When installing the tower connection end of Phillystran guy cables, I
learned that a significant problem can happen with unintended Yagi guy grip
(Preform) antenna interaction.
The problem is this, if your guy grip tower terminations are close to your
antenna mounting position (right underneath), and the terminations (guy
grips) are electrically connected to the tower, they can (in many
installations) interact and cause antenna pattern/VSWR problems. This
problem is worse on the higher bands like10 and 15 meters.
For example at K3LR many Yagi's are side mounted on ring rotors. Phillystran
or fiberglass guy attach to the tower right under each ring rotor. After an
installation several years ago of some 10 meter Yagi's I noticed that there
were three peaks and valleys in VSWR when rotating the antenna mounted on
the ring. Closer inspection and modeling the three short lengths of
"connected" guy grips revealed that in fact the Preform guy grips were long
enough (cumulative effect of 3 together) to impact the near field
environment of the Yagi. The problem is worse the more "horizontal" the guy
wire is relative to the Yagi (lower guy wire connection points heading to
ground mounted concrete guy dead men anchors) and the higher in frequency
the antenna is (Preform length is more "meaningful").
The solution is simple enough and easy at the time of installation. All of
my guy grip installations now include a "bent pin shackle" (tower side) and
a properly sized guy wire insulator - right at the guy attachment point on
the tower. The other side of the insulator contains the proper guy grip for
the Phillystran or Fiberglass guy material. This way the Preform is totally
insulated from the tower.
Once the insulators were installed at the tower connections, the VSWR and
directional pattern of the Yagi antenna remained constant and predictable
(as modeled) no matter where the antenna was pointed.
You can see the tower insulators installed on the K3LR 20 meter tower on
this W8WWV drone video at the 5:18 mark
https://www.youtube.com/watch?v=SA_NG-bVQuc
73
Tim K3LR
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