Hermes Lite 2 as R-390A Companion
Panadapter Integration Without Modifying the Receiver, and HL2 Transmit for CW, SSB and AM

Section 1 — The Hermes Lite 2: Architecture and Capabilities

The Hermes Lite 2 (HL2) is an open-source, open-hardware HF transceiver based on the HPSDR (High Performance Software Defined Radio) protocol, developed by Steve Haynal KF7O. It is a hardware/software system: the HL2 board handles RF conversion, ADC/DAC operations, and signal routing; all demodulation, filtering, and signal processing is performed by compatible software on a connected computer. The HL2 communicates with the computer via 1 Gigabit Ethernet using the standard OpenHPSDR protocol.

Why the HL2 Pairs Well With the R-390A

The pairing works because the two radios have complementary strengths with no significant overlap in their limitations:

• The R-390A’s mechanical filter provides selectivity that no software filter of equivalent simplicity matches: the crystal-lattice passband shape factor and ultimate attenuation are properties of physical precision manufacturing, not DSP parameter selection. On a crowded HF band, the R-390A’s mechanical filter provides clean audio in conditions where software selectivity is fighting against dynamic range limitations.
• The R-390A’s PTO provides frequency stability and dial accuracy that a software VFO matches only with careful calibration. The R-390A knows where it is tuned with high precision.
• The HL2’s panadapter shows the operator what is happening on the band around the R-390A’s tuned frequency — a capability the R-390A cannot provide. Seeing a DX station appearing on the waterfall before it is strong enough to copy in the R-390A’s audio is a practical operating advantage that is difficult to overstate.
• The HL2’s transmit capability completes the station: the R-390A is receive-only, and adding the HL2’s DSP-based SSB/CW/AM exciter gives the operator a complete HF station without adding another valve transceiver to the bench.

Section 2 — Integration Approach 1: RF Antenna Sharing (No Receiver Modification)

The primary integration approach uses a passive RF power splitter to share the antenna between the R-390A and the HL2. No modification to the R-390A is required. The R-390A and HL2 both receive the same RF from the antenna; the HL2 provides the wideband panadapter view while the R-390A handles precision narrow-band demodulation of the frequency the operator is working.

The RF Splitter: Selection and Characteristics

The RF splitter is a passive two-way power divider that connects the antenna to both the R-390A and the HL2 simultaneously. For this application, an appropriate splitter must:

• Cover the complete HF frequency range (approximately 1.8–30 MHz)
• Present 50 Ω to the antenna and to each radio output port
• Provide adequate port-to-port isolation (minimum 20 dB preferred) to prevent the HL2 from loading the R-390A’s input stage and vice versa
• Have insertion loss consistent with the R-390A’s sensitivity specification (every 3 dB of splitter loss raises the effective noise figure by 3 dB)

Suitable options include: Mini-Circuits ZFRSC-42 (or similar), a home-made toroid-wound 1:2 RF transformer on a Fair-Rite or similar core, or a commercial two-way HF antenna switch with simultaneous output capability. The ZFRSC-42 is the community-preferred option: it is inexpensive, provides flat response across HF, and maintains good port isolation. Insertion loss of approximately 3.5 dB per port is acceptable for this application.

HL2 Frequency Alignment with the R-390A

The HL2’s SDR software panadapter displays a frequency window centred on the HL2’s current VFO frequency. To use the HL2 as a panadapter for the R-390A, the HL2 software must be set to the same frequency as the R-390A’s current tuning. Since the R-390A uses a mechanical PTO with a vernier dial, there is no electronic frequency readout to synchronise automatically with the HL2 software; the operator must manually set the HL2 software VFO to match the R-390A dial reading.

For a 384 kHz display bandwidth (a typical HL2 configuration), the HL2 software shows approximately ±192 kHz around its centre frequency. With the centre set to the R-390A’s tuned frequency, the panadapter shows the full 1 MHz band segment the R-390A is operating in plus surrounding activity. A signal visible on the panadapter can be tuned to on the R-390A by reading the frequency from the SDR display and rotating the R-390A’s PTO dial to match.

