Tag Archives: ham radio

Post Field Day Report on K6HX Beacon Operations…

So, part of my incentive on working on my WSPR beacon was one of my coworkers decided to try to do some operating from the local Emeryville Marina, and I thought it might be nice to use the opportunity to try to pack up my WSPR setup and see if I could get some spots using solar/battery power.

I wrote up some of the early experimentation to get the beacon working, but as of then I hadn’t actually gotten on the air. To do that, I needed two things: an antenna and (to be a good RF citizen) a low pass filter to make sure that I wasn’t scattering horrible harmonics everywhere. While it was true that I was outputting a scant 10mw, I thought that being a good citizen was important.

I had some yellow T37-6 cores lying around from a previous bout of homebrew exploration, and dug out an article on constructing harmonic filters by the awesome (but sadly SK) George Dobs, G3RJV. I dug out some copper clad and wired up the filter using some islands that I punched out of scrap copper clad board. I am a total novice at this, so the board layout is terrible and stupid, and the leads are too long: in short, everything I did should be viewed with skepticism.

But as crappy as it is, it seemed to work. I snapped a quick picture of the output as it goes to my antenna, and the previously ugly triangle-ish wave now resembles a reasonable sine wave.

I must admit that I was somewhat befuddled by the Vrms as measured by my scope. The equation for output power suggests that power equals Vrms squared divided by the resistance. If we assume my antenna was a nominal 50 ohms (it’s not, but let’s pretend) then this would indicate an output power of nearly 70mw, which doesn’t really make sense. My antenna had only been “tuned” in the grossest sense, and probably had an SWR of around 2.5:1, which suggests that it’s characteristic impedance was probably a lot higher. For the purposes of WSPR, I configured my output power as 20mw, which I thought was probably still generous, but in the right ballpark. I made a note to myself to figure this out later.

I still needed an antenna. While cleaning my garage, I found a pair of 20m whip antennas and an MFJ 347 mini dipole mount. I mounted these on a small piece of conduit, and then built a quick stand to hold it up, consisting of a 5 gallon bucket, a small round of particle board that I had left over from another project, and four pieces of two by fours to act as feet. Here it is, setup on my back patio. Yes, I know that the antenna is mounted too low to be good or efficient. This entire project was designed to be a hack, and for maximum ease, not permanence.

I set it up and home and set it beaconing. I didn’t get the torrent of spots that I had gotten when I had used a similar setup with 5w of power (using my FT-817) but a few minutes later I did get a spot from KA7OEI from DN31, a distance of about 933 km.

Huzzah! Thought I. I was ready for field day. Mind you, the entire thing was still just a pile of breadboards constructed out of wire. I decided to use my Chinese charge controller and a 20w panel to drive it and to recharge a 7aH lead acid battery. I packed it all up and headed to Emeryville on Field Day.

I’d like to report success, but the fact is that once I got it set up, it was apparent to me (by a lack of blinking lights and LEDs) that it wasn’t working. I wasn’t sure what was wrong, but I had failed to bring my laptop so debugging it in the field seemed like it wasn’t going to happen. After a couple of hours, I decided to pack it in. Sad.

Setup, but disappointingly didn’t generate any RF or even a single spot.

The next day I unloaded it from the car and tested it out. The Arduino and the LCD unit seemed completely dead. I reflashed the code onto a new Arduino Nano, moved it all to a single breadboard, and reset it up in the backyard, and it has been beaconing on 20m, entirely powered by solar since then.

Since then, it has been spotted by 15 unique stations. By far and away the most reliable connection has been to KA7OEI-1, who has heard it 337 times in the last week. ND7M comes in next, with 118 spots. Distance records are pretty much evenly split between AI6VN/KH6 in Hawaii, and WD4ELG in North Carolina.

I made a small map which updates frequently, showing all the spots for the previous 24 hours. You can view that from my home webserver, at least until I get bored with it and take it down. I used the Python matplotlib to generate the maps, this one being an example.

I’ll probably try to get this up more or less permanently, and may add a small RF amplifier to boost its output to around 100mw. I will probably also shift to using an ESP8266 based module so that I can monitor and control the system via WiFi. I’ll update you when I get that going.

Hope you enjoyed.

Recommended Reading: AA7EE’s Boris Beacon…

I’ve been interested in low power beacon transmitters for a long time.  If you’ve followed my blog, you’ve probably read about my experiments with WSPR and QRSS operation.  Most of those took place on 30m using my FT817 transmitter, sometimes with software that I wrote myself.  But I’ve long thought that I should homebrew a transmitter, and the idea of making it solar powered has always been in the back of my mind.

