[TenTec] RF Feedback on Omni VI (opt 3)

[W1GOR]

Mark,

I forgot to mention that coax 'braid' and copper wire are very poor RF
ground conductors, due to their inherent high inductance.  Polyphaser and
other suppliers sell copper 'strap' measuring one and one half inches wide,
and other widths, up to about six inches wide.  This flat strap exhibits
very low inductance and is the only material that I use as my station
ground.  Another item that I forgot to mention is the ground resistance of
your ground rods.  Simply driving copper-clad iron rods into the ground may
not provide the lowest ground conductance.  When my home was built a few
years ago, the electrical contractor simply drove several ground rods into
the backfill next to the foundation.  This area consists of 4 to 6 inches of
so-called top soil, and more than 8 feet of sand (for drainage).  The RF
conductivity of the sand is nearly immeasurable (almost infinity during a
dry spell), and is, for all practical purposes, no ground at all...

When I built the new station, I installed a chemical ground system, based on
a design that is commercially available at $500 or more.  The chemically
enhanced ground system is quite simple to build. Two versions are
commercially available, a straight type and an 'L-shaped' type.  The
straight type is made of an 8 to 10 foot piece of copper pipe, 2 to 3 inches
in diameter, with a cap at the bottom end.  The 'L'-style is 24 to 30 inches
of vertical pipe and 8 to 10 feet of horizontal pipe, joined by a 90 degree
elbow.  Again, cap the lower end.  Drill 20 to 30 small holes (1/8 inch
nominal) along the last 2 to 3 feet of the pipe.  If you have soft or sandy
soil, auger a 5 to 6 inch diameter hole, down to about 7 feet, or use a
post-hole digger.  If you have rocky soil, and are limited as to depth, you
can dig a trench 2 feet down and 10 feet long, and use the 'L' style.  Make
the trench about the width of your shovel.  Attach a heavy ground strap to
the pipe at about 2 feet from the top end.  The best method is to weld the
connection, or use an exothermal device (cad-weld).  Do not solder this
connection.  A solder connection will not handle the high currents of a
nearby lightning strike.  If welding or cad-weld is not available, fasten
the ground strap to the pipe with several stainless-steel hose clamps,
treated with Penetrox or an equal product.  Then completely wrap the
connection with 3M tape, and several layers of mastic to prevent corrosion.
If at all possible, welding the strap to the pipe is preferred.

Now for the chemical part of the chemical ground system.  Mix 2 parts of
calcium chloride (ice melter) with 1 part of sodium chloride (rock salt).
Mix enough to fill the pipe and tap the pipe with a rubber mallet to help
settle the mixture.  Add enough of the mixture to fill the pipe within
several inches of the top.  Cap the top of the pipe with a copper cap that
has been drilled with small holes to admit air.  Calcium chloride and sodium
chloride have an affinity to the moisture in the air.  It is not necessary
to add water to the mixture.  As the chemical mixture acquires natural
moisture, a chemical reaction will cause the chemicals near the bottom end
of the pipe to leech out through the small holes that were previously
drilled there.  As time goes on, the ground conductivity will improve due to
the leeching action.

The actual installation depends on which type system you use.  In the case
of the straight, vertical system, fill the bottom 6 inches of the hole with
cheap kitty litter.  Most kitty litter is composed mostly of Bentonite, a
clay that absorbs moisture (for obvious reasons).  When Bentonite is
moistened, it swells to contain the moisture.  You can buy Wyoming Bentonite
from many suppliers, but the kitty litter is more readily available and a
lot cheaper.  After filling the bottom of the hole with kitty litter, stand
the pipe in the hole, supporting it to keep it centered. Then add kitty
litter around the pipe until you are within a foot to a foot and a half of
ground level.  Make an inspection well from a used 5 gallon plastic bucket,
one with a snap-on top.  Cut the bottom out of the bucket and dig enough
soil from around the copper pipe to allow the bucket to fit down and around
the pipe to a level that will allow about 2 to 3 inches of the bucket to
remain above ground.  Drill or punch a lot of air holes in the lid of the
bucket, but don't drill directly over the middle 4 to 5 inches.  This will
allow rain and snow to enter the bucket lid, without directly entering the
capped top end of the copper pipe.  Rain and snow will leach into the kitty
litter and allow it to swell, forcing a tight fit between the copper pipe
and the surrounding soil or sand.  As reference, the Harger system does NOT
use Bentonite.  Harger backfills their system with a carbon-based mixture
that is conductive to begin with.  The exact chemical composition of
Harger's backfill is unknown.  Harger states that their mixture is superior
to Bentonite because it does not shrink when ground moisture is low, due to
lack of rain or snow.  This is something to consider for those living in
dryer climates.  Harger sells their carbon-based mixture separately, but it
is much more expensive than Bentonite.  Common kitty litter will work very
well in most areas, and you can always water the area from time to time, in
order to keep the kitty litter moist.