Section 3 — Integration Approach 2: CASCADE-390 IF Tap (Enhanced Integration)

The antenna-sharing approach provides a wideband panadapter view centred wherever the operator sets the HL2 software. A tighter and more useful integration is possible by tapping the R-390A’s 455 kHz second IF and feeding it to the HL2 (or a dedicated panadapter SDR), providing a panadapter that is centred on exactly the frequency the R-390A is tuned to and shows only the spectral content that has passed through the R-390A’s first conversion and RF bandpass filters.

The CASCADE-390 kit, designed for the R-390A/URR by VK6ADA, provides a purpose-built IF tap with three selectable extraction points:

• Pre-filter tap: extracts the IF signal before the Collins mechanical filter, showing the full bandwidth of the first conversion. This provides the widest spectral view centred on the R-390A’s tuned frequency — useful for monitoring activity in the band segment without the mechanical filter’s selectivity restricting what is displayed.
• Post-filter tap: extracts the IF signal after the Collins mechanical filter. The panadapter shows exactly what the R-390A is receiving through its selected bandwidth: a direct visual representation of what is passing through to the audio stage. This is the most diagnostic tap for understanding what the mechanical filter is doing.
• RF tap: a high-impedance sample of the signal at the antenna input, similar in principle to the splitter approach but integrated with the chassis.

Section 4 — SDR Software Setup for the HL2 + R-390A Combination

SDR Console v3

SDR Console v3 (Simon Brown G4ELI) is the most widely used Windows-compatible SDR application for the HL2 and provides the most polished panadapter display. In the R-390A companion configuration, SDR Console is used primarily for the panadapter view and for transmit control; the R-390A handles receive audio.

• Add the Hermes Lite 2 as a device: in SDR Console, select Definitions → Radios → Add → HPSDR/Hermes Lite 2. Enter the HL2’s IP address (typically obtained from the network DHCP server or set statically to match your network).
• For the RF antenna-sharing approach: set the SDR Console receive frequency to match the R-390A’s tuned frequency. Use the panadapter display in SDR Console for band awareness.
• For the IF tap approach: set the SDR Console receive frequency to 455 kHz and apply a frequency offset calibration to display the correct HF frequency. The offset value is the R-390A’s current operating frequency minus 455 kHz.
• Configure the audio output in SDR Console to your computer’s speakers or a dedicated monitor output if you want both R-390A audio and SDR Console audio available. For the integrated station, the R-390A audio is primary.
• Transmit (CW, SSB, AM): SDR Console v3 supports full transmit operation through the HL2. Configure the TX audio input (microphone for SSB, or internal audio for AM), enable the TX path, and set up the PTT/VOX controls. The HL2’s relay output can be connected to the T/R switch for antenna switching.

Thetis (OpenHPSDR)

Thetis is the native OpenHPSDR application for Windows, providing full transceiver functionality with the HL2. It has a more complete ham radio transceiver emulation interface than SDR Console and is the preferred software for operators who want the HL2 to function as a complete standalone transceiver alongside the R-390A.

• Thetis detects the HL2 automatically on the local network after the initial network configuration. The HL2 should be on the same subnet as the computer running Thetis.
• The Thetis panadapter display is highly configurable in terms of span and colour. For the R-390A companion role, configure the display span to cover the full band being monitored (typically 200–500 kHz for casual monitoring).
• Thetis supports CW keying via a hardware keyer connected to the computer’s serial port (COMx) or via software keying. The CW sidetone is generated in software and played through the computer audio.
• For SSB transmit, configure the microphone input in Thetis to use the computer’s sound card microphone input. Thetis handles all DSP processing for SSB generation.

