Kelly has a good point. It is possible to manufacture and install an
insulated base but isolating the coax and control lines would be quite a
challenge. If it were me I would just put a ground mounted vertical, maybe
even top loaded with wire like an inverted L, in the back yard. They are
easy to homebrew with DX Engineering tubing.
John KK9A
Kelly Taylor ve4xt wrote:
It seems to me the trick will be electrically isolating that which you
want as an antenna from any conductors (ground wire, rotator cable, coax
shield, etc.) leading down from the system.
Tuned radials for the band(s) of interest would be a start, as would
chokes on the coax and rotator cable.
Without isolating these other conductors, you’re effectively feeding a
long wire at some point above the centre of the wire. Patterns and
impedances may not be what you hope.
Feeding may be an issue, particularly if the structure is grounded:
designing some kind of hairpin or hairpin coil would be one way of
separating the tower from ground at RF but not at DC. The problem with
that, though, is lightning is an RF event, so such coils might negate the
effectiveness of ground, too. Or at the very least, introduce enough
lumped impedance that a significant amount of strike energy is directed
into the coax or rotator cables or the roof itself,
Perhaps some kind of gamma or half-delta feed resolves this? (Run a wire
to the top if the tower like a short shunt feed or at an angle like a
small delta loop.)
It should work, but probably requires a bit more engineering than it appears.
73, kelly, ve4xt
> -----Original Message-----
> From: Edward via TowerTalk <towertalk at contesting.com>
> To: towertalk <towertalk at contesting.com>
> Sent: Tue, Dec 18, 2018 7:20 am
> Subject: [TowerTalk] Rooftop tower as Vertical Antenna
>
> Trying to be creative with my space/visual limitations. I have a Glen
Martin 18’ tower/10’ mast, with tribander and V/UHF Discone, atop of a
35’ roof. The tower legs are on concrete blocks, insulated from the
roof.
>
> Thinking about using the tower as a vertical. I have a remote tuner I
can use at the base. Can throw out a few resonant radials, too.
Thoughts? Advice?
>
> 73,
> Ed NI6S/7Z1ES
> __________________
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|