PACTOR

PACTOR is an HF data mode built to get messages through without errors, even when the band is rough. It came out of Germany in the early 1990s.

Hans-Peter Helfert and Ulrich Strate described it in QEX in October 1991, and it borrowed ideas from two older modes, packet radio and AMTOR. SCS, a German company that calls itself the inventor of PACTOR, holds the name and builds the modems.

It is a connected mode. Data goes out in short blocks, the receiving station answers each one, and anything damaged is sent again, so what arrives is exactly what was sent. The speed steps up when the path is good and drops back when it is not. Its best-known job today is carrying Winlink email, and the same modems also serve boats and other users on marine radio outside the ham bands.

There are four levels. PACTOR I is a narrow two-tone signal with a 200 Hz shift, and its protocol was published; ARRL still hosts a technical description. PACTOR II, III and 4 are faster SCS designs that run in SCS hardware modems. PACTOR 4 fits in 2.4 kHz and can still connect with stations using the older levels.

On the air PACTOR has a steady back-and-forth rhythm: a burst from the sending station, then a short chirp of reply, over and over.

For years, US rules limited HF data by symbol rate, or baud. On November 13, 2023, the FCC replaced that limit with a 2.8 kHz bandwidth limit in the HF data segments, keeping a 300-baud limit on the 2200 m and 630 m bands. You can read the current wording in 47 CFR 97.307(f)(3) on the eCFR. Under today’s rule, bandwidth is what counts, and PACTOR 4’s 2.4 kHz signal fits within it.

A PACTOR modem is a real investment. For Winlink, VARA is a lower-cost alternative, since it needs only a sound card interface. If you already own a PACTOR modem, Winlink Express and Pat can both drive it. As with any connected mode, listen before you connect and keep your transmitter out of heavy ALC.

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