[TowerTalk] Force 12 EF-240 SWR shift

[john at kk9a.com]

A detached hairpin coil or a detached linear loading would raise the  
resonant frequency.

John KK9A


Charles Morrison N1RR wrote:

Linear loading shorting bar disconnected ?
Hairpin break?


N1RR

On Sun, Sep 15, 2019 at 4:32 PM <w5wz at w5wz.com> wrote:

>
> 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