T A RUSSELL wrote:
>
>Several people have correctly indicated that guying a crank up tower will
>add additional vertical load on the cables which needs to be evaluated.
>
>An intermediate step would be to guy ONLY the bottom section which
>does NOT load the cables.
Here are a few thoughts from the land where fixed guyed towers under
100ft are quite rare, and crank-ups are everywhere.
As well as amateur towers, many thousands of the same free-standing
crank-ups have been used for fixed and temporary comms and lighting uses
for something approaching 40 years. More recently the same towers are
being used for temporary cellular radio installations and point-to-point
microwave links until the permanent towers go in, and in these
applications they are almost always guyed because the antennas have
large wind areas and narrow beamwidths.
Almost all of these towers use nested 20ft sections and are based on (or
copied from) an original design concept by G3NJI, whose Strumech company
has done rather well...
There are some important differences between free-standing crank-ups and
fully guyed towers which haven't been spelled out in this discussion.
1. The free-standing crank-up has a base that is designed to take the
sideways (rotation, rocking) forces from a completely unguyed tower.
These forces are designed to be transmitted down to the base by the
stiffness of the tower sections. If such a tower is also guyed, the
sideways wind forces are still being shared between the guys and the
base. This is quite unlike a guyed tower where the base may have very
little rotation resistance - zero in the case of a pier pin - and the
guys have to take all of the sideways forces.
2. You can guy the top of the first tower section without placing any
down-force on the cables (although the combined stiffness of the base,
ground post and tower section may make this unnecessary).
3. If the tower includes a support latch at the top of the first
section, you can also guy the top of the second section without placing
any down-force on the cables.
4. Any guys on higher sections only place down-force on the higher
cables, which are carrying less dead-weight anyway, and possibly have
considerable reserves of strength.
5. The stronger the side-winds, the higher the friction between the
sections, which again reduces the load on the cables.
6. Because the guys are only sharing the sideways forces, not carrying
the whole load, the pre-load tension on the guys can be less than for a
fixed tower.
I don't have direct experience of guying a crank-up (so maybe other
British tower users can confirm this) but the weakest point is usually
where the top section goes into the next section down. Therefore a set
of relatively light, low-tension guys at the top of the top section can
make a big difference to stability without transmitting unacceptable
down-forces to the cables below.
Obviously all the above is subject to detailed mechanical analysis,
which is a lot trickier for a crank-up than for a plain guyed tower, but
maybe it helps to show why guyed crank-ups aren't necessarily a total
disaster.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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