Exploring long distance VHF communication

February 18th, 2019, Published in Articles: EngineerIT, Featured: EngineerIT

During the years in the eleven-year cycle with high sunspot numbers, we have often seen the maximum usable frequency rising above 145 MHz and resultant very high frequency (VHF) long distance (DX) contacts being made over distances in excess of 3000 km. This, however, does not mean that during the rest the 11-year cycle DX VHF contacts are not possible. There are many other modes of propagation not involving the ionosphere and that is why beacons and the study of weather conditions play an important part in VHF/UHF long distance communication.

Brian Jacobs: “A Reverse Beacon Network will result in many new
long distance VHF contacts and a better understanding of the exotic VHF propagation modes.”

There are various propagation modes that play a part in long distance communication. Under the amateur radio spotlight are: tropospheric (Tp), sporadic-E (Es) and trans-equatorial propagation (TEP).

The South African Radio League (SARL) has been holding a number of workshops to focus on the various modes of VHF propagation. Many of these exotic propagation modes are difficult to predict and they often go unnoticed. This one of the reasons that although various VHF beacons have been erected in South Africa, it is not possible to monitor these beacons 24/7. At the February 2019 workshop, Brian Jacobs (ZS6YZ) who serves on the SARL VHF/UHF/microwave committee, presented a paper on the establishment of a reverse beacon network.

The Reverse Beacon Network (RBN) is a project that monitors amateur radio frequencies by using volunteer stations to continuously and autonomously collect data on what and when stations are being received, and how good the signal is. The data is made publicly available on the internet to enable radio amateurs to easily determine overall propagation conditions. It is currently working mainly with CW (Morse code) stations, and mostly on HF. Other modulation modes can be included.

The current RBN stops at 50 MHz, which falls short of the SARL requirement for 144 and 435 MHz. Early investigations show that with minor software adoption the range can be extended to higher VHF and UHF frequencies. That project has already been launched and it is expected that pilot system will be installed at the National Amateur Radio Centre at Radiokop.

Long distance VHF propagation modes

The troposphere is the lowest layer of Earth’s atmosphere, the layer between the surface of the planet and 10 km. Most of the mass (about 75 – 80%) of the atmosphere is in the troposphere. Most types of clouds are found in the troposphere, and almost all weather occurs within this layer. The troposphere is by far the wettest layer of the atmosphere; all of the layers above the troposphere contain very little moisture. If not all, most VHF and UHF propagation has its origin in the troposphere.

Tropospheric propagation is 100% weather-related and makes the study of weather an important element. A Tropo DX mode is any abnormal condition that scatters, reflects or refracts VHF, UHF and/or microwave signals in the troposphere causing changes to their normal coverage. Another name for this is anomalous propagation (AP).

There are six main tropospheric DX modes. The refraction and ducting effects are similar to those that cause visual mirages. Therefore, the distant signals received via tropo can be considered “radio mirages”. Signals that are normally below the radio horizon and out-of-range instead appear above the radio horizon and receivable.

The six modes are line of sight (ground wave), tropospheric scatter (TrS), tropospheric enhancement or tropospheric bending, tropospheric ducting (TrD), elevated tropospheric ducting and tropospheric sub-refraction.

Trans-equatorial propagation

TE propagation is defined as VHF propagation between points on opposite sides of the geomagnetic equator, and at least 1600 km from it, without intermediate reflection from the surface of the earth. Many years ago, Greg Roberts (ZS1BI) wrote in Radio ZS: “It was established during tests with amateurs in 1957/8, particularly in Cyprus, that there were two TEP target zones. In the south, from approximately Pretoria (Johannesburg is just south of this zone) up to a line across Africa cutting through northern Mozambique, central Zambia and central Angola. In the north, this main zone commences at the Southern Mediterranean and ends in a line through northern Italy across Europe. Extension zones for weaker TEP working encompass a much larger area centred on these zones.

“The mode is totally dependent on high solar radiation of the F layer. The ionisation centre is the geomagnetic equator sloping slightly southwards east to west from the horn of Africa to Equatorial Guinea, whereas the geographical equator extends across Africa through the northern tip of Lake Victoria, some 1000 km further south.

“It might be a surprise to learn that a transmission from the better sited northern South Africa area travels nearly 7000 km before making landfall in the Mediterranean, or further, up to about 8000 km (Austria, Hungary) if conditions are better. Therefore, it is not possible to communicate with countries over the greater part of the African continent under typical TEP conditions”

Sporadic E (Es)

Fig 1: A typical Hepburn chart. Purple areas are marginal, blue indicate fair condition and good conditions.

Sporadic-E (also known as Es) propagation is probably the most interesting mode of VHF propagation to study. It is also responsible for most of the long distance (600 km and greater) contacts on the 6 m band. Sporadic-E is a type of ionospheric E-layer reflection caused by small patches of unusually dense ionization. These sporadic E-layer “clouds” appear unpredictably.

Field-aligned irregularities (FAI) is a newly discovered propagation mode that may exist simultaneously with sporadic-E and persists for an hour or more after all evidence of normal sporadic-E has disappeared. FAI signals are generally very weak and may easily be confused with back-scatter signals. Signals propagated by means of FAI have a rough, auroral quality.

There are Es predictions available on the Hepburn charts (available on www.dxinfocentre.com). On 16 December 2018 the predictions showed that contacts between St Helena and the west coast of South Africa was possible and indeed several west coast amateurs made contact with the amateur on St Helena on 144 MHz, a distance of over 3000 km.

Brian Jacobs said: “It is not practical to look at the Hepburn chart a few times a day and that is why the proposed VHF Reverse Beacon Network will be of great benefit and supplement the scarce knowledge we have about exotic VHF propagation. It will result in many new long distance VHF contacts.”

The SARL is encouraging people interested in long distance VHF to monitor the current beacons and if a long distance VHF signal is received, explore what the propagation mode was by studying weather synoptic charts, Hepburn charts and any other weather information available.

 

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