Shortwave

1. What this guide builds

One receiver measures the antenna voltage 64.8 million times each second. So every signal below 30 MHz is in one stream of numbers. A program on an x86 PC cuts that stream into many channels, and each channel is a small stream that any program can read.

This guide stops at a working receiver and one channel that you hear. The decoders are in my other guides: aircraft over the oceans (HFDL), WSPR and FT8, weather by radio, the other shortwave signals, and the sun. Each one adds channels to the receiver that you build here.

 outdoors                 cable entry                                  the PC named hf
 --------                 -----------                                  ---------------
 loop with      RG-6      power       30 MHz        RF         RX-888 Mk II      radiod (ka9q-radio)
 amplifier ----------- injector --- low-pass --- limiter --- HF input ---- USB 3 --- one stream, 0 to 30 MHz
 in its head            (DC goes     filter                  (own USB 3            |
                         up the                               socket, no hub)      +-- channel -- program
                         cable)                                                    +-- channel -- program
                                                                                   +-- channel -- your ears

Three terms that this guide uses:

• Direct sampling. The receiver does not tune. It measures the antenna voltage directly, and software selects the signals later.
• Channel. One slice of the stream: a frequency, a mode and a width. ka9q-radio makes many channels at the same time.
• Multicast group. A network address that a program can subscribe to. Each group of channels goes to one such address. One more subscriber costs the receiver nothing.

2. What you need

A part marked ◆ is what I use. A blank box is for your pencil. Prices are US dollars unless the cell says otherwise. Each price has its seller and the date I read it.

Antenna

Filter and protection

Radio

Computer

Cable and small parts

Lowest-cost path

Section 15 gives the steps and the limits. It is a different station: one tuned radio that hears one slice at a time.

3. Before you start

Skills. You connect coax parts with SMA connectors. You install Linux on a PC from a USB stick. You type commands in a terminal over SSH and edit a text file. No soldering is necessary.

Time. I give no total, because I did not time my build. One step has a known length: the speed tables in section 10 took 36 minutes on my PC. The ka9q-radio documents say that step can take hours.

Accounts. None. Nothing in this guide sends data to the internet.

Values to choose first.

The computer has the name hf in this guide. The receiver instance also has the name hf. So the service is radiod@hf and the control address is hf.local.

The law. This guide receives only. It tells you to transmit nothing. I am not a lawyer, and this is what I read for the United States. US law (47 U.S.C. section 605) says that a person who receives a radio message not meant for the public must not tell others its contents. It makes exceptions for broadcasts to the public, for amateur and CB radio, and for messages about ships, aircraft or people in distress. Other countries differ.

4. The antenna

The antenna is a small loop with an amplifier in its head. I use the MLA-30+. Power reaches the amplifier through the signal cable, from a small box named a power injector.

Where the loop must stand

1. Put the loop outdoors.
2. Put it as far from the electronics in your house as you can. Power supplies, network gear, lamps and computers make most of the noise on shortwave.
3. Do not buy a tall mast for it. For a loop like this, height matters little.
4. Keep it away from metal fences and gutters if you can.
5. Mount it so that you can turn it by hand. A small loop has a null, and you can turn the null toward a noise source.

Distance is the rule that matters. The designer of one well-known loop amplifier says to put the antenna at least several meters from electrical equipment. He ran his own tests with the loop center about 2 m above the ground.²

My own loop is still indoors, next to the computer. That is the worst place for it. Even there, the antenna noise is 20 to 30 dB above the receiver’s own noise. So the receiver is not the limit. The location of the antenna is the limit.

Run the cable

1. Connect the RG-6 cable to the loop head with an F to SMA adapter.
2. Put one snap-on ferrite on the cable near the loop head.
3. Run the cable to the place where it enters the house.
4. Put one snap-on ferrite on the cable near the indoor end.

Result: one cable from the loop reaches the receiver’s location. 10 to 20 m of RG-6 loses under 1 dB on shortwave.

Ground the cable

An outdoor antenna cable needs a path to ground where it enters the building. In the US the rule is National Electrical Code Article 810. Bond the grounding block to the building’s own grounding electrode system. If you add a ground rod, bond it to that system with 6 AWG copper or heavier. Never use a rod that stands alone: a surge then goes through your equipment. I read a published summary of the rule, not the code text.³ Use a licensed electrician if you are not sure.

5. The radio-frequency chain

Build the chain in this order, antenna first.

loop -- RG-6 -- power injector -- F3060 (30 MHz low-pass in,
        shelf filter bypassed) -- RF limiter -- RX-888 Mk II, HF input

Caution. Direct current on the wrong part can damage it or stop the signal. Put the power injector first on the loop cable. Add no second source of direct current. The ka9q-radio document for this receiver lists no setting for a bias tee.

1. Connect the loop cable to the antenna side of the power injector.
2. Connect the radio side of the power injector to the input of the F3060.
3. Set the F3060 to its 30 MHz low-pass filter.
4. Set the F3060 shelf filter to bypass. The F3060 manual shows how.
5. Connect the output of the F3060 to the RF limiter.
6. Connect the RF limiter to the HF input of the RX-888 Mk II.
7. Connect the power supply of the injector last.

Result: the loop has power, and no direct current reaches the filter, the limiter or the receiver.

The reasons for the order:

• Injector first. Every part after it then sees no direct current. I treat the limiter as a part that blocks it.
• Low-pass filter before the limiter. A limiter is a diode, and it makes spurious signals when it clamps. TAPR says to filter ahead of it.
• The 30 MHz filter is not optional. The receiver takes 64.8 million samples each second. So it cannot tell a signal above 32.4 MHz from one below it. A signal from 32.4 to 64.8 MHz folds back onto shortwave. The receiver’s own filter is at 64 MHz, so it does not help.
• What the F3060 does. By its manual, it is 36 dB down at 32.5 MHz and 64 dB down at 51 MHz. It costs 3.7 dB at 28 MHz.
• Leave the shelf filter bypassed. It cuts 28 dB below 1 MHz and 20 dB at 3.5 MHz. Turn it on only if strong AM broadcast stations overload the receiver at night.
• No amplifier. The loop has one. Add another only to pay for a loss that you measured.

Keep the receiver in its case, and keep air around it. It runs hot.

6. The computer

I can only state what I measured. The table says which numbers are mine and which are the project’s.

The ka9q-radio documents say that a Raspberry Pi 5 can also run this receiver at 64.8 million samples each second, “but just barely”. They say that it leaves little for decoders. I did not test a Pi 5. That is why this guide uses an x86 PC.

Prepare the PC

1. Install Ubuntu Server 24.04 LTS. Set the host name to hf. Turn SSH on.

2. Connect hf to your router or switch with an Ethernet cable.

3. In the PC’s firmware settings, set it to start again after a power cut.

4. Give hf a fixed address on your network, set on the computer itself.

5. Log in with SSH and update the system.

   sudo apt update && sudo apt full-upgrade -y && sudo reboot
   

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   Result: the PC starts again, and you can log in with ssh <you>@hf.local or with its fixed address.

7. Install ka9q-radio

Receiver program: ka9q-radio, by Phil Karn. It is free and open source (GPL-3.0). Its radiod service reads the whole stream from the receiver and makes the channels. Nothing here uses Docker.

The project changes every week, and some of its documents are behind the code. These steps build the same version that my station runs (commit b5bbb9aa, 2026-09-25). The project also offers ready-made packages. I have not used them.

1. Add the author’s package source. Two of the libraries are only there.

   sudo wget -P /usr/share/keyrings http://www.ka9q.net/debian/ka9q-radio.asc
   echo "deb [signed-by=/usr/share/keyrings/ka9q-radio.asc arch=amd64] http://www.ka9q.net/debian noble main" \
     | sudo tee /etc/apt/sources.list.d/ka9q.list
   sudo apt update
   

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   Result: apt update ends with no error about ka9q.net.

