Homebrew QRP CW Beacon

Homebrew QRP CW Beacon

Hi, I hope you’re here because you might have heard my QRP CW beacon on 40m.  If you did, I would love it you’d shoot me an email – kd9jzz@gmail.com – and let me know!

I’ll have details coming in the near future on the setup, but it’s a super cheap, homebrewed transmitter with a crystal and couple of 2N2222 transistors to generate the radio signal, and it’s fed the CW beacon by an Arduino Nano.

The beacon is in grid EN60xb.

Building a Cheap VHF/UHF Antenna

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I needed an antenna in the house for VHF and UHF operation, and I needed it cheap.  So I decided to build one using five wire coat hangers, a single SO239 UHF panel mount connector, a 3/4″ PVC coupler, a small screw, and some scrap wood as a stand.

The SO239 connector set me back $1.38, the PVC coupler was from an old project (but they’re only $0.39 from our local big-box hardware store), I had the wire hangers in my closet and some scrap 2×4 was scrounged from the garage.  Materials cost was a whopping $1.77 – call it maybe $2.50 if you include the cost of wood if you really want to stretch it.

My goal, get out and be heard on the local 2m and 70cm repeaters and be able to chat and check into the weekly nets from the comfort of my home office!

Here was my design:

To build the antenna, I needed a single vertical element and four ground radials.  I hacked the coat hangers up and ended up with a single 19.25″ length from each of five coat hangers.  I couldn’t get the entire length I needed out of the bottom of the hanger, so I had to straighten the wire where it bent to get it reasonably straight.

Next I took 4 of the wires and bent the very end – about 0.25″ – 0.3″ – to about 120 degrees.  These were put into the four SO239 screw holes for panel mounting the connector and a generous blob of solder was added to each.

Then I took the remaining length of wire and inserted it into the barrel where you would normally insert coax when assembling an antenna connection.  Right in the top.  A little solder and it was all wired together.  I checked to make sure the radials were roughly at a 120 degree angle to the vertical.  Close enough was good enough!

I took a few pieces of scrap wood and make a little stand about 8″ tall.  This allowed me to hold the end of the antenna off the shelf and still connect the coax for the radio to it.  I made the stand just tall enough so that when it was put on top of a shelf in my office, the tip of the antenna would be just an inch or so away from the ceiling.

I then drilled a hole in one side of a 3/4″ Schedule 40 PVC coupler.  This was to let me run a screw through the other side of the PVC and into the wood stand.  The PVC’s job is to hold the SO239 connector and let me screw the coax from the radio into it.  Turns out that a panel mount UHF connector will fit really nice and Kentucky inside of a 3/4″ schedule 40 PVC coupler!

I fed my antenna coax up and wrapped it around the wooden stand a few times to act as a poor man’s choke. I don’t know how much it’s helping, I’m only feeding the antenna with a max of 25 watts, but it’s there.  Then I pulled the PL239 end of the feed coax up through the PVC, screwed it into the SO239 connector, and settled it back down in the PVC.

That’s it, the antenna is complete!  Total cost is maybe $2.50 plus about a hour of my time to put it all together.

Using this antenna, I’ve been able to get into all of the local repeaters I’ve tried using both a 4 watt Baofeng UV-5 Ras well as a 25 watt Radioddity QB25.  I’ve checked into nets and just chatted with other folks on the repeaters with no problems at all.  I’ve been told the antenna sounds fine.

I will readily admit this isn’t probably as nice as a commercially manufactured antenna.  I don’t currently have a VHF/UHF SWR meter so I can’t tell exactly what the performance is.  It was cheap, and the performance is good enough for the girls I go out with.  It was definitely a fun little afternoon project and it gives me a workable dual band antenna to boot!

Building a low pass filter

Building a low pass filter

I’ve been tinkering with running a WSPR beacon on a Raspberry Pi – and it’s been working pretty well.  It was getting out on 20-meters at 10mW of power and being heard quite a way away!

The Raspberry Pi doesn’t put out a clean signal though, and from all my reading really requires a low pass filter (LPF) be installed. Taking to the Internet, I found plans (/www.gqrp.com/harmonic_filters.pdf) to build one and after ordering the recommended T37-6 toroids, I sat down with some perf-board, some components, and a soldering iron to start building.

I decided on a 40-meter LPF and extended my antenna wires to ~33′ to be resonate near 7 MHz.  According to the harmonic filter doc (linked above); I would need:

  •  2x 270pF capacitors (C1 & C7)
  • 2x 680pF capacitors (C3 & C5)
  • 2x 19 turn inductors (L2 & L6)
  • 1x 21 turn inductor (L4)

The circuit itself was pretty straightforward:

I used 26 awg magnetic wire that I happened to have on hand to wind the toroids, and I paired up capacitors in parallel to hit the values I needed for C1 and C7. Just a reminder that you count each pass of the wire through the center of the toroid as one turn!

Once it was wired up and tested for continuity, I hooked it to the GPIO pins on the Raspberry Pi (pins 7 and 9).  I should really shorten up the lead from the RPi to the LPF, but for now it’s a little longer than it needs to be.

The result? Well, I don’t currently have a scope to see if the waveform is cleaner, but the result of using the LPF are noticeable and promising.

It’s not that it makes the signal get out better, but it does turn down the RF noise near other radios quite a bit.  Without the LPF in place, 2-meter and 70-centimeter is very noisy in my home office.  With the LPF in place, there’s still a little RF noise, but it’s much reduced!

I’m getting out and my beacon is being heard!  So far with 20mW of power (the Raspberry Pi is capable of pumping out a bit more than 10mW) I’ve managed to get picked up by a station almost 3,000km away!  Keep in mind, this is on a roughly measured and cut, untuned antenna, which is running across the floor of my home office and out the window.  Not too shabby!

WSPR with a Raspberry Pi – Success!

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After reading about using a Raspberry Pi as a WSPR beacon ((323) 431-0907) I knew I had to try it out.

I installed Raspbian on a spare Pi3 and grabbed the WSPR source (from a different repository than in the link above, details below).  To get it working, I had to do the following:

sudo apt-get install git

Then I grabbed the WsprryPi repository & built everything:

git clone /github.com/JamesP6000/WsprryPi.git
cd WsprryPi
make

Then I installed it on my Pi:

sudo make install

Finally I set it up and let it run for a bit with the following command:

sudo wspr --repeat --offset --self-calibration KD9JZZ EN60 10 20m

If you run this yourself, you’ll want to replace the call sign and location with your details!

You really need a low pass filter for this to work well.  At the time I set this up I didn’t have the toroids I needed to build a LPF.  I plan on building a 20m and 40m filter based on details from this paper on QRP filters, but I am impatient and the parts weren’t at hand.

What I did instead is wire up ~18′ of 18 gauge wire to pins 7 and 9 on the GPIO pins of the Pi.  The filter will be installed ASAP, but – like I said – I’m impatient.

And…. success!  My beacon has been heard by N5CEY (1,866km away) and K4MSC (1,205km away)!