Most HF remote base stations use some kind of multiband wire antenna. Often these wire antennas must go through and be supported by trees, especially if the antennas are really long and have to be fitted into limited space. For example, a half-wave dipole antenna for the 75 meter phone band will be in the neighborhood of 120 – 125 feet (36 – 37 meters) long. That amount of wire often will need to be threaded through trees to make in fit into the space available.
Although insulated wire is used for the radiating length, the wire is going to be in contact with foliage and branches at some points along its length. As long as the weather is dry and the branches and leaves are not wet, this arrangement works pretty well. It’s when the rain, snow, and ice begin that we see the antenna get detuned by the moisture and increased coupling of the radiating wire to the branches and foliage. Although the insulation on the antenna wire blocks direct current, RF energy is easily passed from wire to tree, especially in wet weather.
With this extra loading, the antenna will no longer be “in tune” with the usual automatic antenna tuner memories that work so well in dry weather. It will be necessary to run the tuning cycle again, sometimes more than once, while observing the SWR reading on the radio interface. Often the band will come alive with received signals after this new tuning cycle is complete. This is a good indication that you are on the right track, and as long as the radio does not return a “HIGH SWR” notice when you transmit, you should be good to go on that frequency.
When the weather dries out, the tuning will revert to normal, so be sure to run the tuning cycle again and make sure the SWR reading is low once again.