On most fiberglass omnis, the lighting spike at the top is an extension of
the last element in the array which is actually a shorted quarter-wave
section. This shorted quarter-wave section at the "far end" of the radiator
serves to put the entire antenna at DC ground.
Some of the cheaper fiberglass omnis have a little metal "cap" on the top,
which isn't connected to anything, it's just there to seal the end of the
fiberglass tube (examples: Diamond, Comet, Maxrad, Antenex, etc.).
--- Jeff WN3A
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of Chuck Kelsey
> Sent: Monday, November 02, 2009 6:43 PM
> To: [email protected]
> Subject: Re: [Repeater-Builder] Lightning Rod (Bolt)?
>
>
>
> Although I've never had one apart, I believe the cap is
> attached to the
> radiating element. Also, I have never seen any fiberglass
> collinear antenna
> survive a direct hit. I have, however, seen folded dipole
> arrays survive a
> lightning strike.
>
> Chuck
> WB2EDV
>
> ----- Original Message -----
> From: "Doug Rehman" <[email protected] <mailto:doug%40k4ac.com> >
> To: <[email protected]
> <mailto:Repeater-Builder%40yahoogroups.com> >
> Sent: Monday, November 02, 2009 5:45 PM
> Subject: [Repeater-Builder] Lightning Rod (Bolt)?
>
> > Looking at some DB Products fiberglass radome antennas,
> they all have a
> > metal protrusion on the top cap; it looks like bronze
> perhaps. I presume
> > that it is some form of lightning mitigation device. Is
> this just a piece
> > of
> > metal in the cap, unattached to the internal antenna element?
> >
> > I'm curious as I need to mount a Maxrad 900 MHz antenna on
> top of a long
> > mast on the top of a tower. The tower takes a lightning hit
> about once a
> > year. The Maxrad is in PVC tubing and it'd probably be
> simple to add a
> > stainless bolt to the top of the antenna if it would do any good.
> >
> > The previous 3 antennas on top were Comet GP-15's. They
> have a thin metal
> > cap on the top of the radome. The first one took a pretty
> significant hit,
> > melting part of the cap and making a mess out of the
> antenna. The second
> > one
> > took a lesser hit, heating the element enough that it
> burned thru the
> > radome. The third strike was even less, just discoloring
> the fiberglass on
> > the top 8" of the radome.
> >
> > I know that I'd be better off with a DB Products antenna,
> but their weight
> > likely well exceeds the mast capability for a top antenna
> (where it needs
> > to
> > be).
> >
> > Thoughts???
> >
> > Thanks,
> > DOUG
> > K4AC
> >
> >
> >
> >
> >
> > ------------------------------------
> >
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>
>
>
>
>
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