2. Install the build tools and libraries. The list is from the project’s docs/INSTALL.md, plus the FFT tools and the clock service.

   sudo apt install -y git avahi-utils build-essential make gcc libairspy-dev libairspyhf-dev \
     libbsd-dev libfftw3-dev libfftw3-bin libhackrf-dev libiniparser-dev libncurses-dev libopus-dev \
     librtlsdr-dev libusb-1.0-0-dev libusb-dev portaudio19-dev libasound2-dev rsync libogg-dev \
     libsamplerate0-dev libliquid-dev libncursesw5-dev libbladerf-dev libhydrasdr-dev libfobos-dev \
     libcsv-dev libsoapysdr-dev chrony
   

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3. Build and install.

   cd ~ && git clone https://github.com/ka9q/ka9q-radio.git && cd ka9q-radio
   git checkout b5bbb9aa
   make -j && sudo make install
   

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   Result: the programs radiod (in /usr/local/sbin) and control, monitor, tune and pcmrecord (in /usr/local/bin) exist.

4. Add yourself to the group radio.

   sudo adduser $(whoami) radio
   

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5. Turn on multicast for the computer’s internal network.

   sudo systemctl enable --now set_lo_multicast
   

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6. Log out. Log in again.

   Result: the command groups shows radio in its list.

The install also creates the user radio, the service files in /etc/systemd/system, the settings folder /etc/radio, and the device rules in /etc/udev/rules.d.

8. Connect the receiver

The RX-888 Mk II has no program of its own in permanent memory. The computer loads a small program, the firmware, into it after each power-on. You do not download the firmware. It comes with ka9q-radio, and a device rule from the install loads it when you plug the receiver in.

Caution. A hub or a USB 2 socket cannot carry 1 gigabit each second. radiod then drops data. Use a USB 3.0 socket on the PC’s main board, and give the receiver that socket alone.

1. Connect the chain from section 5 to the HF input of the receiver.

2. Connect the receiver to a USB 3.0 socket on the PC. These sockets are usually blue.

3. Check that the firmware loaded.

   lsusb | grep 04b4          # expect 04b4:00f1 "RX888mk2". 00f3 means the firmware did not load
   

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4. Check the USB speed.

   lsusb -t | grep -B2 usbfs  # the receiver's line must end in 5000M
   

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   Result: the receiver is at 04b4:00f1, and its line ends in 5000M. A line that ends in 480M means a USB 2 connection.

9. The configuration file

radiod reads one file for each receiver: /etc/radio/radiod@hf.conf. The file has a section [global], a section for the hardware, and one section for each group of channels.

1. Copy the project’s example.

   sudo cp ~/ka9q-radio/config/examples/radiod@rx888-generic.conf /etc/radio/radiod@hf.conf
   sudoedit /etc/radio/radiod@hf.conf
   

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2. Make the first two sections read as follows. Leave the rest as it is for now.

   [global]
   hardware = rx888
   status = hf.local
   data = hf-pcm.local
   samprate = 12k
   mode = usb
   
   [rx888]
   device = "rx888"
   description = "<NAME>"
   samprate = 64m8
   gain = 10
   

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3. Find the section [WSPR]. Delete the two frequencies above 30 MHz from its freq line: 50m293000 and 144m488500.

4. Save the file.

What each line does:

Four rules for this file:

• Always write samprate = 64m8. One of the project’s default files uses the full rate, 129.6 million. That makes more heat and needs a faster PC.
• Write frequencies with a unit letter. 7m074 is 7.074 MHz and 518k is 518 kHz. A number with no letter is a guess: in ka9q-radio, 22500 means 22.5 MHz. For 22.5 kHz, write 22k5.
• A wrong word gives no error. radiod ignores a line that it does not know. Check your spelling.
• disable = yes turns a section off. The example has many sections that are off. A section with no such line is on.

The example has gain = 20. The project’s document on this radio says the default is 10 dB and that its author generally uses +10 dB. I run 10 dB.

10. Speed tables and the first start

radiod sorts the whole stream by frequency with one large calculation, about 50 times each second. The FFTW library does that calculation. It can measure the fastest method for your processor and keep the result in a file. The project names that file “wisdom”. I name it the speed tables.

