[TenTec] Grounds

[kris merschrod]

OK, guys, I will share what was told to me by engineers at Arecibo
Observatory - that is the "big dish" think of the movies "Contact"  and
"Golden Eye." 

My XYL and I lived on the observatory grounds for 6 months in the cabin
shown in Contact and daily looked over the "Instrument" as it is called.  In
the work I was in the dome and on top of the platform, and also on the
ground under the platform. Recall, my field is sociology and not electrical
engineering.

The hemispheric dish is 300 meters in diameter and it is suspended in a sink
hole on steel cables anchored to the rock.  At the bottom the dish is
probably only 25-30 feet above the ground, the rim of the dish is, more or
less at average ground level about 145 meters or so above the bottom of the
sink hole.  Then suspended above the dish is a platform where the radio
receivers are in the Dome.  Antennas for receiving and transmitting are also
suspended from that platform.  The platform is some 150 meters above the
bottom of the dish. The platform is held up by steel cables going to three
pylons that are of reinforced concrete 80 meters high above the average
ground around the sink hole. 
The men who work on the platform told me, as we climbed up an around, that
when electric storms start to come in, their hair stands up and they hear
crackling and they know that it is time to head for ground.  Usually via the
walk way that supports the wave guide for the radar work or via the gondola.

POINT:  IT IS ONE BIG LIGHTENING ATTRACTOR or gives off electrons like mad!

The cables supporting the platform expand and contract with the warming of
the Sun, so to keep focus (we are talking very high frequencies so a wave
length is short) the plat form has to be pulled down or released to keep the
distance between the dish and the receivers on the platform precise.  From
each of the three arms of the platform are a set of cables going down to the
ground under the dish where there are huge concrete blocks where winches
will either pull or release the tension to maintain focus.  The winches are
controlled by microprocessors mounted on the concrete blocks but insulated.
To direct the lightening to ground there are parallel cooper cables to the
tension cables and they go into deep pits next to the concrete blocks and in
those pits is salt water.  Those copper cables and saltwater pits are the
"low impedance path to ground."  
The sort of holy grail of the tech manuals!

The "instrument" runs 24 hours a day and scientists all over the world take
turns using it by remote control - lightening or no lightening it goes on
receiving and sending (depending on the experiment).  During storms the
voltage is rushing up and down those cables and think of the inductance
between those parallel cables yet those micro processors , only a couple of
feet away from the cables seldom get taken out.  Remember that those
receivers on the platform are way above ground (forget the fear of heights
in your second floor shack!) The receivers send the reception to the control
center computers and off to the internet.
So the take away is that if just do the low impedance to ground right, then
we can operate 24/7 lightening or no lightening! Back to the electrical
code?  No, I'd like to see some practical short hand approach.  Jim should
have dumped some salt into that "pit." Warning, take it all with a grain of
salt. 

For photos just Google Arecibo Observatory and enjoy the view.  If you
google Arecibo and Merschrod, then there are some photos inside the
Gregorian Dome where the receivers are mounted. Steve the engineer went from
there to work on the "Square Kilometer Array" in France (We just need more
wire in the air as hams!)

Kris KM2KM

MERSCHROD
123 Warren Road
Ithaca, NY 14850
Skype: Merschrod


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Jim Allen
Sent: Sunday, May 18, 2014 3:17 PM
To: Discussion of Ten-Tec Equipment
Subject: [TenTec] Grounds

While perusing eHam for another topic, I ran across this article which I
share to provoke comments.

http://www.eham.net/articles/21383

73 de W6OGC  Jim Allen
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