[AMRadio] "Hot" Broadcast Towers


sbjohnston at aol.com sbjohnston at aol.com
Mon Jan 12 21:40:03 EST 2009


>was pulling it out of its socket when the old Andrew dehydrator
>about 2 feet behind the transmitter decided to kick on.

I've got a dehydrator on a dish antenna's waveguide and feedhorn at one 
of my transmitter sites that pulls that one on me every so often.  
There's something about being in the transmitter room by myself, 
getting immersed in the work, and then when the pressure has finally 
dropped enough to make the compressor kick on it startles me into orbit!

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
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-----Original Message-----
From: Michael D. Harmon <mharmon at att.net>
To: amradio at mailman.qth.net
Sent: Mon, 12 Jan 2009 7:12 pm
Subject: [AMRadio] "Hot" Broadcast Towers










As a former broadcast engineer (MANY years ago), the discussions of hot
towers, lighting transformers and base insulators caught my attention.
One issue some of you may not have thought of is tower/antenna
maintenance.  Many AM stations never go off the air.  Some of them run
directional at night to avoid interference with other stations, and a
lot more simply have a "night power" restriction, but the tower is fed
with RF 24/7.

You might ask how a worker can climb a tower while it's hot.  Well,
think about the birds perching on the 12,000 volt lines which run
overhead through the community.  If they are on the wire, they're at 12
kV potential, and don't even know it.  On the other hand, I've seen
entire sections of town blacked out because a hapless squirrel or
raccoon tried to climb a pole with a "pole pig" (distribution
transformer) mounted on it, and managed to get between the hot wire and
ground.

The answer is - they jump.  They stand on the ground next to the tower
base, give a giant leap and grab hold of the tower.  Same way with
getting off.  Climb to the bottom, make sure all your belts, etc are
completely loose, and give a giant spring backwards.

I spent my time back in the early Seventies signing logs, trying to
de-ice the elements after a winter storm, and getting late-night calls
from a so called "audiophile" board operator who swore that the
equalization had magically gotten out of whack (since he had come on
duty) on the old Ampex AG-350 tape decks in the control room.  Yes, I
was one of those First Phone holders who were downgraded to the
so-called "General Radiotelephone" license a few years later.  I
remember one cold blustery winter night when I got a call from the 
board
man that the transmitter wasn't making power.  I went in, and found  a
4CX250B driver tube in our RCA BTF-10E FM transmitter that was getting
VERY soft.  After signoff (1:00 AM), I shut down the transmitter and
went to work.  The transmitter room was as quiet as a tomb after the
blowers all shut off, and it was eerie having the station to myself.
The plate supply in the transmitter was around 6,500 volts at about 2.5
amps, and I was painfully aware that I was the only guy there, so the
need for caution was at the forefront of my mind.   The driver tube was
all the way in the back, under a shelf beneath the 4CX10,000D PA, and
required an offset screwdriver to get the anode clamp loose.  I opened
the doors, hit everything with the grounding stick, and reached in and
started loosening the plate clamp on the driver.  I grabbed the tube,
and was pulling it out of its socket when the old Andrew dehydrator
about 2 feet behind the transmitter decided to kick on. "Karoom, CHUG,
CHUG, CHUG ..."

I swear it couldn't have scared me more if it had been a quarter stick
of dynamite.  I peeled the skin from my wrist all the way to my 
shoulder
getting my arm out of that transmitter!  I don't know how, but I never
dropped the tube!  After recovering from my shock (and changing my
underwear), I put the new tube in, checked the driver and PA tuning and
went home.  It's been 35 years since that incident, but I still 
remember
it like it was yesterday.

I don't know if the technique for mounting and dismounting hot towers 
is
still the same or not.  With all the OSHA rules and concern over
workplace safety, I'd be very surprised if there weren't some
politically correct way to do it nowadays, but that's the way we did it
in the Sixties and Seventies.

Hope I didn't bore you with my story!

73,
Mike Harmon, WB0LDJ
mharmon at att dot net
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