Gregg,
It is all about protecting your equipment. Grounding the coax at the bottom
of the tower will help take any lighting induced energy from tower into the
shield of the coax and dump it to your ground system under the tower. You
should also ground the coax at the top of the tower, so the coax shield and
the tower are at the same potential (or close) from the top to the bottom.
This will help prevent arcing between the coax and the tower which will
destroy the coax. Don't forget rotor and any other control lines. You
should have a protection device grounded to the tower right at the rotor for
the control lines, usually an MOV from each wire to the tower and again at
the bottom of the tower. You can make these yourself or buy a commercial
unit. Everything that comes down that tower should be grounded at the
bottom of the tower and where it mounts. The bottom of the tower will be
the "coldest" point, so deal with getting rid of a strike as much as you can
there and then deal the remaining energy at the ground window before it goes
into the hamshack. Going that extra step would be putting lighting
protectors on each coax line at the bottom of the tower as well as at the
shack ground window. The ground window should also have a radial ground
system to spread out remaining energy.
In my shack everything that is powered excluding lighting, goes through a
Polyphaser power entrance box that is grounded to the ground window. This
way, any lightening surge will rise and fall in all the equipment at the
same time and at the potential of the ground window, while isolated from the
rest of the house as well as I can reasonably get it. Many direct strikes
over the last 20+ years have caused no damage. I run an AR Cluster node here
that is on 24/7 with no lightning damage for over 20 years and it is never
off. Follow good practices for grounding your shack equipment to the ground
window as well.
Which reminds me, it is way past time to swap out the MOV's in the system.
Jim, KR9U
__________________________________
Second question is about the need/benefit of grounding coax cables at the
bottom of a tower. I am recabling my antennas after years of using the same
cables. I have read where you should ground the sheild toward the bottom of
the tower so I bought those grounding clamps from DX Engineering and they
same really nice but why do you need to do that? The tower in question is 85
feet tall Rohn 45 and has a 40 meter CC beam on it and also a CC WARC
rottable dipole plus a 75 meter interted V on a metal sidearm. None of
those antennas are at ground potential. It seems to me that there is just
more connectors to go bad at some point. The tower is well grounded with 3
25 foot legs of number 0 copper wire with ground rods every 8 feet along the
25 feet of wire. It works too as the tower has been drilled by lightning at
least once with no damage, cables were disconnected from gear and tied to my
station ground. Just trying to understand and learn why some say to tie the
sheilds to ground at the
tower base.
Thanks
Gregg K9KL
****
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