[LCC] Force 12 EF-240 SWR shift

[w5wz at w5wz.com]

The SWR minimum on my lowest Force 12 EF-240 has shifted from ~7.16 MHz 
down to about 6.7 MHz.  However, the curve appears very similar to 
measurements taken and archived in 2015; it's just that it shifted down 
in frequency.  Examining the antenna with binoculars from the ground, I 
do not see any mechanical damage or oddities.

This is a linear loaded type antenna.  It uses a bead-balun, and has a 
coil hairpin across the feed point of the driven element.

These measurements were taken in the shack, with the AA-55 in place of 
the transceiver; thus the entire transmit RF path is being tested.  I've 
also tested at the base of the tower, such that only 125 feet of LMR400, 
a PL-259-barrel-PL259 connect, and the PL-259-SO-239 connection at the 
balun are involved.  The results are very similar, so I do not suspect 
anything amiss that is not up the tower.

I took a sweep of the antenna today and another identical antenna, 
ranging from 6.0 to 8.0 MHz.  That data can be found at:
http://w5wz.com/files/40mSCAN

The purple line is the top antenna in the stack, and the blue line is 
the bottom antenna in the stack.  Both are Force 12 EF-240.

Lower in frequency = longer elements.  What could have changed to make 
the antenna appear electrically longer?

Any thoughts?

--Scott, W5WZ