1. Make sure that radiod does not run.

   systemctl is-active radiod@hf        # expect "inactive"
   

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2. Make the speed tables. This took 36 minutes on my Core i5-4570. Do not use the PC for other work during it.

   cd /etc/fftw && sudo touch nwisdom && sudo chown $(whoami):radio nwisdom wisdomf 2>/dev/null
   time fftwf-wisdom -v -T 1 -o nwisdom rof1620000 cob1920 cob1200 cob960 cob800 cob600 \
     cob480 cob320 cob300 cob200 cob160 cob150
   ls -l nwisdom && cp nwisdom wisdomf
   

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   Result: the file /etc/fftw/wisdomf exists and is not empty.

   Do not leave out -T 1. Do not copy the file from another computer. The first name, rof1620000, is the large calculation for 64.8 million samples each second. The other names are the small ones that the channels use.

3. Start the receiver.

   sudo systemctl enable --now radiod@hf
   systemctl is-active radiod@hf        # expect "active"
   

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   The service name is radiod@hf, not radiod@hf.conf.

4. Read the start messages.

   systemctl status radiod@hf
   

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   Result: the state is active (running), and the time since the start goes up. If the time stays near zero, the service starts again every 5 seconds. Then go to section 17.

To test a changed file by hand, stop the service and run radiod -v /etc/radio/radiod@hf.conf. It prints its errors on the screen. Press Ctrl+C to stop it, then start the service again.

11. Set the gain and check the stream

The program control shows a live panel for one channel and for the receiver.

1. Start the panel.

   control hf.local
   

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   Result: a list of channels with the columns SSRC, preset, samprate, freq, Hz, SNR and output channel. The last line asks you to choose an SSRC. An SSRC is the number that identifies one channel.

2. Type one SSRC from the list. Press Enter.

   Result: the panel opens. It has a row A/D with a level in dBFS, a row Drops, and a row Overranges.

3. Read the A/D level. The project’s rule is an average of -20 to -25 dBFS.

4. If Overranges keeps rising, lower gain in the file by 3 dB. Then run sudo systemctl restart radiod@hf.

5. Read Drops. It must stay at 0.

6. Press q to leave the panel.

I run 10 dB of gain. That gives about -32 dBFS here, below the project’s rule. It leaves room for strong stations at night.

Test that the antenna sets the noise

1. Open the panel on a channel with no signal. Note the noise level in the row N₀.
2. Disconnect the cable at the receiver’s HF input.
3. Connect the 50 ohm terminator to the HF input.
4. Read the noise level again.
5. Remove the terminator. Connect the cable again.

Result: the noise falls by 6 dB or more with the terminator. On my station it fell by 30 to 40 dB at first, and by 20 to 30 dB after I lowered the gain. If it does not fall on some band, the receiver sets the noise there. Then a better antenna location does not help on that band, and an amplifier may help.

If the cursor in the panel moves to the wrong digit, set a locale that groups digits: export LANG=en_US.UTF-8.

12. The first channel: a time signal

The example file has a section for WWV, the time signal of the US standards institute. It sends on 2.5, 5, 10, 15 and 20 MHz with plain AM. A sixth transmitter on 25 MHz is experimental and can be off. It is a good first test in North America: one of the six frequencies is open at most hours. Outside North America, put a strong broadcast station near you in the freq line.

Add the channels

1. Open the file.

   sudoedit /etc/radio/radiod@hf.conf
   

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2. Find the section [WWV]. Delete its line disable = yes. The section then reads:

   [WWV]
   data = wwv-pcm.local
   mode = am
   freq = "2500k 5000k 10000k 15000k 20000k 25000k"
   

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3. Restart the receiver.

   sudo systemctl restart radiod@hf
   

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4. Look at the new channels.

   control hf.local
   

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   Result: the list has six more lines with the preset am. Their SSRC numbers are 2500, 5000, 10000, 15000, 20000 and 25000. radiod makes the SSRC from the digits of the frequency.

5. Read the column SNR for the six lines. The line with the highest number is the best one at this hour.

6. Type the SSRC of the best line. Press Enter. Watch the SNR row for one minute.

Result: the signal-to-noise ratio stays above zero and moves slowly, as shortwave signals fade. You received a station.

