Topband: Fw: Are stacked verticals feasible?

[Guy Olinger K2AV]

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 at 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
>