These operations have all been on ham frequencies, but even if you are unlicensed in the United States, you can get experiment with low power beacon operation under what is known as “Part 15“.   These rules are part of the Code of Federal Regulations, Title 47, and specify certain bands and limitations as suitable for unlicensed devices.   Rather than reading the boring regulations, you can go to Long Wave Club of America website and read up on experimentation under Part 15.  You’ll generally find operation divided into “LowFER” (operation below 200khz, usually in the 2200m and 1750m bands), “MedFER” (operation in the AM broadcast band, usually between 1600khz and 1800khz) and “HiFER” (mostly centered around 13.55Mhz).  

I was initially most interested in the LowFER bands, but I’ve never really gotten off the ground with the project, but recently have become more interested in operation in the HiFER bands.  It presents some significant limitations.  The most desirable band is limited between 13.553 and 13.567 Mhz.  The total power output is specified in terms of a field strength at 30m distance from the antenna, but is on the order of one milliwatt into a half wave dipole at the frequency.   You also are supposed to maintain frequency stablity to about +/- 0.01% over a temperature range of -20 to 50 degrees C. 

This isn’t a lot of power, so it can be challenging to get an effective signal out, but it also makes for some interesting opportunities for home brew construction.  Most beacons of this sort are small crystal controlled transmitters, and they can be powered by reasonably small solar cells, and controlled by inexpensive, low power, eight bit microcontrollers that are readily available. 

Dave Richards (AA7EE) wrote up an awesome description of his Boris Hifer beacon, named after his cat.  It’s a very straightforward design: a Collpitt’s crystal oscillator, a small two stage low pass filter, and an ATtiny85 microcontroller driven by a LP2950 5V regulator that is powered directly from his solar panel.  (He also points at this $20 kit from Chris Smolinksi at blackcatsystems that would be dead simple to put together).   As it happens, I have almost all the parts in my junk drawer, and could easily put such a beacon together.  I even have a stash of the 13.56 crystals somewhere.

Dave’s construction is gorgeous, and he mounted it fairly low at the top of a fence.  I have a similar fence at the back of my property.   I would be tempted to use a more sophisticated solar setup perhaps utilizing a small battery.  Propagation on this frequency is probably better during the daylight hours, but the solar panel likely generates an excess of power each day, and being able to survive small dips in cloud cover and the like would be good.  But then his experience is that even on cloudy days his beacon wakes up, so perhaps I’m overthinking it.  He also has a good description of using the BOD (brown out detection) in the ATtiny85 to ensure proper startup in varying voltage conditions.  His write-up is really great, and will no doubt save me from a lot of hair pulling.  

Well worth reading.  You might also read his page about making a temperature beacon which was inspired by K7TMG and also this short experiment I did back in 2012 (apologies for the jittery phone video):

Dummy Load/Watt Meter Experiment

Okay, this is pretty basic stuff really, but it’s part of my trip toward additional homebrewing, and it might be of some vague interest, so here we go. If anyone spots anything horribly wrong here, please feel free to help me learn by leaving me a comment here.

Before Thanksgiving, I was having quite a bit of fun with my FT-817. I was getting five or six good 40m/30m PSK31 contacts per night, and was getting pretty regular spots on my 30m WSPR beacon. I had a bit of a hiatus, and only recently got back on the air, but was having much more limited success in PSK31, so much so that I was wondering if my antenna had failed or my coax had gone bad.

The coax seemed a likely culprit. It’s that crappy stuff that Radio Shack sells, and I hadn’t gone to any effort to waterproof the junction where it joins the balun on my 40m dipole. Water could conceivably have gotten into the coax during some of our recent damp weather, and could have caused some additional loss. How could I tell?

Well, the answer is pretty simple: get a watt-meter, and measure the power at the back of the transmitter, and then at the far end of the coax, and see what the difference is. There was only one problem: I didn’t have a watt meter.

So, I built a (crude) one.

The basic QRP watt meter is a combination of a dummy load, and a simple rectifier/capacitor circuit in parallel. The dummy load provides the appropriate 50 ohm load, and the diode rectifies the code, converting it into a DC current, which is then used to charge a capacitor. If you measure the voltage across the capacitor, you can compute the power pretty simply. Power in watts is just Voltage squared divided by resistance, so if you square the voltage and divide by 50, you get the power.

Yes, yes, this isn’t quite right. I haven’t taken into account the voltage drop of the diode, but let’s run with this.

