Funny, I qso'ed a ham in Pisa, Italy last night!
Sometime ago I went through using a transit for plumbing a tower....
You must be careful when using a transit, of unknown history when trying
to determine a plumb plane, as in plumbing a tower.
Notice that the older transits have bunches of little screws,
holes in them that are connected to the mounts of any of the various level
vials on the instrument. DO NOT MESS with these! They are for
calibration labs to calibrate the instrument. Basically , we want the
horizontal plates to be truly horizontal when the level bubbles in 2 axes
say they are level. There are no guarantees that this is the case. Also
we want the axis of rotation of the telescope to be horizontal, and
parallel with the base plates of the transit. We want the crosshairs
inside the telescope (early ones were literally black widow spider web.)
to define a concentric direction with the physical axis of the telescope.
Within limits, these requirements can be slightly out of kilter if the
user knows how to compensate for them. Here is how to test your transit
to see if it can define a plumb plane, as in a tower plumb check.
1. First, Set up the transit a long ways away from the base of the tower.
This allows for more accurate pointing.
2. Level the istrument. Use the brass levelling screws which bear
against the bottom plate of the instrument. GENTLY snug the screws on
opposite sites, approximately level. Gorrilla efforts WILL DAMAGE the
levelling screws. Level the plate bubbles twice, 90 degrees out from each
other. Rotate SIMULTANEOUSLY opposite screws. Using your thumbs to
rotate, ONLY rotate them so that your thumbs either move towards or away
from EACH OTHER. Never move your thumbs in the same direction! While
watching the bubble in the level vials, remember that the bubble will move
across the vial in the direction that your LEFT THUMB moves. Do this on
two orthogonal axes. Check by rotating the instrument 180 degrees, and if
correct, the level bubble will still be centered. Then the bubble will
also be centered an any direction. If not the case something is wrong.
3. Sight an identifiable centered location on the base of the tower. eg:
the center of a a center base pin. If one is not
there, then carefully pencil a line on the concrete base, from one corner
of the tower, to the
center of the opposite side. Do this for all three legs. The three
penciled lines should intersect at a single point.
4. Sight a centered object on the top of the tower eg: rohn top section
sleeve. or perhaps a mast. Should be centered on cross hairs. IF NOT:
5. Either the tower is out of plumb, or the instrument is not accurate.
Check as follows:
6. Tilt the telescope 180 degrees from the first measurement.
7. Rotate the base plate 180 degrees to repoint to the tower.
8 Sight the bottom target, as before, and then the top. Do not move the
horizontal plates between these pointings.
9. If the cross hairs do not land on the very same spot at the top of the
tower, the instrument is not in adjustment, Though all is not lost....
10 The correct plumb point is HALFWAY BETWEEN the first top target and the
second pointing at the same location. Better be less than about 1/8"
difference.....otherwise the instrument is truly unusable.
11. Do this from two locations, approximately 90 degrees from each
73, DX de Pat, AA6EG/N6IJ
"The Contest Station from the Government"
Marina Amateur Radio Contest Station; N6IJ
599 DX Drive
Marina CA 93933
On Sun, 21 Jun 1998, T A RUSSELL wrote:
> Plumb Bob vs. Transit de N4KG
> Frankly, I am amazed at this claim that a plumb bob caused the top
> of the described tower to be off plumb by 2 feet. Could the transit
> be the problem and not the plumb bob?
> I use two plumb bob's, at right angles, to sight the tower and believe
> I can get within an inch or two with this method. I use stranded
> #22 wire and a 5 lb. window weight. Once it is stable it makes an
> straight vertical line to sight. I usually sit on the ground with my
> feet in front
> and arms behind me and move only my eye as I sight up and down the
> tower / plumb bob wire. A portable tri-pod or other support comes in
> de Tom N4KG
> On Thu, 14 May 1998 10:48:19 -1000 Jonathan Starr <firstname.lastname@example.org>
> >A Builders Transit is the only tool that can properly plumb a vertical
> >structure. Any wind or slight convection will make a plumb bob
> >My 100' Rohn 80 tower on sloping ground was plumbed as best possible
> >using a plumb bob, 8' level (while climbing up and down), the horizon,
> >and eyes, and we thought it was pretty good. The only clue that
> >something wasn't right was that the guys on one side sounded
> >duller, when tapped sharply with a wrench. This turned out to be the
> >down side.
> >When I brought a Transit home from the pawn shop, I was shocked to
> >the tower over 2' out from vertical, necessitating two new big grips.
> >Luckily we had left the guy wire ends trimmed long to loop through the
> >Using a transit is easy. There are two bubble levels that must be
> >centered using adjusting screws, and then the crosshairs should ride
> >right up a tower leg or the center of a pole as you elevate the
> >The only thing that makes adjusting guys with a transit time consuming
> >is having to move the transit between two locations. Two transits
> >make it a very quick job.
> >I see used transits readily available now that surveyors and engineers
> >are using laser and gps equipment. Check your local rental or survey
> >equipment supplier for used gear.
> >Jonathan KH6X
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