I use my tower as a quarter-wave vertical on 160. It is on a base insulator
salvaged from a damaged AM broadcast tower, and I use 120 quarter-wave
radials, each about 133' of #12 bare copper wire.
I installed the tower in early 1981 and the radial system was completed in
1982. Radials are brazed to heavy copper strap, using silver alloy solder
applied with a Mapp gas torch. I don't recall whether or not I installed
any ground rods near the tower base for lightning protection before
completing the radial system.
I use a lightning gap across the base insulator, just like an AM broadcast
tower. In fact my tower is identical to an AMBC installation, except the
dimensions are scaled down to 160m, and the guys are broken up every 18' in
order to keep them RF transparent though 7 mHz. I also use the tower to
support a 135' dipole, fed with open wire line and hung at the 119' foot
level.
The only lightning damage I have sustained over the years is when one
near/direct hit took out all the strain insulators in the BOTTOM set of
guys, leaving all other levels intact, and another time I noticed that the
insulators at the ends of the dipole had been heavily damaged by lightning.
I replaced the end support wires and end insulators with Phillystran cable.
For house wiring, #12 copper is rated for at 20 amps continuous if I recall
correctly. Therefore, my 120 radials should take 2400 amps continuous, and
much more than that with the short duty cycle of a lightning hit.
I have never felt any need for ground rods near the tower. I do use them at
the guy anchor points, per the Rohn catalogue/manual. Because of its short
pulse cycle, I believe that lightning should be treated more like RF than DC
or low-frequency a.c., so IMO radials should be more effective than ground
rods, although I have heard claims to the contrary (without any
explanation). I seem to recall from somewhere that the dominant energy in a
lightning pulse tends to be in the vicinity of 10 mHz, so if you are
installing a lightning protection system, radial and conductor lengths etc.
should be chosen that would minimise rf at that frequency.
I have never had any damage to the antenna tuning units at the base of the
tower, although I try to keep them disconnected whenever there is a
lightning threat, but at the same time, lightning has wiped out quite a bit
of my stuff in the house connected to the a.c. power system. Once a jolt
welded a number of the house light switches closed, but they are the old
fashioned kind with ceramic body and access to the guts when the switch
plate is romoved, so I was able to insert a thin blade inside and pry all
the welded contacts apart, and they worked normally again without having to
replace any of the switches. That same jolt did no damage whatever to my
antenna system or ATU's, but it wiped out a thermocouple rf ammeter that
remained connected at the transmitter end, to the buried feedline that had
been DISconnected from the antenna system!
Don, k4kyv
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