[Amps] crossmodulation in PA ? Thanks for all good??advice

[Roger]

>Not quite true.

>Look at MLI-B-5087B, Military Specification - Bonding, Electrical, and
>Lightning Protection for Aerospace Systems and the FAA Lightning Protection
>Handbook, DOT/FAA/CT89/22.  My copies are old, but bonding straps are 
>provided at
>Flap and control surface hinges to prevent welding from lightning 
>currents.
>Many aircraft are hit by lightning while in  flight.  Would you want to be
>riding on one without properly bonded  airframe and control surfaces?

Do they still do that?
I've never seen a plane with the control surfaces bonded like that although 
I don't get to see many military planes. The only thing I've seen so far are 
static wicks on control surfaces

>A good lightning strike at the top beam could weld the rotor  bearings.  Of

With an average of 3 verified strikes a year to the array and the rotator 
was installed March 10, 2001, that's 7 seasons (including this summer) and a 
minimum of 21 verified strikes with no damage except for some badly etched 
coax connectors on top.

>course a strike that welded the bearings would also  probably destroy the 
>motor

When I had the Midland Repeater out here on the old 90' tower the antenna 
took a direct hit. It blew the top 18" of antenna apart and blew out the 
7/8" heliax about 20 feet down the tower. It also removed the "coax seal" 
from every connector at the top of the tower and removed the plating from 
those connectors.  I had a Cushcraft ATB-34 at the top of the tower and a 
Ham IV about 5 feet or so down in the tower. There was no damage to the 
rotator or control (or the ATB 34)

73

Roger (K8RI)

>windings and trash the rotor anyway.  The  bearings would be the least
>problem.

>Bob

K8MLM

In a message dated 9/7/2007 11:47:48 A.M. Eastern Daylight Time,
k0rc at pclink.com writes:

This  misplaced bonding effort applies to bonding around a rotator as well.
We  already dispelled the myth (on the TowerTalk reflector) of rotator
bearnings  being welded by a lightning strike. Common sense tells us the 
rotator is
protected by a Faraday shield anyway (the tower).

73 de Bob - KØRC in  MN


---------------Original thread---------------

Message:  6
Date: Fri, 7 Sep 2007 07:11:41 -0400
From: "Tom Rauch"  <w8ji at contesting.com>
Subject: Re: [Amps] crossmodulation in PA ?  Thanks for all good
??advice
To: "k7fm" <k7fm at teleport.com>, "Nils  Petter Pedersen"
<la7sl at online.no>
Cc:  amps at contesting.com
Message-ID:  <002b01c7f13f$e37c3430$640fa8c0 at radioroom>
Content-Type: text/plain;  format=flowed; charset="iso-8859-1";
reply-type=original

Hi  Colin,

>I mentioned this morning that I had some question about the
>copper bonding
> across the tower joints, and raised the  question that it
> could create
> corrosion.  Copper has a  .35 volt potential and
> hot-dip-zinc has a 1.20 volt
>  potential.  Even though the tower is bolted together with
> bolts,  there is a
> process called "fretting" that can  cause corrosion to
> occur between the
> metals that are otherwise solidly  joined.

My point was that with rare exception bonding a tower joint
is a waste of time. There are tens or hundreds of thousands
of sheer  pressure on the bolts in a typical cross-bolted
tower joint.

While  I agree dissimilar metals should be avoided, placing
them across a tower  joint is meaningless. How good would the
diode be if it is shorted  end-to-end with what we could
consider a zero ohm connection? The same is  true for
lightning. Lightning doesn't care a bit if the joints are
bridged or not.

There are some rumors that bonding the joints helps  things,
but they probably came from looking at early broadcast
towers  with pad joints. In many cases those joints would
have brazed connections  jumpering the joint, but in later
installations that was practice  abandoned after it was found
unnecessary. This probably spawned the idea  Hams should
jumper joints. Anyone who thinks a couple stainless steel
clamps with a few dozen pounds per square inch clamping
force will  significantly change the connectivity in a joint
bearing tens of thousands  of pounds force probably hasn't
thought about the system.

It really  is meaningless. The possible exception is in
systems ready to fall down  anyway.

73  Tom

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