Quisk

Quisk is a Python-based, cross-platform SDR application that supports the HL2 via its HPSDR interface. It is the preferred option for Linux users and for operators who want a scriptable, extensible SDR control environment. Quisk’s interface is functional rather than polished; the panadapter display is adequate for band awareness but less visually refined than SDR Console. Quisk is particularly useful for WSJTX/FT8 integration with the HL2, as it has a well-documented CAT interface that WSJTX can use for frequency control.

Section 5 — T/R Switching and R-390A Input Protection

T/R Switch Options

Three T/R switching approaches are practical for the R-390A + HL2 combination, in order of increasing complexity and capability:

HL2 PTT Output

The Hermes Lite 2 provides a PTT relay output (an open-collector transistor that pulls low when transmitting) that can drive the coil of a suitable relay directly, or through a small transistor driver stage if the relay coil current exceeds the HL2 output’s rated drive capacity. The PTT output activates at the same time the HL2 begins generating its transmit signal — there is no inherent delay. For relay-based switching, a brief software-configurable PTT-to-TX delay in the SDR software (available in SDR Console and Thetis) ensures the relay has time to switch before the TX carrier begins. Set this delay to at least 5 ms for most relay types; 10 ms for older or heavy-duty relays.

Section 6 — Mode-by-Mode Operating Procedures

• CW
  CW Operation — Keying the HL2 with the R-390A on Receive

  CW is the mode where the R-390A + HL2 combination performs best. The R-390A’s mechanical filter selectivity on CW is outstanding — particularly with the narrow CW bandwidth filter position — and the HL2’s DSP-generated CW is clean and fully break-in capable with PIN diode T/R switching.

  Keying method: connect a physical CW key or paddle to the HL2 hardware CW jack (the HL2 v2 has a 3.5 mm CW key input directly on the board). The HL2 hardware keyer generates the CW timing with low latency. Alternatively, use a USB-connected hardware keyer (Winkeyer, K1EL, or similar) through the SDR software. The CW sidetone is generated by the software and played through the computer audio; the R-390A does not need to be used for sidetone.

  Full breakin (QSK) setup: with a PIN diode T/R switch and the HL2 hardware CW keyer, the system can support full breakin CW: the R-390A receives between each dit and dah. The R-390A’s AGC must be set to the fast position for this to work well — the slow AGC release will cause the receive audio to be suppressed for several hundred milliseconds after each TX burst, effectively defeating the breakin. Set R-390A AGC to fast on CW with QSK.

  Frequency precision: set the HL2 software TX frequency to match the R-390A’s dial reading for your current operating frequency. The HL2’s frequency accuracy is limited by the quality of its 122.88 MHz VCTCXO reference oscillator; the HL2 v2 oscillator is typically within ±1 ppm of nominal. For CW DX work where frequency accuracy matters, calibrate the HL2 against a known WWV or CHU frequency standard before operation.

• SSB
  SSB Operation — HL2 Transmit with R-390A Receive Audio

  For SSB, the HL2 generates the USB or LSB signal through DSP in the SDR software. The transmit audio chain is: microphone → computer sound card → SDR software (equalisation, compression, SSB generation) → HL2 DAC → RF output. The receive chain is: antenna → splitter → R-390A (or via IF tap) → R-390A audio output → headphones/speaker.

  Microphone setup: use a standard dynamic or condenser microphone connected to the computer’s sound card line or microphone input. In SDR Console or Thetis, select the correct audio input device for microphone. Apply the audio equalisation and compression settings in the software to optimise the transmit audio for the HL2’s DSP chain. The HL2 generates very clean DSP SSB; heavy compression is not required and will degrade the signal quality.

  PTT for SSB: use the software PTT button, a VOX (voice-operated TX) configured in the software, or a foot switch connected to the HL2’s PTT input. The T/R relay activates on PTT onset. For comfortable SSB operating, the relay switching time (2–5 ms) is inaudible and the transition is seamless.

  Sideband selection: set USB or LSB in the SDR software to match the R-390A’s sideband selection. On 20m and above, use USB; on 40m and 80m, use LSB. The R-390A’s product detector sideband selection (USB/LSB switch) must match the HL2’s transmit sideband for the receive audio to make sense during monitoring of your own transmit on the SDR software’s receive channel.

