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

[Mike Langner]

Please understand that in my case frequently checking the ground system is
not so much based on chemistry as much as it's based on copper theft!!

Mike Langner
Albuquerque, NM
___________________


-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org]On Behalf Of Barry
Thomas, CPBE CBNT
Sent: Friday, February 15, 2008 8:00 PM
To: dynotherm at earthlink.net; Member Discussion List
Subject: Re: [SBE] conversation topic: how often do you check tower
ground


Items like this is why the SBE list is such a great resource. Thanks for a
comprehensive treatment, Phil.


Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: dynotherm at earthlink.net

Date: Fri, 15 Feb 2008 21:33:01 -0500 (GMT-05:00)
To:barryt at sbe.org, sbe member discussion mail list <sbe at sbe.org>
Subject: Re: [SBE] conversation topic: how often do you check tower ground


While the systems are separate, they have the effect of the soil
on copper in common, thus, to an extent, the deterioration of
all buried copper will be somewhat similar.

If the lightning dissipation/mitigation system is combined with
the RF ground as may frequently be the case at the tower base(s),
the condition of the gaps and the lower gap connection to the
common ring tie is generally indicative of damage, however,
careful examination of the ATU components is also critical in
this event. Not only can a strike blow capacitors, it can
reform coils, sometimes in a spectacular way.

The corrosion life of buried copper varies widely as it is
essentially a function of soil chemistry. I have seen radial
systems 60 years old where the finish of the wire had only
evolved from bright copper to heavy brown (CuO) patina. OTOH
I have a very few systems turning brittle blue in less than
ten years.

The difference can usually be found by looking at soil pH,
chloride, nitrate and ammonium content. When a new system
is put into cropland care should be taken to bring the pH
into the mildly alkaline range. If the soil has been heavily
fertilized, a small sample plot of 1/4 to 1/2 acre should
be ripped, deeply plowed, pH adjusted (generally by liming)
and analyzed again a little below the burial depth.

If ammonium or chloride radicals remain present in excessive
amounts, it should be understood that the radial system will
be a high maintenance item requiring annual inspection. At
this point it may be wise to consider an alternative site or
elevated counterpoise.

If the test plot treatment shows an acceptable result the
entire area should be conditioned in the same way before
burying copper (or other metals).

However, most sites in inland areas such as unfertilized
pasture or grazing land or adequately conditioned cropland
may not require replacement literally for generations.

One clue pointing to possible ground system deterioration is
an appearance of base current drift with changing soil moisture
conditions. Another is an apparent long term shift in base
impedance. In both cases, the ground system is suspect, however
the tower(s) may be the cause if not well bonded at section
joints (usually by tack welding). If either of these conditions
appears, the tower(s) should be checked and bonded if this has
not been previously done.

If the condition persists, radials can be checked using a
"sniffer" coil as the external antenna of a FIM. This is
simply a loop coil of several turns 4 to 6 inches in diameter
mounted on the end of a broomstick connected with about 6 feet
of RG-58 and a BNC connector to the FIM's external input.
Moving the "sniffer" over the ground about 45 degrees out
from the tower and comparing results over each radial will
clearly show low induced current in bad radials. One caution
is that this process can show a false good reading when a radial
is broken near the tower. If broken radials are found, it is
wise to check radials by other means such as end to end
resistance at adjacent outer ends, or with a clamp-on RFA
such as was described earlier in this thread.

Unless there is traffic over the radial system on unimproved
roads or paths, or cattle graze the site, it is unnecessary
to check the radials of a system in good soil more often than
every five or ten years unless abnormal operation is suspected.

However, if the soil conditions are poor, annual inspections
should be scheduled. The inspection should be electrical, as above,
for broken radials, and mechanical for visible signs of corrosion
by digging up the ends of a few radials selected at random and
visually observing their condition. Visually, good wire will be
flexible, not brittle, and brown, not blue-green. Cleaning with
sandpaper or steel wool should reveal uniform, unpitted, bright
copper. Pitting, bluish color or brittle conditions are all
indicators of deterioration or failure, however the system will
have a degree of function as long as a low resistance metallic
path remains.

Where cattle graze over a ground system, or where farming or
other vehicles travel over the system on unimproved paths,
annual inspection for broken radials in the affected areas
is essential and consideration to deeper than normal burial
should be considered.

Ground systems and their performance in transmission systems
are one of the topics we cover in the Radio Guide AM Transmission
Seminars.

--------------------------
Phil Alexander, CSRE , AMD
Broadcast Engineering Services and Technology
(a Div. of Advanced Parts Corporation)
Ph. (317) 335-2065   FAX (317) 335-9037



-----Original Message-----

>From: Barry Thomas, CPBE CBNT <barryt at sbe.org>

>Sent: Feb 14, 2008 2:22 PM

>To: A9xw at cs.com, Barry Thomas <barryt at sbe.org>, Member Discussion List

<SBE at sbe.org>

>Subject: Re: [SBE] conversation topic: how often do you check tower

>ground

>

>The two systems would have different characteristics and purposes

>and therefore different inspection schedules and procedures.

>

>For electrical and lightning grounds, they should be inspected after

>every known electrical event. I used to physically check during

>weekly transmitter inspections. I've usually not done a more indepth

>analysis unless there is an increase in storm or electrical damage.

>

>For antenna radials. The system performance usually degrades which

>will initiate inspection but I typically do a full inspection every

>2-3 years if the antenna system is stable.

>

>Sent from my Verizon Wireless BlackBerry

>

>-----Original Message-----

>From: A9xw at cs.com

>

>Date: Thu, 14 Feb 2008 13:12:38

>To:barryt at sbe.org

>Subject: Re: [SBE] conversation topic: how often do you check

>tower ground

>

>

>We're discussing ground system life, how often to check,

>maintenance intervals, etc. Both radial and rods.

>

> Henry


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