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more on masts

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Subject: more on masts
From: (Stan Griffiths)
Date: Sun, 13 Oct 1996 16:58:40 -0700 (PDT)
>     Seems like we may have beat the EMT and other mast material questions 
>     to death.
>       Does anyone have any thoughts as to what might be optimum or even 
>     advisable in how much mast to "bury" in the tower, vs how much is 
>     above it?  How much does using a thrust bearing change this, if any?
>       Thanks for your thoughts / comments .  Frank AA0GG
>FAQ on WWW:     
>Administrative requests:
>Sponsored by Akorn Access, Inc & KM9P

If you are talking about Rohn 25, there aren't a lot of options about how
much mast to put down in the tower.  The rotator will only mount in certain
places since the standard Rohn rotator mounting shelf will only mount where
there are no diagonal members in the tower section.  This usually means
mounting the rotator about 2 feet below the top of the tower, depending a
little bit on which top you have selected, flat top or pipe top.  You can
also do what I have done and mount a rotator plate at the top of the section
just below the top section.  I had to cut out one horizontal member from the
very bottom of the top section to get the rotator to fit into the tower but
I don't think this weakened it much since it has been up now for more than
20 years this way (actually, I have done this to two towers and both have
been up for more than 20 years.)

Here is another option which has come to my attention recently.  Rohn makes
regular seven foot sections which sell for exactly the same price as 10 foot
sections.  The reason for this is so that they can be shipped via UPS.  Even
though they cost the same as 10 foot sections, this does give you a 3 foot
shorter option for distance between tower top and rotator placement if you
put it where I did in the above examples.  You could use a 7 foot section
and a BPL25G as a top section with a thrust bearing on the top.  You would
still have to cut out one of the horizontal members to get the rotator to
fit into the tower.

I currently have a design on paper for a new rotator shelf that would mount
on the horizontal members of any 25G section regardless of whether or not
there are diagonals present.  Any Ham-M, II, III, or IV could be pushed into
the 25G through the triangle formed by a tower leg, a horizontal, and a
diagonal, if done just right.  This new shelf would allow MANY new options
for mounting a rotator at lots of different places in 25G.  Sorry,
Tailtwisters won't fit.  The plan is offer this new mounting shelf for sale
after we try it out first.

As to how much mast should be down in the tower, I think it depends somewhat
on how much is sticking out the top and how much you have loaded it.  I know
this: it is highly desireable to make sure the mast will remain supported
vertically with the rotator removed for service.  A 25AG3 pipe top will do
this just fine with no other support required.  When using a 25AG4 flat top
with a thrust bearing, however, the bearing is not enough to keep the mast
upright so you have to provide another support lower in the tower.

Rohn 45 is a whole 'nother discussion since rotator shelves CAN be mounted
in the same place that diagonals exist.  The problem with 45 rotator shelves
is that they are simply made of material that is too light for what most
people want to do with them.  I have a plan on paper regarding THAT too.


FAQ on WWW:     
Administrative requests:
Sponsored by Akorn Access, Inc & KM9P

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