My experience with 80/160m verticals on the same structure:
I have a 70 ft tall irrigation pipe vertical with two 50 ft top hat wires
for 160m. My radial field is 110 radials most about 135 ft long. I also
have 4 - 8 ft ground rods around it for lightning grounding purposes. To
match, I use a coil at the base and tap up to match, and tap down from the
top to resonate. The band width is only 50 Kz though. The antenna works
great on 160m, I work most of the DX I hear.
To make this structure work on 80m, I added a insulated #10 drop wire about
14 inches away at the 60 foot level. This wire is not attached to the 70 ft
vertical but goes into my matching system that has a second coil for 80m
matching/loading. I tap it the same way as the 160m load/matching coil.
But, in order to match it on 80m* I have to ground* the 70 ft irrigation
vertical at the base. This vertical wire works great on 80m but is rather
narrow banded too, maybe 60 Kz max.
I do not use any computer modeling, so I don't know what it would reveal
about my system.
Ray,
N6VR
Chino Valley, AZ
On Sat, Sep 7, 2013 at 7:33 AM, Guy Olinger K2AV <olinger@bellsouth.net>wrote:
> The short story on a low band collinear supported by or close to a tower is
> that it depends like everything on the details.
>
> One really needs to literally model any long linear metallic conductor
> within a wavelength. Also with typical towers stuck in a blob of concrete,
> and the bottom usually into some degree of dirt below the concrete, it will
> be very hard to characterize what the model should specify at the base of
> any tower as a connection to dirt. It will be necessary to include in the
> model a topping yagi's mast and boom and lowest band elements farthest from
> the mast. If an inverted vee off the tower doesn't have it's coax shield
> grounded to the tower at the top followed by a balun effective on the
> collinear frequency, then the coax plus the vee may be a major detuning
> influence. Again, literal modeling of everything.
>
> The very common answer is that the idea gets dropped after modeling, and
> seeing how much interaction there is, getting into detuning of towers,
> blocking low band common mode current on coax + inverted vees, etc.
> Tricky, tricky, and not simple, unless the area was constructed with the
> low band stuff in mind from the get-go.
>
> It can be done, but not without modeling. A lot of the interactions are
> counter-intuitive, and don't make themselves apparent until all the nasty
> modeling math is run. You can get lucky with what you already have in
> place, but you can't see it with your intuition.
>
> 73 and good luck, Guy
>
>
>
>
> On Sat, Sep 7, 2013 at 1:31 AM, Bob Kupps <n6bk@yahoo.com> wrote:
>
> > Say Paul I am going to try the same setup except ground the 80m vertical
> > as a close (5') coupled radiator and see what the feed impedance of the
> 160
> > vertical is on 80. It models well...
> >
> > On 09/06/2013 09:26 AM, ZR wrote:
> > > I would think that at 6-12' spacing from the tower it would minimize
> > interaction on 160 or 80?
> >
> > I don't know, Carl. I'll leave it to the experts. What I do know is
> > I have made several attempts to erect a vertical for 80 meters near
> > my 160 meter tower, using the same radial system. At 10 foot spacing
> > from the tower, the base resistance of an 80 meter quarter wave
> > vertical was less than 5 ohms. That to me suggests significant
> > interaction with the tower. At 5 foot spacing the base resistance
> > was less than 2 ohms! I don't have the data handy but I seem to
> > recall having to adjust the length considerably from a quarter
> > wavelength to cancel a reactive component.
> >
> > 73,
> > Paul N1BUG
> > _________________
> > Topband Reflector
> > _________________
> > Topband Reflector
> >
> _________________
> Topband Reflector
>
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Topband Reflector
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