[TowerTalk] Fwd: Rooftop tower as Vertical Antenna

[Kelly Taylor]

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 at 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 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
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