[TowerTalk] IDEAS ??

[Dick Green]

I believe the Wilson ST-77B was the predecessor to the U.S. Tower MARB-770
tubular crankup, which I installed with a TH7 last summer. In the process of
installing my MARB-770, I was lucky enough to obtain a copy of the old
ST-77B installation manual from Rob Hummel, WS1A (I wanted to see *some*
documentation, since U.S. Tower provides almost NONE.)

When the ST-77B is fully extended, the top of the top section is about 68'
high. The manual says you can have a mast height above the top section that
ranges from 1' to 9' (max.) With an antenna at the top of a 9' mast, the
antenna would be 77' high. Presumably, that's where the "77" in ST-77B comes
from.

The ST-77B manual includes a table showing the maximum antenna wind load
when the tower is fully extended using various lengths of mast above the top
section:

Antenna         Mast         Max Wind Load
Height          Length       @ 50MPH in Sq. Ft.

  77'              9'            12.00
  76'              8'            12.75
  75'              7'            13.50
  74'              6'            14.25
  73'              5'            15.00
  72'              4'            15.75
  71'              3'            16.50
  70'              2'            17.25
  69'              1'            18.00

However, the manual cautions that most areas have winds in excess of 50 MPH
occaisionally, and recommends that the tower be cranked down at least 15
feet when not in use or during windy seasons. They also recommend a max mast
height of 4' for "large" antennas like the Wilson SY40 (anybody know the
wind load of that one?)

In contrast, U.S. Tower limits the max wind load on the MARB-770 to 10 sq.
ft. at 50 MPH, assuming no significant extension of the mast above the top
section (that's the way my TH7 is installed.) What you really need to know,
however, is that the engineering calculations for the MARB-770 indicate that
the max wind load drops to about 2 square feet at 70 MPH!

Furthermore, it's possible that the U.S. Tower versions of the tower are
somewhat stronger than the old ST-77B (at least that's what U.S. Tower
claims), and your tower has undoubtedly seen quite a few years of use.

Even though U.S. Tower has probably been conservative in its calculations,
and many hams have loaded these towers with stacked beams, if I were you I
would be very cautious. Personally, I wouldn't go over the present U.S.
Tower limit of 10 sq. ft., and would lower the tower to 50 feet whenever the
wind gets above 15-20 MPH, and all the way down when it gets above 25-30
MPH. That's one reason I opted for the motorized version.

No sense in cleaning up a pile of scrap metal if you don't have to...

73, Dick, WC1M


-----Original Message-----
From: Robert W5AJ <w5robert@blkbox.com>
To: towertalk@contesting.com <towertalk@contesting.com>
Date: Saturday, March 21, 1998 12:29 PM
Subject: [TowerTalk] IDEAS ??


>
>Here's your chance to give some free advice!  HI!
>
>Up is a Wilson 77' crankup.  A HDX48 is on the ground ready to go up.
>
>I have available for antenna:
>
>204BA  two of them
>15-5 cushcraft
>105BA  two of them
>TH3
>40-2 cushcraft
>
>Ideas as far as the best placements??
>The complete 77' tower tube rotates so it can support a rotatable stack
>but not fixed wires.
>
>73  W5AJ
>
>--
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