Poking at the amp

I've been poking at the amp tonight, trying to compare it to simulations in LTSpice.  I've noticed at least two places where I actually have different values than the original circuit (some DC blocking caps and the choke on the final), but putting those back into the simulation didn't seem to affect much.  Definitely getting different readings, but haven't found the cause yet.  Frustrating.

On the plus side, still making it really far on wspr :)

Trimming the dipole

I took 20 inches off each end of my dipole (12 inches then 8 inches), and based on my measurements I think I got it to resonance around 6.9Mhz.  Ballparking, I think the SWR there is 1.6, going up to about 1.8 at 7.2mhz.  I was going to tinker with it more, but don't really have time today.

Also, got a new SLA battery (12v, 12 Ah), which means I'm not running through AA like mad.  However, the IRF510 in my amp is running really hot.  Maybe it is because I'm pumping in square waves rather than the sinusoidal I think this was originally designed for, maybe I just have other biases off.  Never did get the 5 watts I expected, so very likely I wired something wrong in this.

In any case, it has started thinking about maybe looking into class E amplifiers here (since I'm starting with a square wave anyway). 

Random reflections

As I was turning things off last night, I happened to run my scope across the outputs of the LPF.  I noticed it was doing like 44 Vpp, which was odd because it was doing more like 22 Vpp into the dummy load.  I realized that  this was indicative of a higher SWR than I was expecting (possibly 4:1, I didn't take exact measurements).  I ran a little experiment tonight, and I *think* my dipole has its peak resonance somewhere in the 6.5-6.6mhz range (where it is more like 1.6:1).

So this weekend I may try to shorten it a bit to get the resonant portion up to where I actually want to use it.

This also makes me want to finish that antenna analyzer I started messing around with last year...

Also, my batteries died part way through my experiment tonight.  I was running off of a back of AA batteries (same as when I was experimenting before), which don't exactly have the power to do this.  I tried with the random wall wart again, but it was noise city.  Am going to look into getting SLA and charger.

Playing with WSPR

Having installed WsprryPi, I've been testing things out.

First issue is I didn't have any wires that had the pin connector end.  I ended up finding some some header connectors that I could cut some pins off of so that I could have single tall pin I could alligator clip onto.

I put that through a 1nf cap into the LPF I built previously, into my dummy load, and looked at the output on the scope.  Didn't like what I saw - definitely some harmonics in there.  Was getting about 2.3v p2p

Next I decided to see if my amp would do better.  I put the output of the raspi into the Mighty Mike amp, into the LPF, into the dummy load.  Much better!  The output looks pretty clean.  Doing 23.3 Vpp, which is about 1.3w/31dBm.

Ran this command:

sudo wspr --repeat --terminate 7 KC9DLM CM87 31 40m

Waited a few minutes, goofing around and what not.

And then I checked wsprnet.

WOWWOWWOW!

This is so exciting I can't even explain.  Especially given I just threw this together in like an hour.

Wow!

The IRF510 is probably a bit hotter than it should be (I don't think I even adjusted the bias level since I put that board in a box six months ago).  

I hopped over to a SF area web sdr (http://69.27.184.62:8901/) and found my signal :)  

The farthest looks to be VE6JY at maybe 1970km away, followed by NO1D at about 1000km.  The closest seems to actually be someone in my town :)

So, that looks to maybe be 941 Miles/watt.  Not the most impressive, but given my antenna is not that high (nor have I ever verified the SWR on it) and it is is around 1pm on 40m, I'm pretty happy with that.

edit: tried again later (8:45pm) - the 40m band is maybe better at night :)

Installing WSPR on Wheezy

Got the urge to try out WSPR.  Installing WsprryPi on a Raspberry Pi I had laying around.

https://github.com/JamesP6000/WsprryPi

Didn't work because my version of gcc was too old (didn't recognize one of the flags).  So went to upgrade gcc

https://somewideopenspace.wordpress.com/2014/02/28/gcc-4-8-on-raspberry-pi-wheezy/

(Truth be told I probably should have just upgraded to a newer version of Rasbian full stop, but this was something I could easily do in the background while I was doing some other things.)

Much happier after that.

Power supply woes?

I built another little preamp today (this time just a CE followed by a CC rather than the shunt thing I used before), just to see if additional drive was the issue.  Did not seem to help.

However, it occurred to me that it might be the power source I'm using.  I've been using alkaline AA batteries.  But calculating things out, for 12v to produce 5 watts, the batteries would need to be delivering around half an amp (not counting what is dissipated in the transistors).  I always assumed that the batteries could deliver more amperage for a shorter time (the amp-hour rating).  However, I forgot that that batteries have voltage curves.  I measured the voltage output when the amp was running, and it was closer to 9V.  So, I think I need to go find a better battery to use :)

(I actually tried a wall wart, but I think I was either getting feedback into it or it was just noisy, was getting a weird output when I tried to bring the power above what I had with the batteries.)

Transmitter progress

I moved the circuit to copper, but no love.  Still when I turn the bias up, it all goes haywire.  Actually worse than before.  Thinking it might have to do with the drive level, I tried adding an amplifier stage ahead of the 2n3053, but again no love.

Then I thought "hey, I sure have a lot of wires running around here..."  I had been using hook-up wire to join my modules, as I didn't have any extra coax.  But I did get some recently, so I added some BNC connectors to the output of the PA and the input/output of the LPF.  Bam!  Pretty output!

I'm getting 20v P-P on about 11.3V power, which is about 1 watt.  I can get it up to about 1.5 watts by increasing the voltage to around 13V.  But I'm hoping for 5W.  Here at least some of the issue I think is the drive level.  I tried the little amp I had, but I think that was overdriving it.  Was getting about 3W out, but it was well ugly.  Will need to have a think about what the drive level ought to be.

I might at some point see about using a different IRF510.  I've read that "lower quality" parts may not work as well, and I'm not entirely sure about this one.

Edit: actually, after playing around in LTSpice, I think I may need to change out some of the coupling caps.  The ones he uses in the Mighty Mike look to be attenuating things too much.  In the simulation, if I change those to larger caps (closer to .1uf) and run it at the same power he does, I get >5W out, even with my current drive level.  With his values and my current voltage, I get closer to what I'm getting on the bench.

Edit 2 (next day): I tried swapping out the .001uf for .1uf coupling caps as I mentioned in the last post.  Raised the power some, but not nearly as much as in the simulation.  I also tried swapping out the IRF510 for a new one from Mouser, didn't see much change.