Jim Lux wrote:
>
>More seriously, those things are pretty cool. There are several
>manufacturers of them. They use them for news vans as well. Do you
>have any information on the air pressure and volume required (some are
>fairly low pressure, high volume.. others are high pressure, low volume).
>
>Properly maintained, they'll cycle up and down several times a day for
>years. They're also pretty fast (as in seconds to extend).
>
>What they may not have is a huge amount of sideload capability,
>especially while extending or retracting. The ones I've seen have been
>designed for a top load of a few square feet (maybe 6-7).
I had two of these things for some years. They work pretty much as Jim
says... but there are some major disadvantages for long-term use at a
home station.
The main problem is that they rely on a very delicate feather-edged ring
seal at the bottom of each section. A typical mast has 8-10-12 of these
seals, and if any one of them fails...
The main problem is water inside the mast. Any rainwater runs down the
outside of each section, and is then trapped inside the next section
above the seal. When you de-pressurize the mast, that water goes right
inside to the bottom. For home-station use, you won't want to keep a
noisy compressor running, so you can lock each section in place... but
then the seals relax, and a *lot* of rainwater runs down the inside.
So every time you lower the mast, it spits out a horrible mess of greasy
water. And you don't dare to raise or lower the mast in freezing
conditions, for fear of tearing the seals.
Because the inside of the mast is always wet, corrosion can make a
strip-down very difficult indeed. The ones I've used are a nightmare of
tiny parts, including more than a hundred 3/16in screws... steel
screws, corroded into aluminum.
Bottom line is that these masts are great for their intended purpose: a
mast that can go up anywhere, for a short time, and then you let it down
and move on. But they require *constant* attention.
The only time I'd recommend one for home-station use is if you're in a
really tight corner and there's absolutely no other way to get an
antenna up.
--
73 from Ian G/GM3SEK
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