Homebrewed FT8 - Success!

Messed around with my antenna this morning.  It would look great, but pulling the feed line to the side a bit made the SWR go sad.  I brought it down and... just kind of messed with it.  Seems to be working now, but I'm not sure I could tell you what I did.  Tightening the N to BNC connection may have been it.

Anyway, had another go at it tonight, and managed to have a QSO!  WA6BJQ down south of me.  So exciting!  Also managed to get picked up all over.

Class D progress - April 10, 2021

 Based on some feedback from the QRZ forum, decided to move forward with a bifilar wound FT50-43 (six turns, 1:1).  Here is the result

Unfortunately, the smoke test let out smoke.  The bottom transistor (not pictured) blew.  I had wired the output transformer wrong, so thought that was the cause.  I put in another one, but it blew also.  My current theory is that I don't have enough decoupling capacitors in there, so the driver isn't able to drive the transistor well, so it is just staying partially on and burning up.  Will add some more and try again (though hopefully that works, because I only have one left!)

I wasn't sure about doing the IC adapters here (probably adds inductance and capacitance - hopefully not the problem I'm having), but being able to easily swap the chips is great.

April 9, 2021 - sort of progress

Someone on QRZ forum has been giving me advice on my Class D design I am working on.  I soldered the tiny, tiny parts onto their breakout boards - I should really avoid parts this small in the future...

I found in my design that the coefficient of mutual inductance plays a huge role in the performance, and I don't really know what that is reasonable to be.  So I decided to make a 1:1 toroid and measure it (based on a procedure in EMRFD).  However, I could not get my Banggood special inductance meter to work right.  It has in the past, but no love tonight.  So I decided to try the function generator/oscilloscope approach where I see where an inductor gives me half the voltage of a known resistor.  Worked well for the primary/secondary windings, but couldn't get it to work for the other connections the book has me do.  I assume the values are low and need to use a high frequency, and the frequency generator isn't very linear in its output over large ranges.  May try some other method here soon.

I tried FT8 again tonight, but didn't get picked up by many receivers.  I checked my signal into a dummy load afterwards (trying to figure out how much power I was putting out), and into a dummy load I was doing about a watt.  However, I see that I am drawing half the power into the dummy load than I am into the antenna, so I should check that again.  I might want to build a little matching network, even though the NanoVNA (the last I checked) seemed to show things fairly happy.

Homebrewed FT8 - Continued

 After having had success receiving and sending separately, today I put together the rest of the picture.  Printed a little box for the TR switch.  Quite pleased with how that turned out - first time I designed a box that closed.  Even chamfered the screw holes so the screws sat right.

Managed to get everything going (WSJT-X already had the patch the guy submitted, so was very easy to get up and running), but guess the conditions aren't as good as the other day.  Not getting a lot of spots, and definitely not making any contacts.  Will have to try a different night.  As an aside, I think my amp might trying to become an oscillator on occasion.  I keep an eye on the amperage of my bench power supply, and sometimes it isn't going back down to the quiescent current level.  Will have to revisit that.  As an aside, I am making great use of the cheap-o infrared thermometer I got for Christmas several years ago - way better than testing the transistors with my finger :)

FT8?

 I recently saw this post where the person made a cheap and cheerful FT8 setup using an Si5351/amplifier and an RTL-SDR.  I thought "hey, I have all those parts", so I thought I'd give it a go.

So far, I've done two parts.  After I got my dipole back up and tested it with the NanoVNA, I plugged my RTL-SDR in and got it working with WSJT-X.  I used a 40M bandpass filter I had built (because I had it), and got results right away.

For my later reference, my setup was

• CubicSDR on “direct sampling - Q"
• Tune to 7.074MHz *upper side band*, bandwidth ~3k
• Turn on Loopback, with the source being CubicSDR
• Turn on WSJT-X, pick “Loopback audio” as the input
• Set WSJT-X to be on 40M/7.074

Today I dug out the "Mighty Mike" amp I built some years back and verified it still worked.  Connected an Arduino to an Si5351 board, that into the amp, and that through a low-pass filter.

I used the software here and didn't use their other one that actually interfaced with WSJT-X yet, as that seems to need a patched version and didn't feel up for getting that to work yet.  As is, took me a bit to get it working, as there were a bunch of libraries I needed to install (wasn't clear which ones when I started), and I had trouble with the "weakmon" library it used (it was trying to load an ".so" file that it couldn't find.  Had to build that and then update the code to reference the exact path).  But after that saw their nice little terminal screen.

So just transmitted from there (not doing any receiving) and looked for myself on PSKReporter.  Bam.  Success!  I probably wasn't even putting out a watt, and my dipole is *really* low, so pretty stoked.

I need to build the relay TR they describe, and need to figure out how to get that patched version of WSJT-X working (as I really don't want to try to build that on my mac...), but that is a project for another day.

Nice day for space birds

My lovely family got me an sdr dongle for christmas, and I had been itching to use it to receive weather satellite images.  But most of the discussions around it talk about building a somewhat fancy antenna, which I didn't really want to do.  So I started looking to see if I could use the rabbit ears that come with the kit, and turns out you can.  Following instructions here and here, I got decent results!

Couple of notes:

• I started off measuring the VSWR with my NanoVNA.  I did as the instructions said (length/angle), and immediately got <1.5, so was happy there.
• Had to restart computer (CubicSDR wasn't putting out sound), and had to switch my SDR back to *not* use direct sampling (I had it setup for HF stations).  Finally managed it to tune in an FM station.
• I was using my compass on my iphone to point the antenna, and I noticed it thought north was very much the wrong direction.  Finally found that sometimes you have to do a funny "figure 8" motion with your phone to tell it to recalibrate the compass.  Weird.
• I set wxtoimg to autorecord, and it started right on time.  However, the image was all black.  Eventually, played around in loopback turned up the "pass through" volume, and it started showing an image!  It had the same artifacts on the side that the blog had, too (not pictured above).  In any case, I should have calibrated from the start like the wxtoimg instructions said (but I don't think were in the blog entry).

A little amplifier success

Charlie Morris had a youtube video on Class E amplifiers, so it made me want to try to get mine working.

I had tried previously using an Ixys driver, but I I think I may have blown it.  It is hard to tell.  I ended up pulling it out along with the IRF510 and putting a new IRF510 in with a bjt push-pull driving it, and a 2n7000 in front of that.

After wiring this up, I started with a lower Vcc and immediately got an output of about a watt.  Encouraged, I started slowing upping the voltage.  But at some point it stopped working - the power supply over current safety was tripping, and I wasn't getting an output, same as before.  But this time I went though and wiggled all the components and did continuity tests and tried again.  Then it started working again.  So maybe this is what was happening before - some stray whisker or something.  In any case, I upped the voltage up to 15v and got out about 4 watts.  I didn't want to go higher than that because I don't know the voltage ratings on the capacitors I used (the shunt cap should end up with about 3x Vcc if I remember correctly).

So, a bit of a success.  However, LTSpice thinks I should be getting out 14 watts at that voltage.  Also, the supply says I am drawing 694mA, which would be 10.4W, so not sure where that other 6.4W is going.  Nothing was getting that hot (I didn't even have a heat sink on the IRF510).  I probably need to go look at the waveforms at all the different points, but don't have time to do that right now.