Data Modes
Mike Richards G4WNC has more about the Raspberry Pi, checking SSD speeds and a neat Spectrum Analyser.
Regular readers will know that I’m always tinkering with the Raspberry Pi and on the lookout for interesting new products. I know many amateurs make use of a Raspberry Pi as a data modes terminal because it works so well. The Raspberry Pi 4 or 400 has plenty of power and makes for a very compact data modes terminal. The other benefit of using the Pi is that you don’t have to load yet more software on your already struggling PC. The Pi is also very flexible as you can completely change its operating system or load a completely new setup by replacing the MicroSD card.
While using an SD card is a great option for price and flexibility, when using the Pi as a data modes terminal, you should consider changing to a solid-state hard drive (SSD). There are a couple of benefits. The first is faster software loading, which means the Pi boots faster and programs open quickly. The other benefit is greater resilience. While I haven’t encountered problems with microSD cards, they do have a shorter life, in terms of write cycles, than an SSD. Native external hard drive support is built into the current Pi models, and the Pi-4’s full-speed USB 3 ports mean that the Pi can make good use of the greater transfer rate available from an SSD. SSD prices have also plummeted over the past year and there’s plenty of choice below £30 from online sellers. I’ve been using a HikVision 128GB SSD for a while and it works well with the Pi, Fig. 1. A 120GB model has plenty of storage and is currently priced at £23. I’ve tested the data transfer speed on a PC with CrystalDiskMark software and it returns a healthy 443GB/s and the Pi delivers similar speeds.
There is, however, a new product around that makes SSD integration even better. One of my favourite enclosures for the Pi is the Argon ONE, Fig. 2, that’s available from several of the Pi authorised dealers. The cast alloy case converts the Pi into a complete MiniPC and routes all the connections to the rear panel. This is complemented with a proper power button (at last!) and an integrated cooling system that conducts heat from the hottest chips on the Pi to the main case. To complete the cooling, the case has an integrated variable speed fan. Argon have recently launched a second version of the case called the Argon ONE V2 that is very similar to the original, but now has full size HDMI ports and a built-in IR receiver for those that like to use remote control handsets.
What really caught my attention was the new Argon ONE M.2 add-on. This is an alternative base unit that fits the original Argon One as well as the V2. As you can guess from the name, the Argon ONE M.2 adds space and connections for an M.2 format SSD storage device, Fig. 3. While most SSDs have been supplied in a standard 2.5in package, the actual electronics are very much smaller. The M.2 format is a more appropriate standard for PC storage devices and is closer to the size of a memory stick, as you can see in Fig. 4. Prices for M.2 format SSDs have also been dropping rapidly and I recently purchased a Kingston A-400 120GB M.2 drive for just £23 from a major online seller. This is an ideal size for use with a Pi data modes terminal because you get the speedier performance of an SSD plus 120GB is more than enough storage for amateur radio applications. The data and power connection between the Argon M.2 base unit and the main case is handled via a moulded USB 3-to-USB 3 connector.
Moving your existing Operating system and programs from your microSD card to the new SSD is simply a case of using the Pi SD card copier that you’ll find via the Accessories menu. Your microSD card will show up as (/devmmcblk0) and your new SSD will probably be listed as (/dev/sda). You can leave the copier settings at their default values and start the copy. When it finishes you can remove the microSD card and the Pi will boot from the SSD. NB: make sure you connect your SSD to one of the blue USB 3 ports on the Pi.
Checking SSD Drive Speeds
While on the subject of hard drive storage, I ought to mention speed testing. I’ve got a few external SSDs that I often use to move data from my main PC to my laptop. I noticed that the transfer rate, when copying files,
Fig. 1: The HIKVISION 120GB external SSD. Fig. 2: Argon One V2 Raspberry Pi case.
Fig. 3: Argon M.2 SSD base unit.
Fig. 4: M.2 SSD and memory stick.
Fig. 5: CrystalDiskMark speed test software. Fig. 6: Pi Disks software disk benchmarking tool. Fig. 7: SATSAGEN spectrum analyser main screen.
seemed to be much lower than it should be. Having looked around for speed test software, it seems that CrystalDiskMark is one of the most popular. This is free software that’s available from: https://tinyurl.com/mdwpad7a
Once installed, it’s very easy to run and the default settings are fine for most applications. You use the drop-down menu to select the desired drive and you can start a test by running the first check on the list (marked SQ1M Q8T1), Fig. 5. This provides a quick result and will tell you if you are achieving the expected rates. One point to note is that the results are shown in MB/s, that’s mega-bytes per second, so the bit rate is eight times that result. For a standard internal SSD or an external SSD using a fast USB 3 port you should see around 400-500MB/s. This test quickly revealed that my problem SSD drive was returning a lowly 43MB/s. The problem turned out to be the USB cable. Although it was a new USB-C to USB-A cable it was wired to the USB 2 standard, hence the poor transfer rate. When I changed to a USB 3.1 standard cable, my speed was restored. If you want to run a disk speed test on the Raspberry Pi it’s the Disks utility you need. This can be installed as follows:
• Open a terminal session (Ctl+Alt+T)
• Enter: sudo apt install -y gnome-disk-utilities
• When complete, you will find Disks has been added to the Accessories menu. Here are the steps to run a speed test:
• Open the program and you will see a panel that lists your connected disk drives, Fig. 6.
