DM6AS - German Amateur Radio Station

The workhorse in my shack for HF is a homemade magnetic loop (about 1700 QSOs). The varicap was an
ebay find for €6. It didn't look like much on the seller's photos, but it turned out I bought 1kg
silver-plated brass. The loop itself is a VGA cord from scrap. Back in the days, the shields were
still made of copper instead of iron or aluminum… The contacts are pole clamps clamping oversize
cable shoes. They are soldered to the split stators using copper tape. Two 3D-printed Ls provide
protection for the rotor and carry the pole clamps.

For working QO-100, I initially bought a POTY kit, but it ate away about 5dB of my already-weak
indoor downlink signal. I gave up on the coaxial feed idea and built a helix with 8.5 turns from
scrap. Poking it out the window yielded the first QSOs.

My QO-100 downlink antenna is relatively boring, just a commercial dish + modified LNB. The dish is
pointed through an old roof (wood and brick). Curiously, pointing it through the roof gives more
signal than pointing it through a window. The signal is still significantly weaker than expected
with a clear sky view. I receive the beacon about 18dB above the noise floor.

I was active as SWL for quite some time before getting a HAM license. In 2011, I built a
direct-conversion I/Q receiver with further signal processing with Linrad. It's heavily inspired by
the designs of YU1LM: A very basic circuit with two flipflops generating a quadrature clock for an
analog switch (74HC4066), in turn switching the antenna to the positive/negative inputs of low-noise
audio opamps. There's no HF gain in this circuit and no preselection. All the heavy lifting is
done by the Opamps (20dB gain) as well as the mic input of the PC.

The flipflops were at first clocked by crystals found in electronics scrap that would result in
interesting mixing frequencies when run at their fundamental or one of their overtones. The most
interesting frequencies were of course the HAM bands. When there was no more room on the PCB for
growing the crystal graveyard, I started abusing the I2S PLL in an ARM microcontroller as LO. It
was slightly noisier than the crystals but much more convenient. In case you wonder what SWL with
this contraption looked like, here's a screenshot of some contest in July 2013.

Just when listening to HF and drawing maps of locators to test antenna experiments started to get
boring, cheap DVB-T-USB-sticks with the Realtek RTL2832 appeared in consumer electronics stores,
extending my shack to VHF/UHF. I had lots of fun listening to technical discussions on the local
repeaters, reverse-engineering ISM band sensors in the neighborhood and drawing maps of ADSB
transponders.

When I was no longer constrained to a student budget, my SWL shack started accumulating more SDR
gear. There was no hesitation buying a RedPitaya for HF, a Hackrf, and later a Pluto, since these
are open source designs. I'm running Bitstreams and Software by Pavel Demin on my Pitaya. The
openness of the platform makes it easy to integrate and exend it with other hardware.

The pitaya is currently on duty on pskreporter unless I need it for something else. With the tuner
modules in the image above as a frontend, the Pitaya is also my RX for working QO-100 with gnuradio.
The reference clock for the LNB PLL is a 25MHz VCXO in turn PLL-locked to a free-running 10MHz OCXO
salvaged from electronics scrap. The tuner module is also disciplined by the OCXO by injecting the
10MHz sine in place of the tuner's original 4MHz crystal.

Having accumulated a sizable shack of SDR hardware during my SWL time made for a tough decision
after getting my new and shiny license: Should I build filters and a PA to get on the bands with the
Pitaya and the Pluto or rather buy a commercial TRX? Looking at the stores made for a strange
picture: Most of these transceivers looked like computers with SDR and PA and filters attached. I
already had the former two, even with open source and hackability that these products would be
lacking…

Further, on VHF/UHF, there is this abyss of digital voice modes where each manufacturer had a
mutually incompatible, patented, propriprietary "standard". I wouldn't have thought something like
this would sell in the HAM world, where accessibility, transparency and homebrewability were key, I
thought. But there was one radio that didn't run into any of these dilemmas: Yaesus FT-818. I will
probably end up building PAs and Filters for my open source SDRs anyway :-).

I also couldn't resist buying kits for monoband CW transceivers from dl-qrp-ag, such as a Mosquita
III and a BTR 18. My CW skills aren't good enough to use them yet, but I'm much more motivated now
for the regular training. Currenlty, I know 32 characters of 42 at 22WPM.

Impressum