A Dual-Terminator Window for 40 m Long-Path SSB Contacts Between Germany and New Zealand

Morning 7 MHz long-path single-sideband (SSB) contacts between southern Germany and New Zealand are sparse and cluster about one hour after civil dawn in Germany, although weaker signals can be received much earlier. We use archived Weak Signal Propagation Reporter (WSPR) signal-to-noise ratios for 2023–2025, audited for transmitter and receiver activity, together with semi-empirical absorption estimates and PHaRLAP ray tracing, to investigate this narrow interval. WSPR was decoded on 498 of 594 mornings with demonstrated station activity (84%). Published speech requirements and an indicative 500 W link budget narrow the 87 min WSPR detection window to an SSB-equivalent interval of 40–60 min, peaking 60–75 min after dawn, consistent with the timing of 60 documented SSB contacts. Modeled D-region absorption is lowest in this interval, when the Sun is low at both ends of the path, a dual-terminator window. The modeled minimum occurs a median of 15 min after the temporal midpoint between German civil dawn and New Zealand receiver sunset (r = 0.99, 512 mornings); because both times shift together on this nearly antipodal path, the season predicts contact time as well as dawn does. Receivers in Australia show that the window closes at receiver sunset or about 154 min after German dawn, whichever comes first. No systematic difference in closure time was detected between 7 and 10 MHz near New Zealand sunset, which argues against both absorption and a falling maximum usable frequency as the cause; the mechanism remains unresolved.