• Click on the drive you want to test, then select a partition.
• Just below the partition map in the righthand panel you will see the gears icon. Click on that and choose the last item, Benchmark Partition.
• Start the benchmark and accept the default settings and provide your password.
• You will then see a performance graph being created that shows the achieved transfer speed and the average access time. An SSD connected via the USB 3 (blue) ports should show a transfer speed of better than 300MB/s and a seek time of 0.25ms or less.
VHF-Microwave Spectrum Analyser
In one of my recent columns, I suggested that we, as radio amateurs, ought to be using our skills to develop radio networks that could help in times of crisis. The crisis could be anything from the increasing occurrence of flooding, through to a major internet service disruption. Whenever, a crisis situation occurs, the mobile network and the internet are put under considerable stress. Temporary data links would be a valuable resource to have available and make excellent learning projects. Developing these resources often requires access to test equipment that can
operate into the gigahertz bands, and these tend to be expensive. However, there are systems available that can fulfil the measurement needs at very reasonable prices. The one I’m going to suggest is a 70MHz to 6GHz spectrum analyser and tracking generator. This simple analyser makes use of the excellent Analog Devices ADALM-PLUTO SDR active learning module. This is available for under £200 and provides transceive capabilities from 325MHz to 3.8GHz using an AD9363 RF agile transceiver chip. While the AD9363 model is specified from 325MHz-3.8GHz, the device is performance selected from the same production line as the higher specified AD9361 and 64 devices. As it’s essentially the same chip, the AD9363 can be coaxed to work to the extended frequency range, but the performance is not guaranteed. To convert the basic Pluto into a full featured spectrum analyser you, need to run control software on the PC and the best I’ve found is SATSAGEN, which is available from: https://tinyurl.com/2mm4fca2
This is an excellent project that works with Windows 7 or later to provide the control and display facilities that convert the Pluto SDR into a 70MHz to 6GHz spectrum analyser with tracking generator. The program has been made available free of charge for noncommercial use, which is very generous of the author. With the addition of a directional coupler, the SATSAGEN can also be pressed into service as a VNA (Vector Network Analyser). Extending the frequency coverage of the Pluto to cover the full 70MHz to 6GHz can be done via the Pluto command line, but that is now done automatically the first time you run the SATSAGEN software. This is a great help for those with no command line experience. I’ve shown the main screen of SATSAGEN in Fig. 7. Here you can see the main display area and the three instrument panels are shown on the right. These provide four basic instruments all with a range of 70MHz to 6GHz:
• Spectrum analyser with variable receive bandwidth, span and gain.
• Spectrum analyser with tracking generator and variable resolution, gain, offset and calibration facility.
• Sweep generator with variable Tx power.
• Signal generator with adjustable power and modulation systems.
As you would expect in this price range, there are a few compromises. The first is the input and output impedance, which are not constant across the range. The simple solution to this is to fit a 10dB masking attenuator to the input and output SMA sockets. The other issue is the Pluto’s uneven frequency response over the 70MHz to 6GHz bandwidth. This is no surprise, especially as we’re operating the AD9363 beyond its advertised specification. However, the SATSAGEN software has this covered with a built-in calibration function. This can be used to correct both the frequency response of the Pluto, the 10dB input and output attenuators and the test leads. This means that the plotted result shows closer to the real performance of the device you’re testing. One shortfall, that cannot be calibrated out, is the crosstalk between the transmit and receive channels of the Pluto. This becomes more of a problem at the top end of the frequency range and is primarily due to the lack of screening from the Pluto’s plastic case. This can be improved by mounting the Pluto in a metal case and the internet will reveal plenty of examples of modifications to re-box and generally improve the performance of the Pluto.
That’s all I have space for this time, but I’ll give some more operating tips another time.
sweep it. The VSWR on 2m is very good, a little bit higher on 70cm, but still usable. Civil and military airbands look just fine (there is a peak of high SWR around 320350MHz) but it will be entirely adequate for reception on these bands and indeed,
Tom Morris tells me that the antenna is already widely used by airband listeners around 220-240MHz.
The antenna comes with a simple mounting bracket made of stainless steel, so that you could easily mount it to a wall. You’ll need to supply your own screws/ plugs as needed. Because the whip is relatively flexible, try and mount it so that if the whip antenna moves around in the wind, it won’t touch anything as it does − which wouldn’t help your SWR and an annoying tapping from outside on the wall might not be welcome either.
The White Knight won’t work as well on 2m/70cm as one of the larger white stick verticals. However, it is decidedly lower profile, which in some instances could be very useful. Because the stainless-steel whip is quite thin, it will not be very visible if mounted at roof level, so could prove a useful antenna where a more stealthy installation is needed. The antenna is rated at 150W (I did not test this) so should cope fine with a 50W FM mobile rig or you could try using it for vertically polarised FT8 on both 2m/70cm. It’s a simple design, but well executed.
Many thanks to Tom GM3HNN for letting me try out the White Knight antenna, which is available, priced £59, from the RadioGeeks website: www.radiogeeks.co.uk