  Transmit power and external amplifier: the HL2’s 1–5 W output is suitable for low-power (QRP) SSB operation. For higher power, the HL2 drives an external linear amplifier directly; the PA must be rated for its input impedance at 50 Ω and the HL2 output must be appropriate for the PA’s drive requirement. Do not overdrive the PA input; the HL2’s output level is adjustable in software.

• AM
  AM Operation — HL2 DSP AM with R-390A Envelope Detector

  AM is the mode that demonstrates the pairing’s complementary strengths most clearly. The R-390A was designed for AM reception and its envelope detector produces excellent AM audio quality when the signal is strong and the selectivity is correctly set. The HL2’s DSP AM generation produces a standard double-sideband with full carrier (DSB-FC) signal that is fully compatible with the R-390A’s AM detector.

  AM generation on the HL2: configure the SDR software to AM mode (carrier + double sideband). The software generates the modulated carrier; the HL2 DAC outputs the AM signal directly. Note that AM efficiency is lower than SSB: the carrier consumes two-thirds of the power for an unmodulated signal. The HL2’s 1–5 W output for AM provides approximately 0.25–1.25 W of useful sideband power at 100% modulation, which is adequate for regional AM contacts.

  R-390A AM receive: set the R-390A to AM mode (the AVC/AM mode selection). The R-390A’s wide mechanical filter position (typically 8 kHz for AM) is appropriate for voice AM. The R-390A’s excellent AVC characteristic maintains consistent audio level across varying signal strengths — a useful complement to the HL2’s panadapter that shows signal level visually.

  Frequency selection for AM: the AM mode is available across the complete HF spectrum with the HL2. Historical AM operating frequencies on the amateur bands (near the band edges in some regions, in the “AM windows” in others) are accessible without modification. The CP-1 crystal pack (if available for the R-390A) extends the possible operating frequencies to the full range that the HL2 covers.

Section 7 — Digital Modes: FT8, WSPR, and PACTOR with the Combined Station

The HL2 + R-390A combination is particularly well-suited to digital mode operation because the digital modes benefit from the R-390A’s excellent front-end linearity and the HL2’s precise DSP signal generation. For FT8 and WSPR (the most widely used HF digital modes in 2026), the operating chain is:

• Receive: R-390A audio output → computer sound card line input → WSJT-X (or JTDX, MSHV) decoding software. The R-390A’s IF output or audio output feeds the computer’s sound card as a standard audio interface; no modifications required.
• Transmit: WSJT-X → HL2 via CAT interface (virtual COM port from SDR software) for frequency control → HL2 audio input for the digital signal → HL2 RF output → T/R switch → antenna.
• CAT control: Quisk and Thetis both provide a CAT interface that WSJT-X can use to control the HL2’s transmit frequency. SDR Console v3 also provides CAT control via its virtual CAT interface. Configure WSJT-X to use the appropriate COM port for the SDR software’s CAT interface.

Section 8 — Frequency Calibration: HL2 Reference and R-390A Dial Correlation

Calibrating the HL2 Reference Oscillator

The HL2’s frequency accuracy depends on the quality of its 122.88 MHz VCTCXO (Voltage Controlled Temperature Compensated Crystal Oscillator) reference. The VCTCXO is specified to within approximately ±1 ppm at operating temperature, corresponding to a maximum error of approximately 14 Hz at 14 MHz. This is adequate for SSB and most digital modes but should be verified and corrected before CW contest operation or digital modes that require tight frequency registration.

Calibration procedure: tune the HL2 software to a known standard frequency (WWV on 10.000, 15.000, or 20.000 MHz; or WWVH on 10.000 or 15.000 MHz; or CHU on 7.850 or 14.670 MHz). The SDR software’s frequency display shows any offset between the indicated frequency and the known standard. Most SDR software supporting the HL2 provides an “oscillator correction” or “clock offset” field in parts per million (ppm); adjust this value until the signal appears at the correct position on the SDR display. Perform this calibration after the HL2 has warmed up for at least 15 minutes.

