Another factor that I have seen happen is the combination of the direct signal
radiating from the antenna and the reflected signal bouncing back from the
metal surface combining out of phase to give a null and resulting in a weird
pattern effect. I think you should try to get your antenna well above the tank
to avoid such an effect. A careful calculation of distance away from the tank
to give phase addition would give you a strong directional effect. No doubt
antenna modelling software would graph this.
Bill
VE5FN
----- Original Message -----
From: N0ATH
To: Repeater-Builder@yahoogroups.com
Sent: Sunday, June 22, 2008 9:26 PM
Subject: {Disarmed} Re: [Repeater-Builder] Antenna mounted near a water tank
on wood pole
Hello Joe - I would try to find at least 90 feet of Rohn 25
and anchor it to the pole - then mount your antenna on
the tower and you would be able to have an earth ground,
a built in ladder, and something to tie your feedline off to.
I recently bought 210 foot for $180 on the ground - I think
you would appreciate the accessibility after a few years.
Just an opinion - Dave / NØATH 73
----- Original Message -----
From: Joe
To: Repeater-Builder@yahoogroups.com
Sent: Thursday, June 19, 2008 1:49 PM
Subject: [Repeater-Builder] Antenna mounted near a water tank on wood pole
Hello Groupies,
I had this question posed to me and I am looking for your opinions. The
question below has been edited to protect the innocent....2 meter
repeater......
73, Joe, K1ike
========================
I have a technical question with which I need assistance........
repeater has been at the water tower site, with the antennas on the
water tower........After some consideration, the town has requested that
we not return to the tower. ..... They have offered instead to install a
seventy foot pole (telephone pole) for our use. The water tower is
seventy-eight feet tall at the side, with a dome that adds approximately
another five feet. We had mounted the antenna to a pole which adds
another five feet. So that means the bottom of the antenna previously
was at approximately eighty-eight feet. The telephone pole would be
mounted outside of the fence which surrounds the water tower,
approximately twenty to forty feet away from the tower. My technical
question is this: how much interference or interaction should I expect
from an antenna mounted seventy-five (or so) feet high, approximately
twenty to fifty feet from a eighty-three foot metal and water
(cylindrical) obstruction? And how high would the antenna have to be
mounted to avoid any interference/interaction with the water tower?
=========================
Here were my thoughts and reply to the question:
You would probably suffer from shading in the direction of the water
tank from the antenna. If the antenna was 20 feet from the tank and the
tank is approximately 40 feet in diameter (not sure of this dimension)
you would be blocked for about 90 degrees. That would be in the
direction of the water tank. If you increased the distance to 40 feet
from the tank, you would block approx. 35 degrees of coverage. (Check
my math, that's just a quick guess.)
So,
-Yes, you would have some blockage of coverage. My estimates may be
overrated. Signal will probably get around the tank, but will not be
line-of-site.
-Moving the telephone pole as far away as possible will minimize the
blockage in coverage.
-I don't think you will see any indications of electrical problems, such
as VSWR, in either case.
-In order to eliminate blockage, your antenna would have to be higher
than the top of the water tank
-Telephone pole installations rarely stay vertical, they usually tilt or
the wood bows after time. You would be best to stay with a low gain
antenna unless you want to periodically climb the pole
a plumb the antenna.
-Telephone poles are difficult to safely climb. See if they will step
it for you with metal pegs. They can start the pegs 15" off the ground
so the kids can't climb it. You would bring a ladder when you wanted to
climb it. I don't think they have a bucket truck that goes 70+ feet?
Who will climb it?
-When they say "70 foot pole" do they mean installed, or is that the
length BEFORE it is installed?
-Make sure that you use an antenna mount that will be easy to adjust if
you have to re-plumb the antenna.
-Have the telephone pole installed so that the shadow is in an area that
is not critical to your coverage. Place it as far from the tank as
possible.
-I don't think that an antenna mounted 70 vs an antenna mounted 88 feet
is going to make a great difference. Both heights should clear the
treetops. The main problem is going to be the steel tank in one
direction.
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