[TowerTalk] F12 C4XL 20m "drop" in resonant freq 100hz?

[Mel Whitten]

Has anyone experienced a significant (approx 100+Hz) and "sudden" drop in resonant frequency for the 20m band -only- on a Force12 C4XL yagi?  This has happened to mine (ant about ten years in air with no prior problems). Before I climb tower to investigate, is there something specific I should look for that may have caused this?   20m is now resonant at 13.980 and was resonant near 14.100.   SWR is 1.06:1 at 14.000 (Z/R = 51.7ohms) and at 14.330 1.58:1 (Z=64.2 R=60.1).  The antenna is at 72 feet and not been touched since I noted this sudden change in SWR.  Still usable, of course but this change may be an indication that something is going bad and may get worse if I don't fix it.  Weather is very spring like (60s and sunny) in Midwest, so no issues with icing.  Looking at elements from a distance, they all look normal. 

Mel, K0PFX
St Louis