HAB Hacks

K1FM Pico Balloon – Part One

The Pico Balloon concept is simple: you build a transmitter small enough to be carried around by one or two common party balloons. As the transmitter floats around the globe it transmits its current position and altitude so you can follow its path in realtime. If you do it right, the balloon will circumnavigate the globe… repeatedly. Great stuff! I had to do it.

After realizing that it was relatively easy to construct a WSPR beacon based on Arduino and a clock generator, a few months back I decided to try building one.

The first attempt was on a breadboard with an Arduino Pro Mini, a uBlox breakout board and the Etherkit s5351a breakout board. After writing a few hacky lines of code (WSPR libraries are already available) WSPR worked! All I needed was making everything small enough to be carried by a party balloon.

I decided I was not going to use the standard Arduino Pro Mini. Rather, I was going to load the Arduino boot loader on a ATM328P myself and have everything else (GPS + Clock Generator) on a the same board.
This unit also worked and, technically, was already small enough to be carried around by a couple of large balloons.

But how was I going to power it? Was a small LiPo battery suitable? After checking the specs I realized that even the smallest ones were too heavy. On top of that, batteries don’t take extremely low temperatures well so it definitely had to be solar power.

In the next revision, which I called 1.1, is where I faced the first serious obstacle. Until that point I was able to successfully load the Arduino boot loader without too much trouble using an ancient Arduino Uno as the ISP programmer. For reasons I still can’t understand, that wasn’t the case anymore with revision 1.1. Fortunately, when I was close to giving up, I remembered that back in the days I had purchased a proper ISP board and tried with that: Bingo! Problem solved.
As you can see, on version 1.1 all necessary programming connections have been moved to a section of the PCB which is supposed to be cut out before launch (the wires soldered on the side were part of my desperate attempts to troubleshoot the ISP malfunction…).

At this point I had a transmitter that – together with the solar panels – weighted less than 10 grams! I really believed it was just about time to buy the balloons when I faced the second problem: I had grossly overestimated how much power solar cells can deliver. Yes, tensions can be pretty high but currents… well, currents are tiny: there is absolutely no way the beacon can be carried around by small balloons while being directly powered from solar panels as I originally envisioned. I needed something light that charged a capacitor and then – only then – I could start transmitting.

By searching things like “Tiny Solar Power Charger” I ended up on Jared’s (N7SMI) page. I was stunned: his pico balloon concept and hardware was identical to mine except that he completed it years in advance and, unlike mine, his actually worked and flew already many times.
Jared had already solved the power issue with a 1.5F super capacitor charged by an SPV1040 controller and two 0.5V solar cells. Brilliant!
Not only that. In order to save power, he provisioned his transmitter to selectively switch off the GPS and/or the clock generator. He also has a temperature sensor and – surprise surprise – a CW beacon too!
Clearly, I now have enough material for version 1.2.

On part two I will publish my current schematics, PCBs and source code.

HF Portable

K1FM Magnetic Loop

After describing my mini loops (here and here), this time I am going to write about the classic “full size” version that I have been using for a while.

At Dayton the Hamvention of 2018 I took my loop with me and I mounted it on a PVC support that fitted a military style backpack. It made quite a splash! Eventually, someone took a picture that ended up on the August 2018 QST and the 2019 ARRL Calendar front-covers. That’s when I proposed the QST editors an article which described how to make the antenna. QST accepted but took its time, finally publishing the article with the May 2019 edition.

The K1FM Loop consists of a LMR400 radiator (124″) fed by an LMR240 exciter (25″). The radiator loop is terminated by a geared, multi-stator variable capacitor made by Alps (14pF to 165pF, P/N 42-36251-1). The exciter, instead, is constructed to be terminated by a 3way BNC adapter.
The geared capacitor and the loop configuration have been inspired by the AlexLoop while the exciter construction and the selfie stick by Alpha Loop.

The exciter loop takes a bit of patience in order to be manufactured this way. Quite a bit, actually. Of course more classic approaches can be used as long as the overall size remains the same.
Both loops are mounted on the selfie stick using a custom 3D printed bracket that fits on standard camera mounts.

The other critical aspect of this project is obviously the capacitor. It is the capacitor which defines the bands you can use, the resolution you get (how “touchy” the loop will be…) and – last but not least – how much power the loop will be able to handle. The 15-165pF geared type is some kind of a sweet spot, a very good compromise for a loop this size when used QRP.
Unfortunately Alps no longer makes this capacitor but from time to time they pop-up on Ebay for about $20 each.
You can, of course, replace this capacitor with equivalent ones (similar, non geared capacitors are more common and much easier to find).

The capacitor enclosure is fundamentally just a box that holds the capacitor against the selfie stick, connecting it to the radiator loop.
The box I used is made by uxcell and measures 3.9″ by 2.6″ by 2″. I mount it against the stick using a custom 3D printed ABS bracket.

The list of all necessary materials, from the main loop to the last nylon screw, is here. Overall the cost for a single antenna should be around $60 but the final cost will vary depending on how much you will spend for the capacitor and for the 3D prints.

Performance of magnetic loops this size have been extensively analyzed and reviewed. Mine is no different than the others.
Please do not expect miracles! Ultra-Portable QRP is hard stuff and you will need to rely the 3 P’s: Propagation, Perseverance, Patience.
In a nutshell this loop performs at around the same level of full sized half-wave dipoles or of a vertical antennas. Yet, you are running 5W watts into it therefore plan for the right time, the right band and the right location if you want to work DX on voice.
All in all, I find it a bit lacking on 40 and I don’t use it on that band unless it is inevitable. 20 and 30 are very fine and – when conditions permit – so are 17, 15 and 10.

Other than the loop, I normally carry my FT-817, a WinCamp Headset adapter, a PC Headset, a Palm Radio CW Pico Paddle, a mini tripod and a military backpack with a PVC antenna mount.
I recently added FT8 and wireless headsets, but that’s going to be detailed in other posts.

Finally, I’d like to spend two words on RF safety.
As a licensed Amateur Radio operator you should already be aware of the following but I, for the sake of safety, I’ll repeat what you probably already know.
Loops can be dangerous around people.
Do not exceed power! By doing so you would irreparably scar your capacitor. If you really go heavy, you basically built a soldering machine: sparks will fly and fires might start.
Moreover, even when QRP, this loop will exceed FCC exposure guidelines when used in close proximity. To be on the safe side, nobody should be closer than 2 meters from this loop, regardless of how much power you are using. Finally, do not touch the radiator while transmitting because you could be burnt, electrocuted or both (~800V at 5W, ~4000V at 100W!).

Hacks SDR

HDSDR OSX version 2.76 released

Once again, with Mario’s permission, I bundled the latest revision of HDSDR to be run on OSX. I also created a new rtl_sdr launcher that should be more stable, easier to use and that incorporates the bleeding edge version of librtlsdr.
I tested this on Yosemite, El Capitan and Sierra. My CPU is a quad i7.

Here is a quick vhf/hf demo that I run using my v3 dongle in direct sampling mode:

In order to download it, please refer to this page.

Latest Release: March 2nd 2017

Hacks SDR

HDSDR on OSX the EASY way – SDRplay edition



Today I updated my HDSDR OSX bundle to include support for SDRplay.
Nothing much has changed… If you want to enjoy SDRplay + HDSDR on OSX, all you need to do is the following:

  1. Install the SDRplay drivers (available here)
  2. Download my bundle
  3. Copy ‘HDSR OSX’ and ‘rtl_tcp SDRplay’ on you hard drive
  4. Run the ‘rtl_tcp SDRplay’ server
  5. Run ‘HDSDR OSX’

As you can see, the bundle still includes the original RTL2832U rtl_tcp server. According to the hardware at your disposal, you can use either use one or the other as their functionalities are practically identical.

The SDRplay support was made possible by the hard work of Tony Hoyle so if you want to thank anybody, please thank him.



Hacks SDR

HDSDR on OSX the EASY way (reloaded!)



A while ago I published a bundle that easily allowed anyone to run HDSDR and the RTL2832U on OSX. To my surprise the bundle was quite successful, a demonstration that there weren’t (yet) many available alternatives to use the cheapest (and funniest!) SDR radio in the world on a Mac.

After a few years things have improved a bit and some valid alternatives have surfaced: Gqrx and CubicSDR. Even SDR# has been reported to work under Mono but, believe it or not, neither of them seem to work well on my Mac. Actually they don’t even come close to the level of stability and/or the number of features that can be enjoyed when using HDSDR.

Last week I installed a new active RX antenna and I tried to reopen HDSDR in order to test it. Guess what? I realized that it wasn’t compatible anymore with the most recent versions of OSX. It was time to do it all over again… so here I am.

How to install

1) Download the zip archive and expand it
2) Copy rtl_sdr and HDSDR anywhere you like on your Mac

3) Make sure your RTL2832U is connected
4) Run rtl_tcp

6) Click on Start


At this point, if things go as they should, nothing else is required and you should be ready to go. I have successfully tested Yosemite, El Capitan, Sierra and they work great.

January 19th 2016:  This bundle now also supports SDRplay. More details here.
February 12th 2017: HDSDR updated to 2.76. Improved rtl_sdr
March 2nd 2017: Fixed problem in rtl_sdr (thanks Walter KC1DVT)

Final Considerations

This is an experiment. I cannot guarantee that it will work nor that it will not damage your system or your data. Download and use the bundle at your own risk.
I am not the author of the software and I can be helpful only up to a certain point. All I am doing is bundling a bunch of different packages in a way that is easily executable on a Mac.