zero when the voltage across is not zero and the voltage across is zero when the current is not zero. Since the power lost in the transistor is the product of instantaneous Volts and Amps, with one of them always zero the power loss is zero giving the theoretical efficiency of 100%.
Phil, it would be great, if you can, to illustrate this with a plot of both the vds and id, perhaps by means of a simulation since measuring the id is difficult without disturbing the circuit.
Rich's post about losses is correct but let's try not to drift too far off the "Class E tuning" topic.
Neil
I have disbursed the wife's beer money to invest in the AMT5000.
I have two transmitter systems running at the present time, one of them an AMT5000, the other an AMT3000.
Both of them have been attatched to Wintennas, the name I give to my technique of loading metal window frames as antennas, extended at the top and bottom with wires to equal the allowed 3-meter length per 15.219.
Grounding for the AMT5000 is a north-south two-radial buried wire, and the AMT3000 attaches to a solid earth ground and is resonated by way of a triangular loading coil per special instructions given at sstran.com.
What's most interesting is playing with the power output control on the AMT5000, which is included to allow precise setting for legal 100mW operation.
Sometimes I set it very low so the radiated signal is confined to an extremely local area, a power of about 36mW. Out in the car that signal only gets to the next yard and then disappears.
The AMT3000 puts a very solid signal for about two blocks and then drops fast.
The AMT5000 set for exactly 100mW goes at least twice as far, although on many days it can be heard in the background several miles away (I have yet to make exact measurements).
Without notice there are times when I decide to crank it up all the way, and the AMT5000 puts out nearly 400mW while I sit at the window watching for suspicious vans. If a neighbor says, "Your lawn looks good," I reply, "What do you mean by that?"
That's when it's time to turn things back to normal.
White Ford Explorer
Worldround Radio on 1680 kHz will be moving to 1640 kHz.
Need for the move is the interference which we "must accept" from the Lutheran iBah noise that splatters over from 1700 kHz, even though that's only a second harmonic of the 5 kW station. While it appears their iBah may be broken and sending more buzz over a wider span than any comparable station, Part 15 requires acceptance of interference, even when it comes from Lutherans.
Not abandoned altogether, 1680 kHz will be used as a testing frequency for our Indoor Antenna Project, results of which will be posted on this website.
The changeover to 1640 kHz becomes official on June 21, 2014 in celebration of the Summer Solstice, the Longest Day of the Year.
Worldround Radio transmits from an AMT5000 Transmitter from SSTran.com, at a power level of 101mW and an antenna length of 3.1 meters.
Yesterday, Sunday, at the Save-So-Much parking lot, that exact van pulled up and the driver pointed at my car and said, "I am with Ford and I can give you a price for the scratches on your car."
The "scratches" on my car are brown leaves that stick due to tree sap.
The FCC wouldn't know that.
Out on the Wild Web the full-time AMT5000 detractor said...
"Neither kit includes an audio interface assembly to connect between the studio (indoors) and the transmitter audio input (outdoors), and that may be the reason you presumed the manufacturer's intention of manufacturing transmitters only for indoor use."
By "neither kit" the fellow who owns neither transmitter was talking about the AMT 3000 and 5000 from SSTran.com.
Were the truth to be known, the AMT5000 does in fact have an interface for balanced audio and power in addition to its regular power and RCA audio inputs.
As the lies and misrepresentations continue, we will follow up with our Word of Truth postings here on the original and only true Part 15 website.
yawn...snore..
I wouldn't worry about it Carl. Egos and biases don't mix well with engineering and science - and Part 15 - and that website is a perfect example.
Bias is very important in the world of semicondoctors and vacuum tubes.