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Fiberglass vs metal for mast material

Jul 16th 2013, 14:33

KB9MDQ

Joined: May 26th 2011, 03:38
Total Topics: 0
Total Posts: 0
I have a 45' tower over a house with a metal roof. I used metal conduit to run a 15' mast on top of that. This gets me to the top of the tree line right behind the house (I live on a river bank on a ridge).

Attached 10' up at the joint is my 160M OCF dipole. Attached at the top is my old Maldol Triband vertical.

The antenna folds over onto the roof making access to the top easy. I am thinking of decreasing the weight of the mast by switching to the fiberglass military mast,

My question is related to antenna performance when grounded to the metal mast (ground plane). the dipole obviously isn't. the vertical is via the antenna clamps by design.

Am I asking for performance trouble by doing this switch?
Jul 29th 2013, 17:05

WA0CBW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The antenna performance shouldn't change as the mast and tower provides little or no effect on the ground plane. However from a "grounding" stand point you need to ground the top vertical antenna to the tower. In the commercial world we would NEVER use an insulating material for a mast. In some instances we would run a separate ground wire from the antenna mounting clamps down the tower to a ground ring buried around the tower. This provides additional lightning protection. Also be sure you follow the NEC guidelines by grounding the shield of any coax entering the building.
If you were extending the point at which you mounted the OCF dipole with an insulating mast you would be Ok as dipoles do not use a ground for their operation and can perform better if they are not near the metal supporting mast. That is why you see most tower mounted dipoles hung from an insulated outrigger arm to get it away from the tower.
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