If you're a follower of this blog, you'll know I have a bit of a thing for chasing high-altitude balloons, be it Project Horus launches, or Bureau of Meteorology Radiosonde launches. I've developed software (based on Zilog80's RS decoder) to automatically scan for, decode, and upload radiosonde telemetry, available here: https://github.com/projecthorus/radiosonde_auto_rx

In Adelaide, the Bureau of Meteorology are still launching the now-obsolete Vaisala RS92 Digital Radiosondes. I've posted on these previously (7 years ago?! Where did the time go?!!!). Sadly, not much in the radiosonde is re-usable, so all the sondes I've collected have been sitting on a shelf, or have been given away as souvenirs.

What has always been interesting to me (and what most people comment upon when they see one of these sondes), is the nice quadrifilar helix antenna, used by the on-board GPS receiver. Ben Zandstra (PE2BZ) had mentioned to me the possibility of using the antenna and the receive amplifiers as a L-band active antenna. Today, Will Anthony (a fellow high-altitude balloon enthusiast) and I finally gave it a go! Thanks go to Will for writing the following segment:

The methods that we've used may not be the quickest method to re-purpose an RS92 radiosonde into an L-band antenna/LNA front end. However we wanted to retain the ability to use the battery packs that come (for free!) with the radiosonde while also ensuring that we don't accidentally and unlawfully transmit with the on-board transmitter. The steps we took don't necessarily have to be performed in the order that we did them, and across the three units we modified we seemed to do it a different way each time. It kind of boiled down to who was using what tools at the time.

Speaking of tools, you're going to need some if you're following along at home. Again, you don't have to use all of what we did, or the same types of things we did, but there are few that you aren't going to want to go without. See if you can spot them as we go along. We used the following tools in hacking these up:

65W temperature controlled soldering iron

Hot-air rework station (temp and airflow controlled)

"Soldapullt" manual "solder-sucker" type vacuum de-soldering tool

Solder wick

Liquid Rosin flux

Side cutters

Wire strippers

"bell wire" single strand copper wire, commonly used in telecommunications and some network cables

Thin (0.56mm) Multicore tin/lead/silver solder

Small "Standard Slot" screwdriver

Small bag for garbage/discarded parts, wire ends, etc

RG-316 coaxial cable

SMA Male RF connector

Needle-nose pliers

Fluke 179 multimeter to check for shorts

Vaisala RS92 Radiosonde (duh!)

Some kind of SDR for testing! We tried both a RTLSDR v3 and a HackRF

Lots of components had to be removed from the main PCB. The above figure shows what was removed, and where we attached the RG316 coax for signal output. What was removed included (in no particular order):

The main CPU

The GPS receiver IC (a uBlox uN8021)

The 16 MHz crystal (next to the GPS receiver)

All of the SAW filters

The balun at the end of the filter/amplifier chain.

Note that you must retain the SOT-23-5 IC that is next to the GPS chip! This supplies regulated 3.3v to the LNAs.

The sensor and radio daughterboards were also removed, using pin-by-pin de-soldering of the pin headers connecting these to the main PCB.

Once all of the parts that need to be removed are removed, it's a good idea to check the preamp-chain rail voltage to make sure you haven't accidentally short-circuited the 3.3V rail to ground. We managed to accidentally do this when soldering on the coax - a few of the resistors surrounding where the GPS chip was are connected to the 3.3V rail - it's very easy to short circuit them with the coax shield!

Once this was all done, we plugged in a battery pack, checked all the rail voltages were correct and plugged our RTL-SDR into the output. A few minutes and some free open-source SDR software later and we were able to see signals on the first attempt. This modification method verified, we then did several more because we enjoy suffering with fiddly things and burning our fingers. So now we've got the most eco-friendly L-Band receive antenna in the neighbourhood, 100% recycled!

These antennas have been successfully used to decode Iridium signals, and observe other L-Band signals. Some screen-grabs from GQRX are below:

Update:

I was able to hack on a SMA connector to the input of the amplifier chain and get a measure of the gain. The Spectrum Analyser I have access to tops out at 1.5 GHz, but around this point I was seeing > 40dB gain.

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