Given that the transmitter hardware includes a GPS receiver, I am wondering if there might be a means of using an output pulse from the GPS to discipline the oscillator frequency, in a future version update to the firmware. This would be highly useful for use a frequency reference, or to facilitate an even higher level of transmission frequency stability/accuracy (that might be helpful for a mode such as FST4W). I guess it depends on how the board is currently hard-wired, i.e. the Arduino would need access to the synthesizer chip output as well as the PPS line from the GPS.

I have thoroughly enjoyed my Desktop transmitter over the past few years. I recently put together the "nanoWSPR" transmitter (QST January 2022), which is a more primitive device, with no GPS, and lacking standalone capability. The output is from a Si5351a chip; the same as I understand the Zachtek units use. Using an oscilloscope (with FFT function) I measured a power output of almost exactly 10 milliwatts (10 dBm) for the raw Si5351a output when feeding a 50 ohm dummy load. I am curious if this is correct, since I see that the description for the Zachtek WSPR-TX Mini says about 20 milliwatts, and it also uses the same Si5351a type chip. It may just be an error in my measurement setup.

I am wondering whether the GPS antenna could be used with an RTL-SDR dongle with the bias-T turned on, for Inmarsat reception, etc. I have the impression that it is an active antenna, so guessing it could be used with other devices as long as the correct bias-T voltage is provided.

I recently added a 200 MHz oscilloscope to my set of tools, and one of the first things I used it for was to calculate the RF power output based on voltage measurements across a 50 ohm dummy load. These were my results... Band RMS Voltage RF Power (mW) dBm 40m 4.06 330 25 30m 4.29 368 26 20m 4.51 407 26 17m 4.96 492 27 I typically use a stated power setting of 23 dBm when running the transmitter without any attenuation in line, which I think is reasonable since the feed line consists of two fifty foot segments of coax (RG-8x and RFC600), and several unions. I also made voltage measurements with the antenna feedline connected, and after taking into account the specific impedance values (measured separately with antenna analyzer) the results in terms of dBm were very similar to those with the dummy load in place. But in general I operate with attenuators to get the power down to the -10 to 10 dBm range, which is the most fun, to see just how low/far you can go. Bruce K4AZ

I have had immense fun this year with my desktop transmitter, especially with attenuators on the RF output, to see how low I could go with the power and still be spotted. I wonder if there might be a future hardware design that would incorporate the ability to control the actual power output, in parallel with the stated power output. I have no idea how that would be accomplished (and at what additional cost), but it would sure be nice to have output power control built in like that.

Yesterday I set my step attenuator to -33 dB, for an output of about 100 microwatts with the desktop transmitter. While I did not cross the ocean, there were three spots in 24 hours...in the states of AL, NC, and VA, with a maximum distance 252 miles (405 km).

Using a variable attenuator I have been probing where the lowest limit for spots will be with my specific location and antenna. At 0.25 milliwatts of RF output I am getting very close to that point, and the low solar flux index (<70) certainly limits HF propagation. Nevertheless at this low level I have been spotted as far as 795 miles away...which amazes me. 73, Bruce KX4AZ

Yesterday I added an additional 10 dB attenuator (20 dB total), so have now reached the 2 mW (3 dBm) power level. Still getting a fair number of spots, including a station in Portugal...6563 kilometers distance.

I am having a LOT of fun with the desktop transmitter. Today I have been using a 10 dB attenuator to reduce the RF output power to approximately 20 mW. Even at this low level I am still getting spots from Europe (Portugal, Denmark etc)...all the way from here in North America. Tomorrow I will add another 10 dB attenuator to get down to 2 mW. Bruce KX4AZ

Wondering whether it would be possible to use an AD9850 synthesizer chip in the transmitter design, which outputs a sine wave rather than a square wave. Wouldn't that avoid the need for low pass filters, and thus a single WSPR transmitter that could cover all of the bands? Or is the output of the AD9850 still not spectrally pue enough?