[SBE] conversation topic: how often do you check tower ground wires?

[Cris Alexander]

At an AM site, you really need two separate ground systems: one for RF and
one for lightning. At first glance, it may seem that a radial ground system
would present a low-impedance path to ground and as such, would dissipate
lightning energy fairly effectively. That is not usually the case, however.
Ground straps, screens and radial wires cannot handle much in the way of
current (they are not designed to), and they can even melt when a heavy
strike occurs. 

You need an array of ground rods around the tower base, all bonded to heavy
(1/0 or bigger) cable bonded to the tower base (ground side of the spark
gap). Mechanical clamps do not provide a good enough connection, especially
in the long term. CAD-welding is the way to go. One or more ground leads
should also go to the ATU chassis to provide a low-Z ground path for
lightning/static currents discharging across the horn or secondary ball gap
there. This same technique can be used at FM/TV or other grounded towers as
well to good effect.

I also advocate an array of ground rods around the transmitter building. The
electrical service entrance often becomes the lowest-Z ground path for
lightning currents that make it to the building, and the shortest route from
transmission line (or whatever) to ground is directly through your
transmitter, remote control, processor, etc. The copper strap between the
transmitter building and tower base(s) is worthless as a building lightning
ground. Tie the electrical service entrance ground to the ground rod array
ring so as not to create disparate ground paths (with considerably different
potentials in a heavy strike!). Check with the local electrical inspector
before doing this, however. I have had to argue my way into it a few times
but have always been successful.

The RF ground system should be inspected as often as conditions warrant. In
dry, non-alkaline soil, a ground sytem may last 40 years without significant
deterioration. Dig up a radial out in the middle somewhere periodically as a
representative sample. If its diameter has been reduced significantly by
corrosion or cathodic action, it may be time to plan a replacement.
***Nearby oil or gas pipelines that employ cathodic protection can
accelerate cathodic deterioration of an AM ground system!***

Perhaps the easiest way to check the effectiveness of an AM RF ground system
is to make a few close-in radial field strength measurements, beginning as
close to the tower as you can get an on-scale reading (10 volts) and
extending out 1 km or so. Plot the measurements on a piece of log-log paper
and graphically analyze for inverse distance field (IDF). If the IDF is what
it should be (use the last full proof or 73.190 Figure 8 as a reference),
the ground system is fine. A deteriorated ground system will show up as
excessive loss and reduced radiation efficiency (reduced IDF).

For more information, go to:

http://www.crawfordbroadcasting.com/~cbc/Eng_Files/Lightning%20Protection.pd
f

______________________________
Cris Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
Denver, Colorado
(303) 433-0104
(303) 433-0905 Fax
Member AFCCE


-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Bill
Burckhard
Sent: Wednesday, February 13, 2008 11:07 AM
To: 'sbe member discussion mail list'
Subject: Re: [SBE] conversation topic: how often do you check tower ground
wires?

We don't have a lot of lightning here in Billings MT. We have been tying the
bottom of the tower into the ground system that is also used for the
transmitter. The transmitter building has ground strap that is tied to
ground rods and goes underground all the way around the building perimeter. 

William R Burckhard
445 South 24th Street West
Billings, MT. 59102
(406)652-4743
billb at khmt.com
www.yourbigsky.com 

-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of
A9xw at cs.com
Sent: Wednesday, February 13, 2008 10:52 AM
To: sbe at sbe.org
Subject: [SBE] conversation topic: how often do you check tower ground
wires?

Those buried radials won't last forever. Copper, turning green, then dust.
So
what is the thinking on how often to dig one up and check the condition, or
measure the ground resistance?  How about those that are not AM towers
(ground effectiveness can be gauged by field strength on radials), but your
typical 1000', 2000' 
tower lightning protection ground system?  Didn't use copper because someone

steals it?  Ground rods vs radials?


:-)

Henry Ruhwiedel 


   
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