I spent a few weeks in Iceland in May 2024, working near the main airport in Keflavik. The area was a US Air Force/NATO base from 1951 until 2006, when it was turned over to the Icelandic government. The US Air Force returned in 2016.

While I was there, I installed a radiosonde_auto_rx station. Iceland has one radiosonde launching site located at the Keflavik airport, and launches two radiosondes a day at 1100 and 2300 UTC. Skew-T plots are published by the Iceland Met Office after every launch.

Hardware

The hardware for this receiving site is the standard radiosonde receiving hardware. A Rasbperry Pi 3B+ and a RTL-SDR Blog v3 are the main components, along with an external antenna. All of the hardware sits on a rack shelf in the building.

Antenna

The air in Iceland is very corrosive from the volcanoes and salt spray. Any metal that isn't stainless rusts almost immediately, and even lower-grade stainless rusts after a few years. Since the receive antenna would be placed outdoors, my homemade quarter-wave ground plane antenna would be a poor choice for a long-term solution because it would not be strong enough for the brutal Icelandic winds, and would rust away immediately.

It's difficult to find commercially made quarter-wave antennas cut for ~400 MHz. Not many manufacturers are in this market, and most of the antennas in this band are high gain antennas for public safety. We don't want a high-gain omnidirectional antenna: the higher the gain, the more the radiation pattern is pushed towards the horizon, which is what we don't want when trying to receive high altitude balloons.

The closest antenna I could find was a Laird FG4060 (local PDF). It's actually a J-pole antenna, which has a radiation pattern close to a quarter-wave vertical. The "0 dB/unity gain" spec is the most important, you can infer that they aren't trying to push the RF energy any particular direction, in either the horizontal or vertical plane. Mouser has these antennas in stock.

This FG4060 antenna is actually advertised as 406-416 MHz, so I emailed them about the actual performance from 400 to 406 MHz. They responded with a S11 plot. At 402 MHz, the SWR is 2:1, which is about a 10 dB return loss. Pretty good to me, especially since the radiosonde is usually transmitting at 404.0 MHz.

Due to space and mounting constraints, the final mounting location of the antenna is not great. Antennas can be detuned by nearby conductors, so getting further away from the vertical mast would improve the system. While there is a camera and Starlink antenna mounted directly above it, that shouldn't matter too much as the J-pole has a null straight up.

Results

The receiver was installed in May 2024, and by the end of August 2024 it had received over 200 radiosondes. All (except for a few) were Vaisala RS41 radiosondes.

Maximum observed range over this time period was 140 km, and longest flight was 115 minutes (1.9 hours). The shortest distance flight was only 4km, with the radiosonde landing in downtown Keflavik (V0551040).

Zooming in on the launch site, we can see that they are launched near the end of Runway 01. Given that the RS41 radiosondes are launched within 1 minute of 1102 and 2302 UTC every day, I'm assuming that it's an autolauncher.

Zooming in on that location on Google maps, one can barely see the characteristic shape of a Viasala autolauncher, with open pedals and circular balloon filling area towards one end of the container (very similar to the Fairbanks autolauncher).

One can also see an above ground pipe to some objects on the west side, which is probably the hydrogen gas storage tanks. Just south of the presumed autolauncher is a much taller building (based on the shadow length) which was probably the original manual launching site, no longer in use.

In the previous three months, there were also four special radiosonde received by my station. Three Mateomodem M20 (local datasheet) radiosondes were launched far out in the ocean, and one of them burst at only 4000 meters.

The three radiosondes are 206-2-00338, 206-2-00228, and 206-2-00177. Additionally, one RS41 V1640688 was launched way out in the ocean, approximately 110 km from the receiving location (shown as the far left radiosonde in this map). I'm curious as to who is launching these.