Propagation is a fun thing — especially when you’re talking thousands of miles on UHF! On October 16, 2020 a 1,950 mile contact was made on 433Mhz FM between the Island of St. Helena (U.K.) and Plumstead, South Africa.
The two stations involved in the contact were Garry, ZD7GWM and Tom, ZS1TA in South Africa.
The power used for the contact was a mere 35 watts using a vertical antenna.
How a UHF contact like this possible
So how was this possible? Looking at predicted tropospheric conditions from October 16, 2020 — a marine duct is likely what allowed the two stations to log the contact.
Marine Tropo (MT) occurs when warm dry air overrides a cooler body of water. Marine inversions often extend the entire breadth of lakes and can extend for thousands of miles over the ocean, like in this case. It also spreads into coastal areas by way of sea or lake breezes.
Marine Tropo can become enhanced or combined with other types such as High-Pressure Tropo which can lead to even stronger signals over farther distances.
Tropospheric ducting is not unheard of between St. Helena and South Africa. However, it’s usually on VHF. During the past few years Garry has made numerous contacts on VHF with South African ham radio operators.
The incredible distance for this 70cm contact may be hard to conceptualize. It would be equivalent to a station in New York City having a QSO with someone in Salt Lake City. It doesn’t break the reported 70cm tropospheric record of 2,493 miles — but it ranks right up there!