More work on the 30 meter MEPT

I have done some more work on the 30 meter MEPT that I have mentioned in my previous post.

The firmware on the ATtiny is done as far as I am concerned. The FSK mode works as expected now.

Initially when putting the MEPT in the case I had an issue, with the transmission cutting out at some point.

It took me a while to figure out what was causing the issue. The symptoms of the issue were, that the transmitter would be running fine for more than an hour when the lid was off. After putting on the lid, the first transmission would usually work, but the second one would cut out. After being silent for a while the transmission started again, only to fail maybe 20 seconds later.

After scratching my head and looking through the code for some weird fringe case that might be causing the transmission to prematurely terminate, I noticed that the AD9850 was getting quite hot during operation without the lid.

I suspected that when putting on the lid, the fact that there was not enough natural convection would probably make the AD9850 go into thermal shutdown. Sure enough, when I covered the DDS chip with a piece of insulating foam the transmission terminated within the first minute, meaning that it was indeed a thermal issue.

My solution was to mount the AD9850 directly to the lid of the case, using the lid as the heatsink. I applied some thermal compound to the top of the chip and insulated the soldered on wires with hot glue that also holds the board in place.

AD9850 heatsink

I also managed to fix the magnetic loop antenna that I had previously built. I found it quite odd that people were reporting that their magloops would seamlessly tune from 7 to 30MHz, because mine would only tune on 40 and 20 meters but no other frequencies.

After carefully inspecting the variable cap I noticed that the outermost rotor plate had been slightly bent inward, maybe only by a millimeter. After straightening out the plate the magloop now tunes up on most shortwave bands from 40 meters upwards.

I also found that the mounting loction of this antenna directly at the window is not ideal, because it makes matching to 50 ohms difficult. I have no issue getting it to match when the loop is a bit further away from the window.

This is a waterfall image generated with baudline plotting the audio output from my FT-857D. The image shows the demodulated output over a 30 minute period.

30MEPT Spectro

All I have to do now is figure out a better mounting location for the magloop and then I can let the MEPT run and see how far its 200mW signal travels.

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Written by Elia on 26 February 2016