Listen to it

The program monitor plays a multicast group through a sound output. A server has no speakers, so choose one of three methods. I did not run method A or method B as written.

Method A: headphones in the PC.

1. Connect headphones to the audio socket of hf.

2. Start the player.

   monitor wwv-pcm.local
   

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   Result: a list of the six channels with frequency, mode and signal-to-noise ratio. You hear all of them mixed.

3. Select a line with the up and down arrow keys. Press m to mute it. Press u to unmute it.

4. Press h for the list of keys. Press Q to stop the player.

If there is no sound, run monitor -L. It lists the sound outputs that the program sees.

Method B: record a file and play it on another computer.

1. Record for about 30 seconds.

   mkdir -p ~/rec && pcmrecord -d ~/rec wwv-pcm.local
   

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2. Press Ctrl+C.

3. Copy the files in ~/rec to your own computer. Play one.

   Result: one sound file for each channel. You hear a tick each second, and a voice that says the time.

Method C: listen on another computer on your network. This needs section 13.

Make a channel at run time

A channel does not need a line in the file. control can make one while radiod runs. This works because [global] has a data line and a mode line. I did not run these steps as written.

1. Run control hf.local.
2. At the question, type an SSRC that is not in the list, for example 7200. Press Enter.
3. Press f. Type a frequency, for example 7m200. Press Enter.
4. Press m. Type a mode, for example lsb. Press Enter.
5. In a second terminal, run monitor hf-pcm.local.

Result: a channel that you tune by hand. The arrow keys change the digit under the cursor. Press h in the panel for the list of keys.

13. The streams on the network (optional)

Each channel leaves radiod as a stream to a multicast group. A program subscribes to the group by its name, such as wwv-pcm.local. radiod announces the names on the network with multicast DNS (the Avahi service).

By default nothing leaves the PC. The setting ttl is 0, so the streams stay on the computer’s internal network. Every decoder in my other guides runs on hf itself, so this is enough. Leave it at 0 unless you need the streams on another computer.

The trap with home switches and Wi-Fi

Caution. With ttl = 1, every stream goes onto your home network. A low-cost switch with no management sends multicast traffic to every socket. A Wi-Fi base station then tries to send it over the air. The ka9q-radio documents say this can cause lost traffic and erratic operation on your entire network. Read this section to the end before you change ttl.

The size of the traffic decides if you have a problem. One voice channel at 12 kHz with 16-bit samples is 192 kilobits each second. The six WWV channels are about 1.2 megabits each second. The channels for the decoders in the later guides are wider, and there are many of them.

Do this before you set ttl = 1:

1. Connect hf to a managed switch that has IGMP snooping, and turn that function on. IGMP snooping is a function that sends a multicast group only to the sockets that ask for it.
2. If you do not have such a switch, turn off every channel section that you do not need on the network.
3. Do not subscribe from a computer on Wi-Fi.
4. For a channel that you only listen to, add the line encoding = opus to its section. The project gives about 15 kilobits each second for sideband voice with this setting. Do not use it for a channel that a decoder reads.

Send the streams to the network

1. Find the name of the wired interface.

   ip address show
   

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2. Add two lines to [global] in /etc/radio/radiod@hf.conf.

   ttl = 1
   iface = <IFACE>
   

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   The iface line is necessary only if the PC also has Wi-Fi. If Wi-Fi starts first, Linux can send all the streams there.

3. Restart the receiver.

   sudo systemctl restart radiod@hf
   

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4. On the second computer, a Linux computer on wired Ethernet, do steps 1 to 4 of section 7.

5. On the second computer, run the player or the panel.

   monitor wwv-pcm.local
   control hf.local
   

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   Result: the same list and the same sound as on hf.

If the second computer has a firewall, allow 5004/udp (the streams) and 5006/udp (status and commands). With the ufw firewall of Ubuntu the commands are sudo ufw allow 5004/udp and sudo ufw allow 5006/udp.

Anyone on your network can then listen to the streams and send commands to radiod. I found no password or other access control in the project’s documents. Do not set ttl above 1, and do not forward these streams to the internet.

14. Run it unattended, and the load

The command systemctl enable in section 10 already makes radiod start with the PC. If radiod fails, the service starts it again after 5 seconds.

