[TowerTalk] Gas Tube protection for remote antenna switch's

[K1TTT]

the ONLY function of a lightning arrester is to equalize the voltage between
conductors.  It clamps the voltage between conductors to some low value that
is designed to prevent damage to other components.  They are really only
effective if they are close to the equipment being protected since any
exposed conductor may allow induced voltages to accumulate.  And they have
to be on ALL power and signal conductors at the same location.  

 

There is a real danger in choosing the wrong location for an arrester.  For
instance, you say you were going to put them at the base of the tower.  Lets
assume you do that and then run the coax into a radio in the shack without
another arrester at the entrance spg.... when lightning strikes anything on
the tower and reaches that lightning arrester the voltage between the shield
and center conductor is equalized, but its NOT zero!  suppose your tower
ground has a good 10 ohm impedance, and the stroke current reaching it is
10kA... that is still 100kV that is now on the center conductor of the coax.
(voltage reference to distant 'ground').  The arrester at this point is
protecting the cable and connector at the bottom of the tower by equalizing
the voltage between the conductors and all is well.... that is until that
100kv travels down the coax to your shack.  now here if you don't have a
proper spg entrance with another arrester that 100kv goes straight to the
radio, sees the nice low voltage path out through the power line to the
distant ground at 0v and pop goes the radio.  now, lets say you do have a
spg but without arresters.  in this case the voltage gets equalized between
the shield of the coax and the ground of the radio, but not between the
center conductor of the coax or the power conductors of the radio, so now
the high voltage on the center conductor either arcs over the connector to
the shield or goes through the radio to the low voltage power conductors.
So no matter where you put an arrester outside, you STILL need them on all
conductors at the spg so that everything in the shack is at the same
voltage.

 

 




Jul 27, 2010 08:33:29 AM, jim.thom at telus.net wrote:

Date: Mon, 26 Jul 2010 11:07:38 -0700
From: "Jim Brown" 
Subject: Re: [TowerTalk] Gas Tube protection for remote antenna
switches;

On Mon, 26 Jul 2010 05:46:46 -0700, Jim Thomson wrote:

>## putting a shorted pl-259 in pos #1 just used up pos #1. 
>Why not use a torroid.. like 45 uh... using 10-12 ga magnet wire. Then,
>the center conductor is always at DC grnd. [ on the cable heading towards
>the station.. ie: input side] 

Repeat after me -- LIGHTNING IS NOT A DC EVENT, IT IS AN RF EVENT. Write it 
100 times on the blackboard! :) That choke only discharges DC. 

73, Jim K9YC

## Point well taken.... and I thought the same thing... abt 2 x hrs after I
made that comment.

## The problem with the ameritron RCS-10 box is.... all the un-used ports
are floating.. and not grnded
[like the as remote box's]. With the rcs-10 box.... with the power shut off
to the controller, the default is 
port #1. Their book sez if you stuff a grnded pl-259 in there... then ur
coax center conductor is bonded to
tower [ via the remote box casing] 

## ok... this brings up another point. What about your super high Z choke
baluns, when used on a yagi ? 
They will also look like a high Z.. and probably 45-90 uh. Have you ever
measured the uh on them...say 
from braid of input to the choke.. to the braid on the output side ?? What
kind of readings do you get ?? 
say with a run of the mill simple LCR meter. Or is the resistive component
so high, that there is little uh.

## what happens when lightning hits the DE of the yagi..or will it opt to
hit the boom /other ele's instead ?? 

## On the INPUT to my remote switch box on the mast, abt 3' above the top of
the tower, is where I have the 
type-31 choke balun. This is to just supplement the existing bead baluns at
each yagi's DE. The idea here was,
if RF spillover gets past the existing type 43 bead baluns.... it won't get
past the type-31 balun on the input to the
remote switch box. The type-31 balun of course, is being used on all
bands..since it's on the input to the remote switch 
box. 

## speaking of lightning arrestor's.... mine was slated to go at the base of
the new crank up tower.. and tied in to the
3 x cadwelded 8' rods, buss system, etc. Should it be at the top of the
tower instead.... or should we be using a lightning 
arrestor at both the top and bottom of the tower ?? If I install a 2nd
lightning arrestor at top of tower.... which side of the
type -31 balun should it be on ?? As is... the braids of all coax's are
bonded to top of tower [via remote switch box] 
Braid of single coax is also grnded at base of tower

Tnx.... Jim VE7RF 


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