R-390A PTO Calibration Correlation

The R-390A’s PTO dial accuracy is typically within ±500 Hz across the full 1 MHz band segment after correct alignment. When using the R-390A as the frequency master for the combined station, the operator reads the R-390A dial and sets the HL2 software to the same frequency. Any remaining offset between the two (due to PTO calibration or HL2 reference error) will be visible as a small discrepancy in where a signal appears on the SDR panadapter relative to the R-390A’s indicated frequency. This discrepancy can be noted and mentally compensated, or it can be corrected by adjusting the HL2’s ppm offset calibration until signals on the panadapter align with the R-390A’s indicated frequency.

Section 9 — Complete System Architecture

  ┌──────────────────────────────────────────────────────────────────────────┐
  │   R-390A + HERMES LITE 2 INTEGRATED STATION — COMPLETE ARCHITECTURE     │
  └──────────────────────────────────────────────────────────────────────────┘

  APPROACH A: RF ANTENNA SHARING (No R-390A modification required)
  ─────────────────────────────────────────────────────────────────────────
                           ANTENNA (50 Ω coax)
                                   │
                    ┌──────────────┴──────────────┐
                    │  PASSIVE RF SPLITTER         │
                    │  (Mini-Circuits ZFRSC-42     │
                    │   or equivalent; 3.5 dB      │
                    │   insertion loss per port)   │
                    └──────────────┬──────────────┘
               ┌───────────────────┴──────────────────────┐
               ▼                                           ▼
       ┌──────────────┐                         ┌──────────────────┐
       │  R-390A/URR  │                         │  HERMES LITE 2   │
       │  (receive)   │                         │  (RX + TX)       │
       │  Mechanical  │                         │  1 GbE Ethernet  │
       │  filter      │                         │  to computer     │
       │  PTO tuning  │                         └────────┬─────────┘
       └──────┬───────┘                                  │
              │ Audio out                                 │ Network
              ▼                                          ▼
       [Headphones/Speaker]                     ┌─────────────────────┐
       Primary receive audio                    │  SDR SOFTWARE       │
                                                │  (SDR Console v3 /  │
                                                │   Thetis / Quisk)   │
                                                │  Panadapter display │
                                                │  SSB/CW/AM TX DSP   │
                                                └─────────┬───────────┘
                                                          │ PTT relay out
                                                          │ (controls T/R switch)
                                                          ▼
                          ┌──────────────────────────────────────────────┐
                          │  T/R SWITCH (relay or PIN diode)             │
                          │  RX position: RF splitter → R-390A          │
                          │  TX position: HL2 TX output → Antenna       │
                          │  R-390A input ISOLATED during TX             │
                          └──────────────────────────────────────────────┘

  ─────────────────────────────────────────────────────────────────────────
  APPROACH B: CASCADE-390 IF TAP (Optional enhanced integration)
  ─────────────────────────────────────────────────────────────────────────

                           ANTENNA → R-390A RF input
                                           │
                                    [R-390A RF section]
                                           │
                                  [First IF / Bandswitch]
                                           │
                           ┌──────────────┤
                           │ Pre-filter   │
                           │ tap (wide)   │
                           │              ▼
                           │     [Collins mechanical filter]
                           │              │
                           │ Post-filter  │
                           ├◄─────────────┤ (filtered — shows what R-390A hears)
                           │              ▼
                           │     [R-390A second IF → detector → audio]
                           │
                           ▼
                    CASCADE-390 buffer amp (high-Z tap, isolated)
                           │
                           ▼
                    HL2 Rx input ← set SDR to 455 kHz
                           │        Apply -455 kHz display offset
                           ▼        to show true HF frequency
                    [Panadapter centred on R-390A tuned frequency]
                    [Automatically follows R-390A tuning — most useful integration]

