JTCAT (Built-in FT8 & FT4)

JTCAT is POTACAT’s built-in digital-modes engine. It decodes and transmits FT8, FT4, and FT2 entirely on its own — you do not need WSJT-X installed. JTCAT runs as a pop-out window on the desktop, and the same engine powers the FT8 screen in ECHOCAT on your phone.

While it runs, JTCAT:

• Decodes your radio’s USB audio and shows decodes on a waterfall
• QSYs the radio and switches it into its data mode automatically
• Auto-replies to callers and can run Auto-CQ / Auto-POTA sequences
• Auto-logs completed QSOs and forwards them to your configured logbooks
• Reports decodes to PSKReporter

Note: JTCAT takes over radio control while it is running — POTACAT’s normal CAT connection is suspended until you stop it.

JTCAT vs. WSJT-X vs. ECHOCAT

These three names get mixed up constantly. Here’s the map:

Warning: To use JTCAT, the WSJT-X mode checkbox under the CAT pill must be unchecked. WSJT-X Mode hands rig control to an external WSJT-X and disables POTACAT’s own CAT — JTCAT cannot run in that state. See the WSJT-X page for how that mode works.

Audio Setup

JTCAT listens to — and transmits through — your radio’s USB audio codec. Audio devices are configured per rig under Settings → Radio → Edit:

• Audio input (RX): the radio’s USB audio device, selected explicitly (for example USB Audio CODEC for an IC-7300, or FT-710 USB Audio). Don’t leave it on “Default” unless the radio genuinely is your operating system’s default recording device.
• Audio output (TX): the same radio USB device, so TX audio reaches the rig instead of your computer speakers.

Tip: The classic symptom of a wrong input device: the waterfall reacts to the radio’s speaker volume knob or to room noise. That means JTCAT is listening to a microphone, not the rig.

Platform and rig specifics:

• macOS — grant Microphone permission to POTACAT in System Settings → Privacy & Security → Microphone. Without it, decoding silently fails; the log shows Cycle boundary: ... max=0.0000.
• FlexRadio — use the Flex Direct (VITA-49) audio source; no DAX program is needed. PTT requires the SmartSDR API host (port 4992) configured in the rig settings — this is separate from the SmartCAT connection.
• Rigs without built-in USB audio (for example the Xiegu G90) — use an external USB sound-card interface.

Radio Mode (DATA) Setup

The rig must be in its data mode — USB-D on Icom, DATA-USB on Yaesu, DIGU on Kenwood, DATA A on Elecraft. JTCAT sets and enforces this automatically (the log shows post-reconnect mode enforcement: FT8), but a few radios need extra attention:

• Yaesu — the data mode is labeled DATA on most models and PKT on some older ones; either way, make sure data-mode audio comes from the rear/USB input (DATA MOD = USB — some rigs default it to the ACC jack).
• Yaesu FTDX10 / FTDX101 family — in DATA mode these rigs can transmit offset from the dial by the carrier point (roughly 600 Hz) with a narrowed filter. Set MENU → OPERATION SETTING → GENERAL → DATA MODE to Others (or DATA-STD, depending on firmware) so TX lands where the dial says.
• Elecraft KX2/KX3 — per-band memory recall can override DATA A on a band change. Select KX2/KX3 as the Radio Model in Settings → Radio so POTACAT re-sends the mode after each QSY.

Clock Sync

FT8 lives and dies by your computer clock — an offset of even a few seconds kills decoding entirely. JTCAT shows a clock-sync banner when your system clock is off; if you see it, sync your clock to internet time before troubleshooting anything else.

The JTCAT panel also has a Latency (ms) control — the equivalent of WSJT-X’s “Audio: Soundcard time delay.” It is auto-calibrated, so you normally never touch it, but if your DT readings look consistently offset it’s the knob to check.

Making QSOs

Click a decode (or a highlighted POTA activator) to reply. JTCAT runs the exchange — reports, RR73, 73 — automatically.

