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
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