[TowerTalk] GUYED TOWERS vs. SELF SUPPORTING

[n4kg@juno.com]

GUYED  TOWERS  vs.  SELF  SUPPORTING  de  N4KG

Several TowerTalk subscribers have pointed out the limitations
of small (narrow) lots as reasons for considering self supporting
or crankup towers.

All of the Rohn catalogs recommend guy anchor spacings of 
80% for their guyed towers but this is not the only option.

One of the major limitations in guyed tower loading is the
downward load on the bottom section.  At the maximum
rated height, guy wire tension is a major consideration.

At heights below 100 ft, it is feasible to trade off maximum
antenna load for guy anchor spacing.  Look at the recommended
spacings for the Rohn tilt over towers which are limited to 48, 58,
or 68 feet.  Less than 50% ! 

I know of a pair of 130 ft Rohn 25 towers in Michigan which have 
been supporting a large Lazy H wire array for decades with the 
guy anchors at 60 ft.

When deviating from the Rohn recommendations, it would be wise
to do the calculations on the downward forces under maximum
wind and load conditions.  Until Rohn offers us more options, we 
are on our own to either perform the calculations or hire a PE to
do it for us.

For large antenna loads, you may want to increase the tower size
(R45 vs. R25 or R55 vs. R45) and use heavier guy cable (1/4 inch
vs 3/16) at least for the top set of guys.  Anchor size and strength
should also be considered.

Remember, a steep guy is better than NO guy !

Be sure to calculate the maximum vertical load for your proposed
installation and choose a tower / guy wire that are rated for that load.

And remember, higher is NOT always better, especially on the 
higher bands.  With the increasing sunspots, 40 to 60 ft is usually
better than higher antennas on 10 through 20 meters.  On 40 M,
60 to 80 ft works well.  These heights are managable on small
city lots with properly guyed towers.

de  Tom  N4KG



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