Grounding coax at the base of a tower would help to reduce the voltage
difference between the shield and the tower. in the event of a nearby
lightning strike or even electrical transients in the area (noise) the small
gap between the tower leg and shield acts like a loop which would create a
voltage difference all along the length of the coax and the tower. by
grounding the shield to the tower leg you short out that loop which 'should'
help keep the voltage across the outer insulation lower and 'may' prevent
damage to the insulation from an arc caused by that voltage... it would also
help prevent noise picked up by that loop from following the shield into the
shack where it may get into the receiver through leakage in cases or pin1
problems... but note there is still usually another loop that can pick up
induced voltages and noise that is the coax shield to the tower to the ground
lead back to the shack, this is often bigger than the tower leg loop and could
generate higher voltages which is why it must be grounded again at the shack
end. Note, the voltages are present with or without arresters.
Arresters act to equalize the voltage between the shield and center conductor,
or on control cables between the ground and the signal lines in the event of a
large fast rising transient such as lightning. This helps protect equipment
connected to the cables from becoming the path that equalizes that voltage,
note that they do not reduce noise other than that big bang when your radio
blows up.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: Jorge Diez CX6VM [mailto:cx6vm.jorge@gmail.com]
Sent: Sunday, December 28, 2014 14:56
To: David Robbins
Cc: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Arrestor placement
Hi
And grounding coaxials is good for other porpoise than protecting equipment,
for example to reduce noise? Or doesn't help to reduce noise at the station?
Thanks
Jorge
CX6VM/CW5W
Enviado desde mi iPhone
> El 28/12/2014, a las 09:51, "David Robbins" <k1ttt@arrl.net> escribió:
>
> The PRIMARY place for arresters is at the equipment you want to protect.
> Other places like at the base of the tower are only protecting the coax or
> maybe a remote switch. To protect the radios put them at the entrance
> panel, anywhere else is a waste of money.
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt@arrl.net
> web: http://wiki.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://k1ttt.net
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
> Thomson
> Sent: Sunday, December 28, 2014 04:19
> To: towertalk@contesting.com
> Subject: [TowerTalk] Arrestor placement
>
> Date: Sat, 27 Dec 2014 18:48:41 +0000 (UTC)
> From: David Gallatin via TowerTalk <towertalk@contesting.com>
> To: "TowerTalk@contesting.com" <TowerTalk@contesting.com>
> Subject: [TowerTalk] Arrestor placement
>
>
> Hello all,Somehow I have gotten the idea that one's lightning arrestors are
> best placed at the bottom of the tower. This seems logical to me. Then I came
> across another piece that said, no, arrestors are to be placed at the shack
> entrance panel. Which is it? Or does it matter???73, David, AA9G
>
> ex W5DCG and KC9EEV
>
> ## Install one at the base of the tower AND also where the coax enters the
> home. In my case, the two arrestors are 65 feet apart.
> The one at the base of the tower is aprx 4 feet above ground. Base of
> tower is earth grounded via 3 x 8 foot deep copper clad
> ground rods..all cadwelded. 2 ga bare cu wire to each leg of the tower.
> Another 65 foot of bare 2 ga cu wire, C clamped to
> basement entrance + SPG...where a 4th ground rod is installed...and also
> cadwelded. The HOFFI brand lightning arrestor at the
> base of the tower serves other functions. It’s the interface between the
> rigid coax from the shack to the base of the tower.
> Flexible coax from tower lightning arrestor going up the side of the HDX-689
> crank up tower. Lightning arrestor in mounted to the
> spg is the 2nd hoffi brand arrestor. Remote coax switch box is mounted to
> the mast. All braids from each yagi coax is bonded to the
> mast via the remote switch box. The lightning arrestor at the base of the
> tower bonds the coax braid to the base of the tower... via the
> casing of the lightning arrestor.
>
> ## Like K1TTT points out, you need a 2nd arrestor where the coax enters
> the home. Now if the tower base is close to where the
> coax enters the home, then you could get away with only the one arrestor.
> If hairpin or beta matches are used on the yagis, the center conductor
> will be bonded to the braid of the coax anyway..right at the feedpoint..via
> the hairpin or beta match. In the case of the beta match, both the center
> conductor and braid are bonded to the boom. Either way, the center
> conductor is bonded to the braid..which is then bonded to the top of the
> tower.
>
> ## If it gets past all of that, the arrestor at the base of the tower
> should do the trick. If it gets past that, then it has to get past the
> arrestor at the SPG.
> If you have a OWA yagi, the center conductor is not bonded to anything...
> and the arrestor is def required. You could always install a 15-50 uh coil
> across the feedpoint of the OWA... which may not be effective for lightning
> hitting the center conductor side of the DE. A 3rd arrestor at the top of
> the
> tower would also work with any OWA yagi.
>
> ## A 35 uh coil or 12 ga magnet wire wound on a torroid like ICE uses....
> at the spg will also dc ground the center conductor. ICE uses the back emf
> across the coil to fire the gas discharge tube that is in parallel with it.
>
>
> Jim VE7RF
>
> _______________________________________________
>
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