Installing the 'L' style pipe is slightly different than the vertical style.
Dig the trench deep enough to accommodate the 'L' shaped pipe.  Again, set
it deep enough to place the vertical portion of the pipe in the same
relative position as the straight style pipe.  Use the plastic bucket scheme
to protect the top of the pipe and allow inspection of the system.  Again,
when drilling air holes in the plastic bucket lid, avoid drilling these
holes directly over the perforated top copper cap, to keep rain water from
directly entering the pipe.   Before placing the "L" shaped pipe in the
trench, line the bottom of the trench with several inches of kitty litter.
Place the pipe in the trench and add kitty litter to fill the spaces around
and over the copper pipe.  Cover the copper pipe with about 2 to 3 inches of
kitty litter and then fill the remaining trench with soil, packing it down
with a tamper or the back of a shovel.  You can plant grass or other ground
cover over the horizontal portion of the system.

The commercial versions of these chemically enhanced ground systems are
available from Harger (http://www.harger.com) and
Lyncole XIT Grounding (http://www.lyncole.com).  I have used these systems
in many commercial installations and they work very well.  At $500 plus,
they are over-budget for most Amateur Radio Operators.  Copper pipe and
related fittings are expensive
when new, but affordable if you buy used materials from salvage companies.
The structural integrity of the pipe is more important than physical
appearance, since you're going to bury it anyway.  If using salvaged copper
pipe, be sure to remove all paint and other coverings before burial.  I
check my system once a year by opening the plastic bucket lid, and removing
the perforated copper cap.  If the calcium chloride/sodium chloride mixture
has settled, I simply add more of the mixture to bring the level up to the
top of the pipe.  Keep unused quantities of the mixture in a tightly sealed
bucket.  The calcium chloride is also available as DOW FLAKE, a chemical of
much higher purity than common ice melter.  If you can obtain DOW FLAKE
locally, I'd advise using it in place of the less effective product.  Sodium
chloride, or common salt, is about as pure as one would need.  I use coarse
rock salt to keep the mixture from spilling out of the leech holes at the
lower end.

Be aware that the commercial versions of these systems are covered by
patents and other manufacturing rights.  Using the design for personal,
non-commercial purposes is allowable.  Just don't decide to manufacture and
sell your own version...

73, Larry - W1GOR





----- Original Message ----- 
From: "Mark Erbaugh" <mark at microenh.com>
To: "Tentec (E-mail)" <tentec at contesting.com>
Sent: Monday, May 19, 2003 23:34
Subject: [TenTec] RF Feedback on Omni VI (opt 3)


> My Omni VI option 3 upgrade has developed symptoms of RF feedback. I got a
> report of RF in my transmitted audio. I'm also hearing muffled audio in
the
> speaker or headphones when transmitting. I get this problem even when
> transmitting into a dummy load. The muffled audio in the speaker seems
> pretty much the same whether I'm  on 75m or 10m.
>
> My Pegasus is sitting next to the Omni VI. I moved all the cabling from
the
> Omni VI to the Pegasus. The Pegasus doesn't appear to have the RF feedback
> problem.
>
> My ground isn't ideal. I have about 15 feet of RG8 coax braid going to the
> copper ground wire that runs from the electrical panel to grounding rods
> (2). There is about 8 feet from where the braid joins the ground wire to
the
> ground rod. So I guess my total length to ground is about 23 feet.
>
> I do plan on shortening the path to ground and using heavier wire, but
> that's going to require drilling through the poured walls of my basement.
> The two ground rods for the electrical system are just outside the wall of
> my shack (about 6 feet apart). By drilling through the basement wall, I
> should be able to get the total length to the first ground rod down to six
> feet or so. Is it likely that a third ground rod for the shack (but
> connected to the other two) would help?
>
> Any ideas?
>
> Thanks,
> Mark
>
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