It is indeed true that variations in liquid soil moisture will dramatically affect ground system performance and base fed vertical impedance when an on, or below grade ground system is used. Frozen ground moisture is on a par with dry ground. This is one of the reasons for going beyond the point-of-diminishing-returns number of radials--the more radials used, the more stable the feedpoint Z is, over varying soil moisture in the radial field. Antenna efficiency is related to radiation resistance relative to the ground system. If you have a short vertical (inverted L) with a low resistance that you cannot increase, you can improve it by lowering the ground resistance by employing more radials.
73 Rob K5UJ _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector