Gold plated contacts in salt water

I've been in the remains of an AT&T microwave relay station. The fitting that brought the waveguides through the wall and provided the dry-air connections for them were still there (with a stamped date of

The phone company also used to supply small quantities of dry nitrogen for free from large roadside tanks, but now that they have decided to let the copper infrastructure rot, I hardly ever see them anymore.

Why not leave the bottle at the bottom and climb with a hose? Maybe the hose is harder to deal with than the bottle, or there's nobody at the bottom to work the valve...

Matt Roberds

I don't climb such towers any more. That was when I was young and stupid.

The box only needs a few PSI above atmospheric pressure to keep the water out of about a 2-3 cu-ft box. That can be handled by a very small bottle about the size of a camping propane cylinder. The dry air is delivered in heavy 120(?) cu-ft cylinders, that are difficult to handle. The safety and insurance people insist that I don't transfer a few cubic feet of dry air to a non-certified tank. What is allowed is air transfer to a 7 cu-ft tank about the size and weight (17 lbs) of a recreational propane tank certified to 240 psi max. That tank, and regulator gets hoisted up the tower on a rope to repressurize the NEMA box. It's not as bad as it sounds as long as the wind doesn't bang the tank against the tower.

When nobody was looking, I filled the tank with my air compressor, after filtering through a desiccant. If I was desperate, I used my bicycle pump. To avoid the climb I wanted to install a permanent plastic or vinyl hose up the tower and into the box. However, the site manager decided to charge extra for the hose, which he considered the same coax and control cables. Sigh.

Some of the newer systems include small built in air compressors to maintain positive pressure. I haven't seen any of those yet. I'm told the problem is if someone opens the box, the air compressor must run for many hours to purge all the moist air that gets in.

I actually like pressurized enclosures, as long as someone else hauls the tanks and climbs the towers.

Thanks for the responses.

Can't use the pressurised heliax as a pipe?

Sure, if I want to also drill holes in the 7/16 DIN coax connectors at both ends. The boxes I was dealing with didn't have any coax cable connections, except for the antennas. Most cellular TMA's (tower mounted amplifiers) already use Heliax or other coax but are not pressurized.

In my situation, the boxes on top of the tower were designed to be powered via shielded and gel filled CAT5 for up to 100 meters or an overpriced umbilical cable for longer distances. They were custom built for land mobile and wi-fi frequencies about 10 years ago before TMA's became fashionable. We were running 3 or 4 boxes on a single cable. More correctly, they would be a tower mounted radio, not just an amplifier. Pressurizing the package is a retrofit and is not standard but over the years, has proven to be a good way to prevent moisture and ice damage.

With a cellular TMA, there are one or two coax cable runs coming from a DC injector, where presumably the pressurized dry air might be introduced through the coax. I haven't seen any that do this. These are expected to be swapped and not serviced on the tower. The NEMA boxes I was dealing with were too big to be easily swapped. Sorry, but I couldn't find any photos.