[RFI] More info

tduffy tduffy <tduffy@sygnet.com>
Sat, 02 May 1998 12:50:51 -0400

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May 1998 23:00:50 -0500 From: dale and sue Reply-To: svetanof@mc.net
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Robert Smithwick" CC: RFI Reflector Subject: Ground Rod Placement
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7bit Hello, Smitty. Thank you for the kind words. There are LOTS of
excellent experts on this reflector, which is one factor that makes it
so good. You have already had very good advice from W4DES and K0IL on
this subject. I can only expand upon their comments a wee bit: 1. I
notice that ALL of the antennas listed in your original message are
balanced (Yagis and dipoles). You did not specify the feedline
arrangement, but I will assume use of coax with baluns up at the
antennas. Such a configuration does result in minimum RF ground current
from the rig. Your "simple ground" is, most likely, adequate in that
respect. 2. As I understand your message, you have two (2) ground rods:
one for the rig, and another for the tower. You don't say if the two are
bonded together in any way. They should be if they aren't. 3. I, too,
hope that the ground rod for the rig is not in your basement or house,
but is at least a little ways outside of the house and/or foundation
wall. The concern, as the others have stated, is for lightning reasons.
4. K0IL has mentioned the Polyphaser book. I have it, too, and it is
excellent. As he says, it is imperative that a unipotential system be
maintained to minimize damage to both the equipment and your house. With
that tower, you also do want to be certain to have lightning suppressors
on each of the coaxes coming thru the wall and into the shack. (As
well-illustrated in the Polyphaser book, you have to regard the tower
structure and the vertical cable runs as parallel inductors. When
lighting current flows off the top end of the tower, a considerable
potential difference occurs between the top (antenna) end and the ground
end (product of the huge [>50kA] lightning current and the Xl value of
the "inductors"). That difference appears across the cables, between
center conductor and shield braid. (For whatever it's worth, the
connectors on your RG-8 or RG-213 will break down somewhere around 5 kV,
but that's too late for the rig. I have seen RG-8 used as high voltage
wire in systems operating as high as 25 kV, so it may hold up.) 5. For
whatever it is worth, it may be interesting to note that the Polyphaser
suppressors work on a different principle than the Alpha-Delta,
Cushcraft, and other suppressors that use a spark gap mode (sealed or
not); the Polyphasers actually limit the total energy that can be passed
onto the rig. They do it by capacitive divider action. (The gap style
units can not limit thru energy until the gap fires and shunts it.) That
is why you must specify frequency range and power when buying them. They
are truly DC blocked because 1 cap is in series with the center
conductor and the other is in shunt to ground on the rig side of the
device. 6. The best tower grounding concept I have seen comes from my
days back with the "original" Ma Bell: EACH leg of a tower had its own
ground rod, and all rods were bonded together to maintain unipotential
operation. Also, the leads from each leg to their respective rods were
gently curved out away from the concrete base, as you want to minimize
any current flow through the concrete; due to its inherent moisture
content, it can and will explode (from steam pressure) if too much
"juice" passes thru it. I am glad you have had no problems to date. But
with lightning being the unpredictable critter that it is, being
prepared makes sense. Happy DX. 73, de Dale, WA9ENA EMC Engineer
Lindgren RF Enclosures

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