At 11:51 PM 6/29/2005, Ian White GM3SEK wrote:
>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.
The systems I've seen keep a small positive pressure inside at all times;
just a fraction of a psi is enough to keep the water out and the seals
loaded. But, as Ian says, you need to have some source of that
pressure. Although you don't need a compressor running continuously.. a
high pressure bottle as a reservoir works, you do need to run the
compressor periodically (as in every few days). This isn't necessarily a
problem (your refrigerator has a compressor that runs intermittently for
years, and is quiet, etc.), but IS a complexity that doesn't apply to the
usual ham setup.
They also don't use just any old compressed air. It has to be dried, just
like any other compressed gas setup, or you get condensation and corrosion.
>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.
In theory, if they're always pressurized with dry gas, then this shouldn't
happen. There's a little bit of leakage, so the new dry gas tends to flush
out the moisture.
>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.
Exactly that... constant, regular maintenance (like an older British sports
car<grin>). They're not something you bolt to a trailer and use once a
year at FD.
>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.
Jim, W6RMK
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