1. Restart the PC.

   sudo reboot
   

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2. Log in again after two minutes. Check the service and the USB speed.

   systemctl is-active radiod@hf        # expect "active"
   lsusb -t | grep -B2 usbfs            # expect 5000M
   

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3. Check the clock service. The decoders in the later guides need correct time.

   chronyc tracking | grep "System time"
   

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   Result: an offset far below one second.

The load on my computer

The large calculation is the main cost, and it does not grow with the number of channels. One more channel adds little. One more subscriber to a channel adds nothing to radiod.

Do not run other heavy work on hf. The ka9q-radio documents report lost data on a small computer when other programs took the processor.

15. Lowest-cost start: one tuned radio

You can try shortwave for the price of one small radio before you buy any part in section 2. This is a different kind of station. Be clear about what it is.

What you do

1. Buy one tuned radio that covers shortwave. The lowest-cost one in the table in section 2 is the RTL-SDR Blog V4L.
2. Connect it to the best antenna you have, outdoors. The rules in section 4 apply.
3. Install the driver that the radio’s maker gives for your system.
4. Install one SDR program with a screen on the computer you own. I name SDR++ because it is free and runs on Windows, Linux and macOS.
5. Tune to 10 MHz, AM. Listen for the time signal.

I have not tried the V4L on shortwave. Its seller says that it needs updated drivers and that some programs do not support it yet. Read the seller’s page before you buy.

What this path cannot do

• It hears one slice. A tuned radio sees a slice from a few hundred kilohertz to about 2 MHz wide. Shortwave is 30 MHz wide. The radio I first planned to use sees less than 1 MHz.
• Jobs take turns. One kind of signal now, another kind later. The bands open and close with the sun, so a job that waits its turn misses traffic.
• The later guides do not fit it as written. They assume that every channel exists all day. With one slice, you can run one decoder on one band at a time.
• A low-cost radio has less range between weak and strong. The ka9q-radio documents say that the main drawback of an RTL radio is its 8-bit converter. Strong broadcast stations at night can cover weak signals.

The middle path

ka9q-radio also runs tuned radios. It has a driver for the Airspy HF+. Its example file radiod@airspyhf-generic.conf runs that radio 768 kHz wide. So sections 7 and 9 to 14 work with that radio, for the channels that fit in one slice. The project’s own example turns on one WSPR frequency for this reason. I did not run this. The project’s document for RTL radios lists a range that starts at 24 MHz. So I do not know if a V4L works with ka9q-radio on shortwave.

16. Check that it works

Then note the A/D level and the N₀ level on one quiet frequency, by day and by night. Those numbers are your baseline. When you move the loop, compare with them.

17. If it does not work

18. What I did wrong

• I planned for a narrow radio. My first plan used a radio that sees less than 1 MHz at a time. The jobs had to take turns: weak beacons, then weather charts, then aircraft at night. I wanted all of them, all day. One wide receiver removed the schedule.
• My loop is still indoors, next to the computer. That is the worst place for it. The station works, and the antenna noise is still 20 to 30 dB above the receiver’s own noise. The fix is distance from the house, not a new part.
• I started with more gain than I needed. I lowered it to 10 dB. The antenna still sets the noise, and strong stations at night have room.
• I wrote a frequency with no unit letter. 22500 meant 22.5 MHz, not 22.5 kHz. The channel was silent, and radiod gave no error.

