> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
> K8RI
> Sent: Wednesday, August 07, 2013 9:11 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] FIRST TOWER & 2ND FLOOR SHACK --HELP
>
> On 8/7/2013 1:56 PM, Jim Brown wrote:
> > On 8/7/2013 9:59 AM, Earl Morse wrote:
> >> Whether you run the conduit to the top of the tower and put your
> >> polyphasers at that point OR run the conduit to the base of the tower
> >> and place the polyphasers at that point OR run conduit through the
> >> house and place the polyphasers at the entrance point its all about
> >> providing that alternate path for lightning that doesn't pass through
> >> your hamshack.
> >
> > It DOES matter where Polyphasers are located -- their function is to
> > protect the equipment to which they are connected, and they work by
> > shorting the center conductor of the coax to the shield. For that
> > reason, they should be as close to the equipment as practical.
> >
> > That is VERY separate from the issue of grounding, which is the most
> > critical aspect of lightning protection. I think most experts would
> > advise that the cables go down the tower all the way to its base, be
> > bonded to the tower at the top and bottom, and go along or in the
> ground
> > to the house, where they should be bonded to the building ground, then
> > come up to the shack. There must also be a bond from your shack ground
> > to the building ground.
>
> Another aspect of grounding the shield at the top and bottom of the
> tower as well as at your SPG at the base of the house is the
> considerable capacitance from the center conductor to ground/shield in
> that length of coax. I believe this tends to bypass much of the pulse
> coming down the line "before" it reaches the rigs, so theoretically
> it's not only better for the rigs but reduces the work the Polyphasers
> have to do.
>
> 73
>
> Roger (K8RI)
The main reason to ground the coax at the top and bottom of the tower is so
it doesn't arc to the tower. The tower being large will have a lower
impedance to ground than that of the coax and will allow less voltage
differential between top and bottom.
There will be a propagation delay between the center conductor and the
shield of the coax therefore there will be a voltage differential at the
bottom of the coax between the center conductor and shield. The lightning
energy will propagate inside of the coax just like a regular radio wave
does.
If you have a short at the top of the coax between center conductor and
shield such as a vhf loop antenna, the lightning energy still will be
present on the center conductor by the time it reaches the bottom of the
coax. This is due partly because the skin effect isolation between the
outside shield and the inside of the shield falls apart at lower frequencies
and allows some current to flow on the inside of the shield as well as the
outside of the shield.
Also the resistance of the shield will allow a voltage drop along its
surface which will allow coupling to the inside of the shield. This is
called the transfer impedance of the cable. Google "transfer impedance" for
more info.
These factors are why a protection device (polyphaser) is needed at the end
of the coax run near the equipment at the single point ground.
73
Gary K4FMX
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