[TowerTalk] Vertical antennas and lightning

[Jim Thomson]

Date: Sun, 01 Aug 2010 18:14:35 +0000
From: "K1TTT" <K1TTT at ARRL.NET>
Subject: Re: [TowerTalk] Vertical antennas and lightning


Unfortunately in this case the 'bleeding' when there is a charged cloud over
head is putting charge ONTO the antenna from the ground to equalize it with
the rest of the charge being attracted to the base of the cloud.  So it make
it MORE likely to start the upward streamer that completes the path.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

##  I think it was W0un,/w3lpl who said, that his property had loads of 
well grnded towers... and each tower had loads of well grnded yagi's 
[els bonded to boom, etc]  .   The story went, that every time  lightning started hitting
the grnd.... as it approached his ant farm... it would stop... and pass right over top,
then  continue hitting the grnd... a 1/2 mile away.    This apparently happened,
regardless of whether the lightning path  was E-W  or W-E... or N-S/ S-N
 etc.   The theory was... the well grnded ant farm was draining off the
charge overhead. 

Someone mentioned using 500k to 3 meg reistor's  to dc grnd the center
conductor + control lines.    IF the control lines all just have MOV's  between
each conductor and grnd.... do you parallel  one reistor with each mov ?? 

OK.... what abt on a new tower... b4 concrete is poured.   On a previous install
back in 1980... we  jack hammered 3 x grnd rods at the BOTTOM of the hole..then cadwelded
cables to em..then one cable bonded to each tower leg.[2" away]    In that case... the tower
was 34"  wide, straight sections... and angle steel legs... and the 1st 6'  of the 20'
section  went right to the bottom of the 6' hole. 

The idea was,  if the tower got hit with lightning....the path would be down the 
3 x legs... and  to the 3 x 8' rods.... which were 14' below grnd level.   IE:  rods
started at 6' below grnd..and ended up at 14' below grnd.   That puts all 3 of em well below the
frost line in winter.   In dec/jan... with -17 deg C temps... the 1st 3'  of grnd would be frozen solid.

IMO... I'm not fond of the ufer grnd scheme.   In the above case.. NONE of the rebar cage was
bonded to the tower legs/rods.   I find it hard to believe that concrete  will conduct at all. 
One of the tower reps up here told me one of their towers got hit... and that tower did have the
cage bonded to the tower legs  [ ufer grnd].. no rods... and that chunks of concrete were
flying skyward every where. 

On my old trylon 8 yard  tower base.. the anchor stubs are still sticking up 7"  above the cement...
and none of em are bonded to the rebar cage.   I can't draw any current between any two of em..
using an isolation xfmr... and variac... and stuffing 240 vac  between  any 2 x anchor legs. 
Perhaps concrtete conducts  with several kv through it.   I could try the experiment again,
with a small, spare  46 lb dahl plate xfmr.. [2700 vac sec]. 

On the trylon tower... we jack hammered in 3 x 8' rods..next to the concrete..and cad welded 
all  of em to 2/3 ga cu wire.   Then compression lugs and bolts on each tower leg.   I will
re-use  em.. and re-do the metal work... then bond to adjacent new tower. ..+ add 1-2 x new
8' rods. 

i take it the SPG...is still  60' away... just outside the basement ?    I have another rod there..
and  60' of  2 ga cu  bonded between the tower base and my single rod  outside the basement
window.   I used a large  1/4"  thick al plate just inside the basement.... and bonded to rod outside
with 2 ga wire [ rw-90] .   The old 3/4"  copper water line coming into the house cracked.. and was replaced
with 1" plastic pex pipe.   So the bonding between  200A main panel and inside basement cu pipe is
not doing anything..since the inside 3/4"  CU pipe.. morphs into the 1"  plastic pipe.  I re-routed grnd
from the 200A panel... and ran it over to my  2 sq foot  AL plate in basement.   So I guess the al plate
now becomes my spg ?   The same al plate is drilled and tapped for  other stuff..like henry radio LP filter
lightning arrestor's   etc. 
  
later... Jim   VE7RF