Using an adjustment on board the transmitter, a fine tuning oscilator trimmer that could vary the frequency by maybe 2klz +or - just to eliminate the beating with the out of sync interfering stations. I hear this on AM sometimes when 6 stations are fighting to come in on one frequency and there's the wavering because some of the commercial stations are not completely in sync. with each other from different places.
Several years ago PhilB, a member here, proposed a dummy load for transmitters which simulates the antenna and ground system.
He wrote "A "part15.us standard" antenna simulation circuit can be defined by forum consensus (I suggest a 30pF 500V 5% mica capacitor and a 30 ohm 1/4 watt 5% carbon composition resistor)."
These would be connected in series from the transmitter output to transmitter ground. Original post is here: http://www.part15.us/comment/19419#comment-19419
Neil
I built one of those PhilB "Dummy Loads" as just described and use it for testing the transmitters "on the bench".
Using it to evaluate the "sync" with far aways stations is a good idea that I never thought of before, but now I will do it.
I want to just clarify that the warbling problem discussed in this thread is not a normal, acceptable phenonomon that the user can tweak. Signals that are 1-5 Hz off from each other are well within the realm of normal and can be minimized through the frequency fine tuning capacitor. Interfering signals that are off by a few Hz are not going to cause warbling in the audio in fringe areas, but inadequate shielding or buffering can. I used the SSTran recommended dummy load on the Procaster. It weakens the signal enough to produce nulls on a walkman with a directional loopstick two or three feet from the box.
Additional prediction: If Gerry can find a solution to this problem, it may also improve the audio clarity of the Procaster even when other signals are not present.
OK I will try to as clear as possible. I would take a manila looking file folder and cut it into a rectangular shape and cut at an angle at the four corners. Then I could bend the paper downward to make a cover over the circuit I wanted to shield. I should add that I covered the exterior of my paper circuit house with copper foil tape. Then using as short as possible a wire soldered to the foil covered house to a ground trace on the board. I used this technique many times when trying to shield a circuit from either radiating or to shield the circuit from radiation back in my EMI/EMC days. I hope this was clear enough.
Information on progress made looking into this problem has been posted on our website here:
https://www.chezradio.com/misc
A slight improvement will be made on the next run of boards although we don't know if this is contributing to this problem. Could it be a bad/damaged main board?
ChezRadio That Sounded Great Playing Pink Floyd
you made a Believer out of me that it is possible to have album rock on AM and sound good. I've got to try and save up for this transmitter for my station. I think it would make it sound great while still being legal.
I've got another idea for you you should come up with a AM stereo transmitter and call it the procaster album rocker. That version could be in C-Quam AM Stereo. Even though that thing was mono it did sound good from what I heard. Did you use any special audio processor other than the processor that was built into it?
Thanks for posting.
Add me to the growing list of readers who have been following the response to this illusive hard-to-replicate problem by Chez Radio.
The video clearly showed a stable and professional AM signal being subjected to music one might expect to hear on the radio.
We are thankful for your contribution the part 15 experience.
Gerry thanks for the great support of this great product. While I am not experiencing this "warbling" problem with my Procaster, it's great to know that you are not just another "one and done" manufacturer.
Just to note: The FCC considers that a carrier ratio of about 20 dB (100:1) is needed between desired and undesired AM broadcast co-channel stations to produce acceptable S/N for the stronger station in the audio output of an AM receiver tuned to that carrier frequency.
The carrier frequency tolerance for licensed AM broadcast stations is ±20 Hz. Most of them operate within a tolerance of ±5 Hz, with very low phase noise/jitter.
But when co-channel stations are present with nearly the same net field strength at the receive antenna as the desired station but on a slightly different carrier frequency, the AGC circuit of the receiver will see the net signal it is sampling to be changing in amplitude. The various carrier signals constantly add to and reduce the amplitude of the signal used by the AM receiver AGC (automatic gain control) circuit.
That changing r-f level produces an AGC change in the r-f amplification of the receiver, which results in a similar change in its audio output level. The audio result of that has been described here as "warbling."
Probably its a bit more than should be expected for a network including a "Part 15 AM" transmitter to perform as well under these conditions (or even nearly as well) as a network using only licensed AM broadcast transmitters.
Even so, if the received carrier of the Part 15 AM station exceeds that of the interfering signals sufficiently, it can produce useful nighttime service -- typically just closer to its transmit antenna.
Below is a plot of field intensity vs. distance for a Part 15 AM station using about the best legal hardware permitted under FCC §15.219, and better-than-average soil conductivity.
The nighttime skywave of licensed AM stations can range from maybe 0.5 mV/m to 3 or 4 mV/m. If the skywave interference is 0.5 mV/m, then the required signal of a Part 15 AM station that needs to be just 10X greater would be 5 mV/m.
Notice in the plot that the radius to the 5 mV/m signal of that installation is less than 50 meters (50 meters = 164 feet).
Other conditions can be examined by referring to the chart.
Here is a link to a 13 1/2 minute narrated audio tour of the night time coverage area of Expresso 1620, one of the Part 15 stations in Worcester, MA that uses a Procaster transmitter. It was recorded on Sunday, 3/25/2018 when signal levels from interfering stations were more than usual. The warbling is quite evident:
http://www.t1700.net/Expresso_Procaster_at_night.mp3
Nighttime Dreams
During many years in hobby radio I have wished, along with many others, for a legal AM nighttime signal that would match the daytime signal.
Knowing that the rules don't allow the small power increase that would make such equalibrium possible, I have wondered purely academically how much power it would take to do it.
Post # 28 gives a ballpark estimate, which may vary:
"The nighttime skywave of licensed AM stations can range from maybe 0.5 mV/m to 3 or 4 mV/m. If the skywave interference is 0.5 mV/m, then the required signal of a Part 15 AM station that needs to be just 10X greater would be 5 mV/m." - Rich
10X greater is a convenient number of about 1 Watt.
Somebody does make a 1 Watt AM transmitter, but this is only a dream and we don't recommend it.
An Available Nighttime Transmitter