So, I went into the junk box (which I’ve been trying to add stuff too, dug out a rectifier diode, one of my larger 8ufd caps, and a pair of 10w, 100 ohm resistors (more on this later). I also dug out an SO-239 socket that I picked up from Radio Shack. Here’s the resulting gadget.

img_0424

Using my cheap old Radio Shack digital multimeter, I clipped some test leads to measure the voltage between the diode and the ground. I took a 1 foot cable, hooked the load to my FT-817, and tested it out. Setting the FT-817 to its max power level, I turned on the transmitter and measured the voltage. I got a reading of 19.2 volts with this (essentially non-existant) cable run. If you multiply this out, we get 19.2*19.2/50 yielding about 7.372 watts (a bit higher than the nominal 5 watts that the FT-817 lists, but we’ll get back to that later). On its low setting, the voltage read right around 6 volts, yielding about 720mw.

Allright, that’s my baseline. I then took the dummy load outside to the far end of my cable run, and hooked it up there. The results were 17.8 volts (yielding 6.337 watts) and 5.5 volts (yielding 605mw). How does that compare with the nominal result we’d expect from RG-58 over a 50 foot run on 40m? The coax calculator can tell us. Plugging in 50 feet of RG-58 with an SWR of 1:1 on 7.07Mhz and an input power of 7.37 watts, we find we get about 0.5db of loss, and an output power of 6.587 watts. Similarly, if our output power was 720mw, we’d find about 643mw as the output power. That’s pretty darned close to what we expect from RG-58.

So, my conclusion is that my coax, while not the greatest stuff in the universe, isn’t a complete dummy load. If I’m going to blame my relatively poor recent performance, I’ll have to look elsewhere for a scape goat.

Okay, here’s some open questions. First of all, the resistors I chose. They were $0.99 at Fry’s, but their body seems to be made out of some kind of ceramic. There is a possibility (even a good one) that these resistors are internally wirewound, which means they aren’t necessarily entirely resistive: they could have an inductive component. Does anyone have any experience with this sort of resistor? Should I find some others?

Secondly, I didn’t account for the voltage drop for the rectifier diodes. The diodes were something that I dug out of my junk box, and I’m not even sure what type they are. I imagine they have somewhere between 0.5v and 0.7v drop. Technically, this means that the voltage measured at the cap is low by that amount, so the power should be a bit more.

Thirdly, I am confused by the values that I got. Especially considering the above, these power values seem much higher than the nominal values that we are supposed to get from the FT-817. The nominal maximum power should be 5w. I am suspicious that we are mixing peak and RMS voltages/powers here, since the values that I got are almost exactly sqrt(2) off from the values that I might expect, but I haven’t worked that out entirely in my head yet.

Anyway, that’s enough for today.

Addendum: This site has a circuit which is essentially what I built (different diode, and I am using a volt meter, rather than a current meter). It does indeed seem to indicate that the voltage that I’m reading out is the peak, not the RMS voltage. I’ll have to think about it some more.

qrpmeter

Addendum2: Wes Hayward, W7ZOI has a writeup based upon the exposition in EMRFD which would be good for me to understand.

Addendum3: Yes, the resistors are probably wirewound (these cement power resistors apparently almost all are). I’ll have to go find some 200 ohm, 2 watt resistors to gang up in parallel.

Addendum4: I can’t stop! Another potentially useful link for measuring low powers, might be useful when I get to my QRPP beacon project.

K6HX beaconing on 30m with WSPR/MEPT

Okay, I hadn’t been doing any WSPR beacon operations since before Thanksgiving (which was also before my new callsign) and I was kind of bored today, so I dug out my power supply, tuner and interface and set my computer beaconing again on 30m. It’s a combination beacon: using WSPR above 10.140100, and a visual “MV” written as part of a sawtooth in the visual MEPT portion of the band (between 10.140000 and 10.140100 Mhz). Output power is about 4 watts, split between both signals.

I’d be interested in any reception reports.

Addendum: Click this link to examine the reception reports I’m getting via the automated WSPR logging.

Addendum2:

Today, on Jan 2, I got a reception report from W1BW, and I could faintly see my “MV” appearing on his grabber:

MV on the W1BW grabber

You can see his “flying W” very strongly, and if you stare really close (click on the smaller image) you can see my MV which looks like part of a sawtooth around 23:22 (and other places).

Addendum2: Alan, VA3STL in Ontario also noticed my signal on the 2nd. Here is his screen grab, showing both my MV and my MEPT signal.

va3stl