Alastair Beaton wrote:
>
>Lo,
>
> Pneumatic masts are also quite popular with the military, so it might be
>worth checking surplus stores and similar sources. Most are designed only to
>take a small UHF array, a couple of lamps or a wire dipole. Most types use
>key collars to lock each section in place (so you can disconnect or switch
>off the compressor while the mast is up).
>
> GTE produce a 115VAC hydraulic "Magic Mast" for the US Army. Radiation
>Systems Inc. in Sterling, Va., is one of the USAF's major suppliers.
>Hilomast is probably the biggest supplier of civilian pneumatic masts in the
>US. Their URL is:
>http://www.aoa-gps.com/hilomast.htm
>
> Clark Mast Teksam at Binstead IoW has the largest range of telescopic
>tubular masts in Europe. Racal is another major supplier to the military.
>Just as broadcasters tend to buy a "turnkey" OB/ENG truck fitted out to
>their spec with the mast included, so the military buy their masts as part
>of a specific antenna system (I think the Magic Masts go with the Patriot
>anti-aircraft/bird-scarer system) so they tend to arrive surplus when a
>system is being decommissioned.
I have two 40ft military surplus air-operated Clark masts here. The
section length is 6ft and the whole thing nests down to about 7ft which
is very convenient to work on.
The handbook says it can be raised to 40ft in two minutes by two people,
using the hand pump provided. Believe me, this is a short-cut to the
cardiac unit. I use a small electric compressor.
Air masts are great for the intended purpose of short-term operation.
For mobile use they would probably last as long as the vehicle they're
mounted on, but they do have problems for long-term home use.
The whole mast rotates in the base cage, but these masts are not
intended for large rotary antennas. In a 40ft mast with a 6in bottom
section, the top section is only 1.5in. Even if there is a key-way to
prevent rotation, it will not handle the wind torque of even a single
VHF long yagi, so you must use the locking collars. That in turn means
that the whole head-load has to go up un-guyed when the mast is raised
(although that problem is not unique to air-operated masts). The Clark
masts have very substantial spread legs that can be bolted down, and
will hold the whole thing in calm air to a light breeze until the guys
are on.
The normal method for long-term use is to lock the sections as they go
up, and then let the air leak out. In the Clark masts, the sections
engage very solidly when they're fully extended, and then they will stay
locked forever.
The main problem is with the ring seals between the sections. These can
dry out or warp without the air pressure to keep them in shape, and then
you'll have trouble next time you want to raise the mast.
Perhaps worse is the fact that rain runs down the sections, inside the
mast, down past the relaxed seals and collects in the bottom. After some
months it's quite common to find a gallon of greasy water in the bottom
section - which jets out when you lower the mast! Most serious is that
in a cold climate this water will freeze and split the bottom section.
Maintenance... forget it. They use millions of small steel screws in
each section, and after the mast has been wet these corrode solidly into
the aluminum. The design is such that you need to strip quite a lot to
get at any one part of the internals... and one corroded screw will stop
you cold.
That's why I have two of these things - the first one is lying under the
hedge because the seals failed and I couldn't get into it. However, I
had to buy another (surplus) because it is probably the only way I could
raise a 40ft mast at this QTH. The ones here have lasted 10 years each,
so the experience isn't all bad.
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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