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
Sent from my iPhone
> On Dec 18, 2018, at 22:59, Hans Hammarquist via TowerTalk
> <towertalk@contesting.com> wrote:
>
> I don't think those concrete blocks have any good insulating value but other
> than that, go for it. You have to figure out some type of common mode choke
> for the feeders to the existing antennas, though. (That's my only advice. The
> rest sounds good. You should arrange to put the tuner close to the mast. If
> you use a coax and put the tuner in the shack you will get rather high losses
> unless the mast is at resonance. (If it is, you probably don't need a tuner.)
> Hans - N2JFS
>
>
> -----Original Message-----
> From: Edward via TowerTalk <towertalk@contesting.com>
> To: towertalk <towertalk@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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|