19. Not run and not confirmed

• No step in this guide was run on a new, empty computer as written. The settings are the ones on my station, checked against the project’s documents on 2026-10-10.
• That the newest ka9q-radio builds with these exact steps. I built the commit named in section 7. The project had new commits on the day of this check.
• The ready-made ka9q-radio packages. I have not used them. The project’s own source list file now selects Debian 13 by default, and you must edit it for Ubuntu.
• Which firmware file the device rule loads at the commit named in section 7. Today’s code has a setting file for it, /etc/radio/rx888_bootfile.conf. On my station the receiver shows 04b4:00f1 after I plug it in.
• The line data = hf-pcm.local in [global], and section 12, “Make a channel at run time”. The project’s documents describe both. I did not run them as written.
• Section 12, methods A and B, and the names of the files that pcmrecord writes. I read the options in the program’s source, and I did not run these commands as written.
• The six WWV channels as a first test. The section is in the project’s example file. I do not state which of the six you hear at your location.
• The chronyc command in section 14. It is a standard command of the clock service. I did not run it for this guide.
• Section 13 as a whole. On my station ttl is 0 and every decoder runs on hf. The warnings about switches and Wi-Fi are from the ka9q-radio documents. The ufw commands are standard Ubuntu commands that I did not run for this purpose.
• That the RX-888 Mk II has a bias tee on its HF input, and what state it is in at power-on. The maker’s page does not mention one.
• That the TAPR RF Limiter blocks direct current. Its product page does not say. So the injector goes before it in every case.
• The F3060 figures in section 5 are from its manual, which I read on 2026-10-07. Its product page gives no figures. I did not measure them. I also do not describe how to bypass the shelf filter: read the manual.
• The loss of the RG-6 cable. I did not measure it.
• The rule on loop height and distance. It comes from two loop designers. I re-read one page on 2026-10-10. I did not test it, because my loop is still indoors.
• A Raspberry Pi 5 for this receiver. The project says it works “just barely”. I did not test it.
• The price of the RX-888 Mk II, and the US price of the MLA-30+.
• Everything in section 15. I have not used an RTL-SDR Blog V4L or SDR++ on shortwave, and I did not run ka9q-radio with an Airspy HF+.
• The load figures in section 14 are from my computer on the named days. A different processor gives different numbers.
• The block time of 20 ms (“about 50 times each second”) is the default in the project’s documents. I did not read the value on my station.

20. Sources

Read on 2026-10-10, unless the line gives another date.

ka9q-radio. https://github.com/ka9q/ka9q-radio: README.md, docs/INSTALL.md, docs/SDR/rx888.md, docs/SDR/rtlsdr.md, docs/SDR/airspy.md, docs/FFTW3.md, docs/ka9q-radio.md (the [global] section), docs/ka9q-radio-2.md (hardware), docs/ka9q-radio-3.md (channels), docs/NETWORK-NOTES.md, docs/notes.md, config/examples/radiod@rx888-generic.conf, config/examples/radiod@airspyhf-generic.conf, ka9q-sources.list, share/help.txt and share/monitor-help.txt (the keys), the service files radiod@.service.in, set_lo_multicast.service.in and rx888_boot.service.in, the device rules 70-rx888-boot.rules and 71-rx888.rules, and the option lists in src/control.c, src/monitor.c, src/tune.c, src/pcmrecord.c and src/rx888.c. The manual pages for control, monitor, tune and pcmrecord are empty templates today. License: GPL-3.0 (read 2026-10-09).

The full-station guide. Build the station, sections 2, 3, 4, 11 and 17. Its commands were checked against my station on 2026-10-07.

Parts. RX-888 Mk II: https://www.rx-888.com/rx/. TAPR RF Limiter: https://tapr.org/product/rf-limiter/. Turn Island Systems F3060: https://turnislandsystems.com/product/f3060-filter-shelf-30-60-mhz-lpf/ (the page today, the manual on 2026-10-07). RTL-SDR Blog V4L: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/. MLA-30+: the listing linked in section 2 (2026-10-07).

Antenna location. Chavdar Levkov, “Wideband Active Small Magnetic Loop Antenna”: https://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm.

The lowest-cost path. SDR++: https://github.com/AlexandreRouma/SDRPlusPlus (its README).

Background. NOAA on the ionosphere: https://www.swpc.noaa.gov/phenomena/ionosphere (linked from the episode, not re-read today). Heat in the RX-888: https://ka7oei.blogspot.com/2023/08/improving-thermal-management-of-rx-888.html (read 2026-10-07).

The test signal. NIST, radio station WWV: https://www.nist.gov/pml/time-and-frequency-division/time-distribution/radio-station-wwv.

Rules. 47 U.S.C. 605: https://www.law.cornell.edu/uscode/text/47/605 (read 2026-10-07).