On Tue, 02 Dec 2008 13:51:29 -0500, Bill Winkis wrote:
>We know for good protection we want the path of least resistant for a
>charge to make it to ground dissipation
The requirement is for a low IMPEDANCE, which is the combination of
resistance, inductance, and capacitance. All of these components vary
with frequency. Nearly all earth connections are dominated by INDUCTIVE
REACTANCE, NOT resistance. The primary importance of resistance is that
the conductor should not melt (or vaporize) before it conducts a
lightning strike.
>Point #1 ......I know for RF it's NG to use stranded cabling for
>ground rod tie ins etc....usually most Hams would use solid 4.0 for
>tie ins...but is there some point that a stranded cable would equal
>a solid cable...for instance would a number 2 stranded equal a 4"
>solid....?????
It is NOTHING to do with RF, and everything to do with CORROSION. As
compared with a solid conductor, stranded conductor has more exposed
surface area; braid has far more than that. That's why braid and stranded
copper are bad for use outdoors. Both are fine indoors, and outside if
well sealed against corrosion. Wide solid copper strap is widely used
because it provides the least inductance and the least resistance (least
skin effect) with the least surface area for corrosion (as compared to
stranded or braid).
>Point #2 ......We see an awful lot of low inductance braided flat
>tinned copper ....from 1/2 inch and up to 2 inch plus being sold as
>RF grounding cable...so what's the difference flat or round????...
Wide flat conductors generally have lower inductance and resistance than
a round conductor of the same cross sectional area, thanks to skin
effect.
>Point 3 ..... How do we think/plan for a lightning ground protection
>system versus a RF radial reflective ground system when they are all
>tied to Mother Earth in one fashion or another...??
Radial systems provide a return for the currents and EM fields produced
by certain antennas (like verticals). To do this, they do NOT need, nor
do they benefit from, a connection to earth. Radials CAN reduce the
impedance of the earth connection by providing capacitance to earth. For
this reason, they SHOULD be bonded to the ground system.
>I have a new tower ready for the grounding system ..... plan is to
>set up a spark gap on each tower leg.....from the gap run #4 solid
>copper to an 8 foot copper clad rod ...then out 16 feet to another
>copper rod....now on the this rod 2000 feet of # 14 copper stranded
>will be laid in 60 foot fan.....(one of these radial fans will meet a
>similar radial system under a Hygain HyTower.....so it ends up being
>a 240 foot radial patter....
>Why the spark gap?? Dissimilar metals...
That's a new one on me!
>But instead of that #4 solid copper I sure would like to use 5/8 inch
>diameter #1 or 2 stranded copper.....
>Comments..???
You will, of course, need a low impedance bond (beefy copper, short)
between the tower ground electrodes and the building's power system
ground electrodes.
The idea of separate radials suggests some confusion. The energy in
lightning is NOT DC! It has a DC component, but the energy is broadly
concentrated in the MF spectrum (300 kHz - 3MHz). Thanks to inductance,
lightning hitting your tower is unlikely to see the end of those radials.
Lightning will primarily see conductors in contact with the earth CLOSE
to the tower. More ground rods is good, more copper in contact with the
earth is good. All that means parallel paths, which reduces both
inductance and resistance.
Also, be sure to bond the tower to structral steel in the concrete tower
base, bond all elements of that structural steel together, and bond all
of that to the ground electrode system. That concrete base now becomes a
Ufer ground electrode, which, in general, tend to be a rather good ground
because they have a lot of conductor (concrete) in contact with the
earth.
The primary benefit of long radials is as a return for RF fields and
current from the antenna. For antenna radials, #18 or larger copper will
work fine. You can use stranded copper if it's insulated well enough that
it doesn't corrode.
2,000 ft of wire is not a lot of wire for a radial system. You pay for
copper by the pound. A good radial system needs more wire, but doesn't
need a lot of beef. In general, if the total length of copper is a
criteria, you're better off with more short radials than a few long ones.
That's because the RF current is greatest closest to the base of the
antenna. I suggest that you study the excellent material on N6LF's
website. It is absolutely definitive on the topic of radial systems for
HF antennas! The radial system for my 160M vertical is a mish-mash of #
18-#14 insulated copper that I was able to buy cheap.
73,
Jim Brown K9YC
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