Hello Ray,
I have a DX-Engineering 8040VA-1 vertical. This is a ~57 foot vertical
with a small hat assembly to tune at the low end of 80. There is a trap
for 40M.
http://www.dxengineering.com/parts/dxe-8040va-1
DX Engineering offers an optional 30M add-on kit which is a 1/4 wave
wire on 30M, spaced 8" (?) from the main vertical. The 30M wire is
simply attached to ground at the antenna base and it floats at the top.
http://www.dxengineering.com/parts/dxe-8040-30aok
In other words, the 30M added wire is not driven directly and forms a
parasitically driven element. Think "sleeve dipole".
Anyway, it works extremely well on 80-40-30M. I wonder if something
like this would work in your situation? It would greatly simplify the
feed system. The bandwidth is very good on all bands too. You could
probably try it without having to take anything down...it's all at the
base of the antenna.
On 9/7/2013 1:48 PM, Ray Benny wrote:
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
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73,
Gary K9GS
Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com
CW Ops #1032 http://www.cwops.org
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