  ─────────────────────────────────────────────────────────────────────────
  T/R SWITCHING TIMING SEQUENCE (PTT relay, typical values)
  ─────────────────────────────────────────────────────────────────────────

  Operator presses PTT
        ├── t=0:    HL2 PTT relay output activates
        ├── t=2ms:  T/R relay coil energises; contacts begin moving
        ├── t=4ms:  T/R contacts complete; R-390A isolated, HL2 TX path active
        ├── t=5ms:  Software PTT-to-TX delay expires; HL2 begins generating RF
        │            ↑ This gap ensures relay is switched before TX arrives
        │
        [Transmit period — R-390A isolated from antenna]
        │
  Operator releases PTT
        ├── t=0:    HL2 PTT relay output deactivates
        ├── t=2ms:  T/R relay releases; contacts return to RX position
        ├── t=4ms:  R-390A reconnected to antenna; HL2 TX path open
        │            HL2 TX output will be ~0 during this period (software cutoff)

  ─────────────────────────────────────────────────────────────────────────
  SOFTWARE COMPARISON FOR R-390A COMPANION ROLE
  ─────────────────────────────────────────────────────────────────────────
  ┌──────────────────────────────┬───────────────────┬──────────────────────┐
  │  Feature                     │  SDR Console v3   │  Thetis (OpenHPSDR) │
  │  ───────────────────────────  │  ───────────────  │  ────────────────── │
  │  Panadapter quality          │  Excellent        │  Very good           │
  │  TX modes (CW/SSB/AM)        │  Full             │  Full                │
  │  CW keyer (hardware)         │  Limited          │  Native              │
  │  IF offset calibration       │  Yes              │  Yes                 │
  │  WSJT-X CAT interface        │  Yes (virtual CAT)│  Yes                 │
  │  Linux support               │  No (Windows)     │  No (Windows)        │
  │  Cross-platform              │  No               │  No (use Quisk)      │
  │  Ease of setup               │  High             │  Moderate            │
  │  Best for                    │  Casual/monitor   │  Active operating    │
  └──────────────────────────────┴───────────────────┴──────────────────────┘

Complete R-390A + Hermes Lite 2 station architecture. Approach A (antenna sharing) requires no receiver modification. Approach B (CASCADE-390 IF tap) provides tighter panadapter integration centred on the R-390A’s tuned frequency. Both can be used simultaneously. T/R relay timing values are indicative; verify against specific relay and software delay settings. All connections at 50 Ω impedance throughout.

Section 10 — Parts, Sources, and Construction Notes

Hermes Lite 2 Hardware Sources

The Hermes Lite 2 is open hardware; the complete KiCad schematic and PCB design files are available on the HL2 GitHub repository (github.com/softerhardware/Hermes-Lite2). Assembled boards are available from several community-supported vendors including N2ADR (USA) and others identified in the HL2 community documentation. When ordering, confirm the version (v2.x is current and recommended) and whether the PA board is included.

RF Splitter

The Mini-Circuits ZFRSC-42 covers 1–500 MHz with approximately 3.5 dB insertion loss and 20 dB port isolation. It is available from Mini-Circuits directly and from common electronics distributors. Alternative: an equal-split power divider wound on a Type 43 or Type 61 ferrite toroid covers the HF range with similar characteristics at lower cost; construction details are available through the QRP and HPSDR communities. Connectors: the ZFRSC-42 has SMA connectors; use quality SMA-to-BNC or SMA-to-SO-239 adapters as needed for the R-390A and HL2 connector types.

T/R Relay Options

Suitable coax relays for the T/R switch include: Tohtsu CX-140D (BNC connectors, 1 GHz rated, fast switching), or a generic HF relay rated for the operating power level. For the HL2’s 1–5 W output, any quality coax relay rated for at least 50 W at HF is adequate. If driving a linear amplifier, the relay must be rated for the amplifier’s output power. The relay coil voltage must match the available PTT drive voltage; the HL2 PTT output can typically drive 12 V relay coils through a small transistor (2N2222 or similar) switching circuit.