[time-nuts] Freestanding mast

[Oz-in-DFW]

On 9/2/2010 7:46 PM, Charles P. Steinmetz wrote:
> I'm curious what the best freestanding mast is for a timing antenna
> (think Lucent timing antenna or marine "mushroom" GPS antenna -- light
> and pretty small).  The mast would have its highest support at rooftop
> or chimney-top level, and could extend from there as far downward as
> the ground with additional supports as required.

How far up do you need to go?  Do you need to clear dense trees or lots
of adjacent buildings, and if so, how high are they?

If you get about all nearby structure and obstructions you need to start
thinking about lightning protection in a vary serious way. 

> Should be able to survive at least Category 2 winds and heavy snow and
> ice.

The definition of heavy snow and ice is very regionally dependent.  I'm
in the DFW are and heavy = any.  I used to live in Laramie and worked on
mountaintop radios where heavy was measured in feet.  Where are you?
Likewise the structure required to support survivability is heavily
dependent on worst case ice load and height.

110 mph/50 m/s isn't that hard for a few feet of pipe clamped securely
to a structure to survive.  Even ice load isn't much of a factor as it's
more structural than load for a small antenna and short pipe at some
point .  Falling ice clears all bets. Literally. 
>
> What reasonably available mast material no more than, say, 3" in
> maximum cross-section would allow the most vertical extension above
> the highest support, and how much extension would that be?  I'm
> thinking 10 feet of 2" or so thin-wall steel tube may be OK, but
> beyond that I don't know.  Tubing is probably not the optimum shape,
> but I assume the availability of other engineering shapes (say, "+"
> cross-section) is likely to be limited.
>
> Ideas?

Most of the telecom targeted antennas are made to screw on to 3/4" or 1"
water pipe with the feedline in the pipe.   Typical application is either:

   1. A short (1 - 2 foot)  piece of rigid conduit of the correct size
      is fit to the shelter with a sweep bend to feed the antenna
      feedline directly into the building.  These are often not clamped
      at all, though frequently clamped to an eave.
   2. A short (1 - 3 foot)  piece is clamped to a larger mast and a
      longer feedline is run into the building.

>
> Thanks,
>
> Charles
>
I suspect you may be over thinking this and a foot or two of pipe on an
appropriately located eave will do fine.  If you need to go on a
chimney, get a chimney strap kit and four feet of pipe sized to fit the
antenna.  Strap it at points two or three feet apart.

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