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1. Mobile Horus Binary v2 and APRS Receiving Station with Raspberry Pi

   After the previous December HAB launch, I realized that I needed to spend some time on my receive hardware setup in the car. I previously had a VirtualBox VM running Horus-GUI and headless horusdemodlib, but it was very much a kludge. As an upgrade, I wanted to add APRS reception for the balloon 2 meter APRS tracker.

   In order to keep track of all the hardware required for this, I created a block diagram using draw.io desktop. Click to make bigger.

   

   The next sections will describe the different parts of this setup.

   RF Filtering

   I quickly realized that having two RTL-SDR dongles in close proximity to a 10 or 50 watt transmitter might cause problems. It would certainly desense the receivers during transmission, but it also might cause permanent damage to the dongles. Permanent damage is hard to diagnose because there's no error message saying "broken," it just doesn't receive weak signals (or any signals at all).

   Filtering the 441 MHz voice transmission out of the 144.39 MHz APRS receiver is pretty easy, as those frequencies are very ...

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2. Building a UHF Band-Stop Stub Filter

   During a recent balloon launch, I communicated with the other chase teams with my mobile rig, transmitting around 441 MHz with a transmit power of 10 watts = +40 dBm.

   For the next balloon launch, I also wanted to put a 144 MHz APRS receiver on my car, but was worried about damaging the RTL-SDR dongle I would be using for receiving. RTL-SDR dongles have a maximum input of around +10 dBm with absolutely no filtering on the input, so I could potentially permanently damage the RTL-SDR dongle when I transmitted.

   Moving and separating the antennas around on the car roof and trunk might give 20 dB of isolation, but we're still at risk of permanently damaging the receiver.

   I needed a band-stop or band-pass filter for my APRS receiver. I already had a band-pass cavity filter for 144-148 MHz, but the 3rd harmonic of 145 MHz is 432 MHz, so this filter won't really keep 441 MHz transmissions out of my 144 MHz receiver. Bummer.

   Open and Shorted Stubs

   Fundamentally, stub filters are based around the concept of constructive and ...

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3. Digital Communications Inc DCI-146-4H Band-Pass Filter

   I recently acquired a Digital Communications Inc. DCI-146-4H band-pass filter. This is a 4-pole cavity filter for the 2 meter Amateur radio band, with a pass band of 144 to 148 MHz. Physically it's about 12" long, 6" high, and 3" deep, and is very light. The connectors are UHF SO-239.

   

   I purchased this band-pass filter for operating Summits On The Air (SOTA) radio events. Operating 2 meters FM simplex on the top of mountains is a lot of fun, but many SOTA mountains also have high-powered broadcast transmitters, public safety repeater systems, etc. These strong transmitters cause desensitization in receivers, and I can't hear other stations unless they are very strong. This type of band-pass filter removes all out-of-band signals, but won't remove any in-band transmitters like amateur radio repeaters.

   Close-In Performance

   The datasheet (local pdf) shows the insertion loss at around 0.4 dB. I measured this at about 0.7 dB, and the extra loss was probably the short RG-58 jumpers and RF adapters I used.

   

   Zooming out a bit, rejection at 151 MHz is approximately 20 ...

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4. Adding a Bodnar GPS Disciplined Oscillator to a KiwiSDR

   With the release of WSJT-X 2.3.0 in September 2020, Steve Franke K9AN, Bill Somerville G4WJS, and Joe Taylor K1JT included a two new digital protocols called FST4 and FST4W. These two digital modes are the next evolution in low-power and long distance digital communications. These modes use four-level Gaussian frequency-shift keying (4-GFSK) modulation, which has smooth transitions between the four tones. This modulation is a bit easier to decode than regular WSPR, which uses 4-FSK modulation that has abrupt tone changes.

   FST4 is designed for keyboard-to-keyboard QSOs (similar to FT8), and FST4W is designed for beacon transmissions like the regular WSPR protocol that we use for picoballoons. The time periods for a single transmission are 120, 300, 900, and 1800 seconds, with longer periods requiring less SNR for decoding.

   In Spring 2022, Rob Robinett AI6VN included FST4W decoding in Version 3.0 of the wsprdaemon software, which is what I run for WSPR decoding at my home in San Francisco and in Inuvik, NWT. That's cool, I thought. I can just "check the box" in the configuration file ...

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5. Installing a WSPR Receiver in Inuvik, NWT

   I recently set up a Weak Signal Propogation Reporter receiver in the town of Inuvik, NWT. I was there in the beginning of June 2022, just as the world was opening back up from the COVID-19 travel restrictions.

   

   I thought Inuvik would be an interesting place, as it's at 68 degrees North latitude, which is above the Arctic Circle. During the summer, the sun never sets from May 25th to July 19th. In the winter, the sun doesn't rise from Dec 6th to Jan 5th. But the sun does get close enough to the horizon to have civil twilight, where the sun is less than 6 degrees below the horizon, so it's not completely dark all the time in the winter.

   From an RF perspective, I thought this would be a really interesting location for several reasons. What happens to HF propagation when the D layer is always present? Would 20 meters be alive all summer long? Conversely, what happens in the winter when it's only the E and F layers?

   Hardware

   The hardware for this receiver is almost identical ...

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6. SOTA Activation of Vollmer Peak, W6/NC-298

   The San Francisco Radio Club organized another Summits on the Air event on 23 April 2022. For this "Simul-SOTA" event, there was 11 amateur radio operators activating 9 summits around the Bay Area. Steve W1EGG made a handy map with who was activating each peak:

   

   I activated Vollmer Peak, which is at the southern end of Tilden Park in the hills above Berkeley. Parking at the trailhead was easy in the morning, but by the time I left parking was nonexistent.

   The hike up was short, and the views were excellent. I could clearly see Mt. Tamalpais across the bay, which was activated by Vlad K6VVP.

   

   A lot of the other peaks that were activated that morning were visible from from the top, including my previous activated San Pedro Mountain.

   HF Radio

   This was my first HF activation. I borrowed a Xiegu G1M radio, which is a small QRP SDR. I also borrowed a lithium power pack, and a 49:1 balun for an End-Fed Half Wave antenna. I cut a piece of two conductor zip-cord 33 feet long, which ...

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7. Automated Recording of the HamSCI WWV/WWVH Science Signal for Sunrise Festival

   I'm planning on participating in the Sunrise Festival, which is a citizen science campaign run by the HamSCI group. Over the 30 April 2022 weekend, hams from all over North America are encouraged to record the WWV and WWVH Scientific Test Signal, transmitting at 8 and 48 minutes past the hour.

   I previously used my KiwiSDR to receive these signals, and luckily there is an easy API for the KiwiSDR that will allow us to save recordings from the SDR.

   kiwirecorder.py

   kiwirecorder.py, written by John Seamons ZL/KF6VO, is a simple python program that remotely connects to a KiwiSDR and saves spectrum locally. The output file can be demodulated audio (SSB or AM) like you get from a traditional radio, or raw IQ files which are spectrum recordings. We are much more interested in these raw IQ files, as any type of demodulation (such as AM or FM) removes information from the recording.

   To check out kiwirecorder in your home folder with git clone https://github.com/jks-prv/kiwiclient. I'm running this on a standard Ubuntu 20.04 ...

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8. Receiving the HamSci WWV Scientific Test Signal

   I was perusing the latest edition of QST and noticed a short article on some scientific experiments that hams were doing with WWV and WWVH. I tune in to WWV occasionally to check propogation, and also for frequency calibration of my Kenwood TS-2000.

   

   Wow, this seems cool! The HamSCI WWV webpage has a bunch of information, including a great presentation by Kristina Collins KD8OXT.

   Receiving the Signal

   I recently added a KiwiSDR at my home station, so I twisted the dial over to WWV (in software, pretty anticlimactic), and started listening. I barely heard the test signal from Hawaii because propagation that evening was very poor. There was several solar flares over the next few days, causing lots of interference on HF.

   Eventually, on 10 April 2022 at 2208 UTC, propagation was good enough to grab a clean screenshot of the test signal. This was transmitted from WWV on 15 MHz.

   

   Comparing my received signal to the what was actually transmitted shows the voice, tones, and chirps are very easy to hear when propagation is good. But interestingly enough, I ...

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9. Simultaneous Multi-Band WSPR Decoding with KiwiSDR and WsprDaemon

   I've been interested in receiving Weak Signal Propagation Reporting (WSPR) signals recently, for two reasons. The first is that the picoballoons that we launch use WSPR for position information. Not only do I directly receive the picoballoons after we launch them, but I also use the WSPR network to check on their location around the world. I don't have a receiver in Europe, so I rely on other amateur radio operators to receive the balloon and post its location online, and I want to give back to the network.

   The second reason is that I'm curious about how far I can receive signals with my somewhat compromised city antenna. While my fan/parallel dipole antenna will never perform as well as the huge antenna farms other amateurs operate, I can work on making my station a bit better by reducing local noise or improving my antenna. Incremental changes is what I'm going for.

   Hardware Radio

   Soon after we launched the first few picoballoons, I started receiving WSPR packets with my Kenwood TS-2000 using WSJT-X. This radio is a good basic ...

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