• Late-join TX — you don’t have to wait for the top of a cycle. Click partway into a TX window and JTCAT starts a shortened reply immediately, matching WSJT-X’s late transmit start.
• AP decoding — a-priori decoding is supported, which helps pull out weak replies directed at you.
• Hold TX — this checkbox is the equivalent of WSJT-X’s “Hold Tx Freq”: your transmit frequency stays pinned to the value in the TX Freq box while RX follows the other station. The setting persists between sessions.
• dB numbers are confidence scores — JTCAT’s per-decode signal figures are the decoder’s own confidence score, not calibrated SNR. Don’t expect them to match WSJT-X’s dB readings.

Tip: Keep the TX Power slider low — around 10–20% typically produces clean audio with little or no ALC action.

Auto-Logging

JTCAT logs each QSO automatically when the report exchange completes and forwards it to any logbooks you’ve configured — see Logbook Forwarding. There’s nothing to click.

That includes interrupted endings: if you click a new station while the previous QSO is still in its 73/courtesy phase, the completed QSO is logged before the new one replaces it.

Note: Some users report a logging bug where a QSO is logged immediately after the report is sent, with the sent RST recorded as both sent and received. If your logged reports look mirrored, check and correct the entry in the logbook.

Troubleshooting

Nothing decodes

Work through these in order:

1. Audio input device — must be the radio’s USB codec, not “Default” or a microphone. See Audio Setup.
2. macOS Microphone permission — System Settings → Privacy & Security → Microphone.
3. RX gain — too high (clipping) or too low both kill decodes. If the RX meter shows a red CLIP indicator, lower the rig’s USB audio output level (or the OS recording level) rather than only the app slider; some IC-7300 users report running the rig’s USB output level as low as about 10%.
4. Clock offset — look for the clock-sync banner and sync your system clock.

Radio transmits but nothing shows on PSKReporter

The five-step checklist:

1. Watch the ALC meter during TX. Zero deflection means audio isn’t reaching the modulator at all.
2. IC-7300: set MENU → SET → Connectors → USB MOD Level to about 50%.
3. DATA MOD = USB — some rigs default data-mode audio to the ACC jack.
4. TX gain slider in JTCAT/ECHOCAT is not at 0.
5. Cross-check with WSJT-X on the same audio devices to separate an app problem from a radio configuration problem.

Waterfall quirks

• A waterfall that follows the radio’s volume knob or room noise means the wrong audio input device is selected.
• Some users report the phone (ECHOCAT) waterfall not displaying even while decodes flow — decoding happens on the desktop, so QSOs still work.
• If decodes or the waterfall stall after a long or resumed session, restart POTACAT (or close and reopen the JTCAT window). Newer builds include a watchdog that restarts audio capture automatically after several silent cycles.

Running from source

Packaged builds ship with the fast native C decoder (ft8_lib). If you run POTACAT from source on macOS, install the Xcode command-line tools (xcode-select --install) and run npm run build-ft8; otherwise JTCAT falls back to a slower WASM decoder. The terminal shows which one loaded: [FT8 Worker] Native C decoder loaded (ft8_lib) versus Native decoder not available, using WASM fallback.

FT8 on Your Phone (ECHOCAT)

ECHOCAT’s FT8 screen drives the same JTCAT engine on your desktop, so everything above still applies — configure audio and rig mode on the desktop first.

One behavior trips up WSJT-X veterans: an FT8 transmission needs about 12.5 seconds of the 15-second window, so you have roughly a 2-second grace window at the start of a TX cycle to tap a reply. Tap within it and the reply goes out immediately (late-join TX may still squeeze in a shortened reply if you’re slightly late); miss it and JTCAT waits for the next cycle. This is FT8 timing physics, not a bug.

Note: ECHOCAT is a remote control — every tap travels to your desktop before anything happens. The latency figure at the top of the app shows that round trip; the higher it is, the less of the grace window you have.

Two more phone-side pointers:

• In FT8 mode, the PTT/transmit controls live inside the FT8 banner. Press Stop there before switching back to the Spots or VFO views.
• To call a directed CQ (such as CQ POTA), set FT8 → Target → Custom in ECHOCAT.