The Hermes Lite 2 QRP SDR

The Hermes Lite 2 (HL2) is an open source Software Defined Radio project – yes, that means everything from the circuit design and PCB layout to the SDR software needed to run it are open source designs.

The radio comes as a mostly pre-built kit, consisting of two PC boards, a PCB connector and a metal case. As you can see, the radio is a small black box with connections front and back and a few LEDs to look at while you’re using it. The rear panel carries the RF outputs and has space for optional connections.

The HL2’s features include:

• Coverage from 130 kHz to 34 MHz
• TX output power of 5W
• Up to 4 ‘receivers’ active at any one time, each from 48 to 384 kHz wide
  • monitor 4 different amateur bands simultaneously
  • decode FT8 or WSPR on all 4 bands simultaneously
• Runs from 12-16v, taking around 2A max.
• Alternate low power +17dBm output (useful to feed a transverter)
• Output to key an external linear amplifier
• With some SDR software, Controlled Envelope SSB, like the Flex and ANAN radios, which doubles talk power.
• Trials ongoing to synchronise two units for optional diversity & antenna beam steering

Like the ANAN radios, the HL2 connects to a PC running SDR software via Ethernet. All audio goes in and out of the PC and all RF goes in and out of the Hermes Lite.

The radio has its roots in the openHPSDR (High Power SDR) project, the basis of ANAN radios. The main board for that project is the Hermes board and the Hermes Lite is a derivative, based on the AD9866 chip originally designed for broadband modems. It turns out the RF performance is great and, like a modem, the radio is full duplex so you see your own transmitted signal on the software spectrum display. Having full duplex also means that you can clean up TX intermod, even from an external linear, using Pure Signal which pre-distorts the signal to cancel out the intermod.

The ‘brains’ of the radio is a Field Programmable Gate Array (FPGA). Chip manufacturers often use FPGAs to test chip designs but they can also be used in small volume manufacturing. The firmware programming on the FPGA is called Gateware and can be upgraded from the PC. Aside from occasional updates there are also Gateware variants, including one receive-only version providing 10 simultaneous software receivers. (Yes, you can apparently use CW Skimmer on 10 bands simultaneously.)

The radio itself comes in the form of a small Hermes Lite board and optional N2ADR filter board and case. In reality the filter board is pretty much essential unless you are intending to build it into a box with a higher power linear amplifier. The boards both have rows of I/O pins that line up with one another and are connected together 1:1 with a PCB jumper board, which is supplied. The filter board not only contains filters to clean up the transmitter output but also an SWR bridge and power sensor and the all-important RF connections. (Output from the Hermes Lite board is otherwise on that row of pins on the right hand side of the board.)

The Hermes Lite 2’s internal parts. Radio PCB on left, HF low-pass filters & power/SWR sensor PCB on right
You can download PCB design files, order your own boards from a PCB fabricator, order a set of parts (there’s a prefilled Mouser cart you can access) and make your own. Rather more handily you can order ready-made working boards and case from a Chinese company, Makerfabs. They’re made in batches when they think there’s enough demand and they’re currently on Build 9. At current exchange rates the boards and case cost around £235 including shipping. You can expect to pay around £70 in import VAT & handling charges etc on top but there are reports some have arrived with no charges and no extra delay.

Although the boards are fully populated, tested and working there is a small amount of DIY to do. The case comes in two halves and has end panels made of PCB material which hold it together. The edges of the end panels can feel rough, so may needed a little sanding. The PCBs slide into grooves in the case, so no drilling is needed to mount them. The case is used as the PA heatsink though and a metal ‘shim’ is supplied to make thermal contact with the bottom of the case. You need to remove some paint from the case base and PCB slot where the PA is located and drill a hole for an M3 bolt to hold it in place and the board in tight contact with the case and shim. The bolt isn’t supplied and a 12mm bolt is about the right size. It is also a good idea to use a smear of heatsink compound on top and bottom of the shim and in the slot. The bottom of the case is probably dissipating most of the heat, so it is a good idea to fit some feet to allow air to circulate underneath.

Improve heatsinking by removing paint & bolting the PCB down to a shim

Once case mods are complete the PCB simply need to be slid into the case. It is a bit fiddly if you are using the supplied shim, which fits tightly between PCB and case but it is worth the effort for peace of mind. At this point the purchaser will probably wish they’d thought to order a coaxial DC connector (5.5mm OD, 2.1mm centre pin) and an inline fuseholder as neither are supplied.

There are a number of different SDR programs you can use with the HL2. SparkSDR is a great one to get you started. It discovers radio on the network immediately and as well as regular modes like SSB, CW, FM etc you can also use it for WSPR, FT8 & FT4 as long as you already have WSJT-X installed. It’s a real hoot realising you’ve got simultaneous decoding of FT8 on 4 bands at the same time! How well does it work in transmit? Well a quick 2min blast of WSPR on 40m using 2w output produced a report from Australia…

Alternate SDR software packages you can use include an openHPSDR version of PowerSDR, Thetis (also based on openHPSDR) and SDR Console. Another option is Quisk, which is optimised for CW but is also great for test purposes. There’s also a program for the Raspberry PI, PiHPSDR, which can be used as a regular program but is designed to allow connection of LCD touchscreen, rotary controller and buttons so it becomes a standalone unit. (Commercially made units based on this software are available for use with ANAN radios and cost £699.)

All very different programs, each of which make it feel like you just got a new radio!

We’ll cover software in another article but to whet your appetites, here’s a screenshot of an openHPSDR version of PowerSDR with two receivers in operation. One is tuned into 40m, the other 10m. Both are tuned into CW transmissions, with the 40m one going to the left audio channel and the 10m one to the right channel.

 73

Mike
G4RAA