Antenna Loading Coil…Watching the Paint Dry
Posted on September 6, 2012
In another thread an outdoor antenna system details about a loading coil and a rain cover were presented. The rain cover (Photo) is made of cellular core PVC which may not be an ideal material for this but it should be satisfactory. A discussion related to coil materials is found at this LINK. The outside of the cover was painted with acrylic latex paint which should form a moisture barrier but if the outside painted surface is wet from rain then signal attenuation is expected and this was observed.
The problem presented when after two days of dry weather following rain the field strength did not return to the previous “dry” value. The inside surface of the cover was not painted and it is a possibility that the PVC absorbed some moisture from the humid air. It is known that regular high density PVC will absorb moisture and such data are available but I found none for the cellular PVC. It would be reasonable to assume that this material will also absorb moisture.
As a solution the PVC cover was replaced with one made from a one gallon ice cream container (Photo) which is made from a plastic which should have low absorption. This container was also chosen because of its large diameter which should keep the surface moisture from rain farther away from the coil. Unlike the situation observed when the first cover installation detuned the coil requiring the removal of 1 turn to lower the inductance this cover had no effect at all on the tuning when it was installed.
Satisfied that this cover would work I applied two coats of acrylic latex paint for aesthetics and this is where “watching the paint dry” came into play. With wet paint the field strength was down 3 dB from the dry condition. As the paint dried the FS slowly rose to the initial reading.
It remains to check the new cover’s recovery after a rain. The first cover drying problem perhaps could have been solved by painting the inside surface but the observation that even when dry the cover changed the tuning indicates some interaction with the loading coil which could be detrimental so the choice was made to switch to the new larger cover.