Antennas

Antenna selection is a very important aspect of the trans-oceanic tropospheric ducting station. The criteria for selecting antennas are similar to those used for beacons. The biggest difference is that the station is usually manned while a beacon site is often unattended. This difference makes antenna rotations and beam optimization a lot easier. Maximum antenna gain is highly desired as the paths to be worked re very long range, and path loss may be high when using marginal or weak tropospheric ducts.

Most stations will elect to use a large yagi or arrays of yagis. The resultant narrow beamwidth is not a problem as the operator can rotate the antenna to point at the desired beacon or collaborative stations at the other end of the path.

Some stations may want to automatically monitor a number of beacons spread out along the other end of the trans-oceanic path. There are a number of solutions for the receiving equipment but the narrow beamwidth of the high gain antennas presents a problem. One solution is to use an array of short yagis stacked vertically. Such an array is described in the section on beacon antennas. Arrays can consists of 4, 8, or more yagis, depending on the amount of tower height available. The horizontal beamwidth remains the same as for a single yagi, but as more yagis are stacked one above another, the vertical beamwidth becomes narrower. The antenna system gain is increased overall by giving up gain at high elevation angles, resulting is a wide but very flat main lobe. A five element yagi array will have a -3dB beamwidth of 50-55º regardless of how many yagis are added to the stack. However, the vertical beamwidth of the single yagi, will be reduced to 25º for two yagis, 12.5º for four, 6º for eight, and so on.